Issue |
A&A
Volume 505, Number 1, October I 2009
|
|
---|---|---|
Page(s) | 29 - 44 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200912126 | |
Published online | 03 August 2009 |
H I line observations of 2MASS galaxies in the zone
of avoidance
,![[*]](/icons/foot_motif.png)
W. van Driel1,2 - S. E. Schneider3 - R. C. Kraan-Korteweg4 - D. Monnier Ragaigne5
1 - GEPI, Observatoire de Paris, CNRS, Université Paris Diderot,
5 place Jules Janssen, 92190 Meudon, France
2 -
Station de Radioastronomie de Nançay, Observatoire de Paris,
CNRS/INSU, 18330 Nançay, France
3 -
University of Massachusetts, Astronomy Program, 536 LGRC, Amherst,
MA 01003, USA
4 -
Department of Astronomy, University of Cape Town,
Private Bag X3, Rondebosch 7701, South Africa
5 -
Laboratoire de l'Accélérateur Linéaire, Université Paris-Sud,
Bâtiment 200, BP 34, 91898 Orsay Cedex, France
Received 20 March 2009 / Accepted 27 April 2009
Abstract
Aims. A pilot survey has been made to obtain 21 cm H I emission line profiles for 197 objects in the zone of avoidance (ZoA) that were classified as galaxies in the 2MASS all-sky near-infrared Extended Source Catalog (2MASX), as well as a further 16 2MASS pre-release working database sources that did not make it into 2MASX.
Methods. One hundred sixteen of the 2MASX sources and the 16 working database sources were observed using the Nançay radio telescope, usually in the 325 to 11 825 km s-1 range, and the other 81 2MASX sources were observed with the Arecibo radio telescope in the -500 to 11 000 km s-1 range, and for 9 also in the 9500 to 21 000 km s-1 range.
Results. Global H I line parameters are presented for the 22 and 29 2MASX objects that were detected at Nançay and Arecibo, respectively, as well as upper limits for the undetected 2MASX objects. Another galaxy (ESO 371-27) was detected in the Nançay beam centred on an undetected target, ESO 371-26. Nançay data on 12 sources could not be used due to high rms noise levels, most likely caused by strong nearby continuum sources. None of the 16 working database sources were detected at Nançay. Whereas object 2MASX J08170147-3410277 appears to be a very massive galaxy with an H I mass of
and an inclination-corrected rotation velocity of 314 km s-1, it is clear that only radio synthesis H I imaging observations will allow a firm conclusion on this.
Conclusions. Overall, the global properties of the detected galaxies match those of other ZoA H I surveys. Although the detections are as yet too sparse to give further insight into suspected or unknown large-scale structures in the ZoA, they already indicate that an extension of the present pilot survey is bound to quantify filaments, clusters, and voids behind this part of the Milky Way. It is shown that the number of candidate 2MASS-selected ZoA galaxies to be observed in H I could have been reduced by about 15% through examination of composite near-infrared images and the application of extinction-corrected near-infrared colour limits. Present results confirm that the Galactic extinction values from Schlegel et al. (1998) are valid for latitudes
,
but increasingly less so for lower latitudes.
Key words: galaxies: distances and redshifts - galaxies: general - galaxies: ISM - infrared: galaxies - radio lines: galaxies
1 Introduction
Because of dust extinction at low Galactic latitudes, resulting in the so-called zone of avoidance (ZoA), redshift surveys have generally concentrated on regions farther than 10



The interstellar extinction in the -band (2.2
m) is 12 times
smaller than in the B-band and 5.5 times smaller than in the
I-band (Cardelli et al. 1989).
We therefore decided to select probable ZoA galaxies from the - at that time (in 2000-2002)
- emerging systematic 2-Micron All Sky Survey (2MASS, Skrutskie et al. 2006). Many of these
ZoA galaxies are invisible on the Palomar Sky Survey (DSS) images and
even in the I-band have several magnitudes of extinction, whereas the extinction in the
-band remains relatively modest in most cases. Follow-up H I-observations of these
heavily absorbed galaxies are then the most efficient - if not only - tool to obtain
a distance estimate and map large-scale structures across the ZoA.
Nevertheless, identification of galaxies at very low Galactic latitudes remains difficult
even using 2MASS, not so much because of dust extinction, but due to stellar crowding
close to the Galactic plane.
In particular, the wider Galactic Bulge area makes the automatic identification of extended
sources increasingly uncertain, or even impossible when star densities of log N=4.00/(deg)2
are reached for stars with
,
leading to the so-called NIR ZoA (see
Fig. 9 in Kraan-Korteweg 2005).
2MASS used two identical telescopes in the north and the south to observe the entire sky at
J, H, and ,
providing an opportunity to select a much more uniform sample of galaxies
than has been possible previously. Note that although we initially used the 2000 and 2002 versions of
the working database for our source selection, all data presented here are updated to the
final release catalogue values. Figure 1 shows an all-sky map of 2MASS-selected bright
(
4), extended sources in Galactic coordinates (centred on the Galactic anti-centre direction),
demonstrating that we can select sources quite uniformly deep into the ZoA. The heavy
curves in the figure show declinations of -40
,
0
,
and +35
which correspond to the declination limits of the radio telescopes (Nançay and Arecibo;
see Sect. 2 for further details) used in this pilot project of observing 2MASS-selected
ZoA galaxies in H I.
![]() |
Figure 1:
All-sky plot of bright (
|
Open with DEXTER |
2MASS has a 95% completeness level of
at high Galactic latitudes (Jarrett et al. 2000),
and has little dust extinction at low latitudes. However, close to the Galactic plane, the
numerous faint (red) foreground stars effectively create a high sky-brightness noise level, and
the fainter (low surface brightness) galaxies are extremely difficult to detect because of confusion
noise. We therefore preferentially selected galaxies with
where the 2MASS galaxy
sample still remains essentially complete (cf. Huchra et al. 2005). A
completeness
limits corresponds to an optical B-band magnitude limit of
to
respectively,
for typical B-K galaxy colours ranging from about
for ellipticals to
for spirals
(e.g., Jarrett et al. 2003).
In practice, a
selection was possible for the Nançay sample only, where 92% meet
this criterion, whereas finding enough sources not yet observed in H I in the considerably
smaller area observable at Arecibo resulted in selecting fainter sources, 93% of which have
.
It should be emphasized here, however, that a NIR selected ZoA galaxy sample may not necessarily
be identical to objects selected at high Galactic latitudes. It likely is biased in the sense
that such a survey favours galaxies with higher (redder) surface brightness, or shorter
scale lengths, hence early type or bulge-dominated spirals, against low surface brightness
bluer galaxies. This is difficult to determine directly because of uncertainties in estimates
of Galactic extinction and stellar subtraction. In fact, this is already playing a role at high
Galactic latitudes where 2MASX underestimates magnitudes of galaxies by 0
2 to 2
5 (for
total magnitudes) due to the loss of other LSB features of extended dwarf galaxies, hence being
increasingly incomplete for LSB galaxies, and fails to detect the lowest LSB galaxies entirely
(Kirby et al. 2008; Andreon 2002).
The near-infrared is less affected by a galaxy's star-formation history. A -selected
sample therefore provides a more accurate indication of the total stellar content of the galaxies,
and is, for instance, less biased by galaxy interactions which can trigger star formation.
Collecting redshifts and H I parameters for these galaxies, in particular in the ZoA, also
contributes to broader efforts to build complete all sky samples of bright 2MASS galaxies,
such as the 2MASS Redshift Survey (2MRS, see Huchra et al. 2005).
Future studies can use the results from the present project to systematically identify a subset
of 2MASS ZoA galaxies with good H I characteristics, such as appropriate inclinations and
strong H I emission, that will allow the determination of peculiar velocities based on the
NIR Tully-Fisher (TF) relation, and will complement ongoing efforts by Masters et al. (2008)
for the whole sky 2MASS Tully Fisher relation survey (2MTF). In combination with the large-scale
structure information provided by the 2MRS, such a survey will allow a significant improvement
over dipole anisotropy derivations as well as reconstructed density and velocity fields from the
2MASS Redshift Survey (Erdogdu et al. 2006a,b) for which the lack of data in the ZoA still remains
one of the major contributors to the uncertainties in understanding the dynamics in the nearby
Universe as well as the Local Group (Loeb & Narayan 2008).
This paper is structured as follows: the selection of the sample of 2MASS ZoA galaxies observed by us in H I with the Nançay and Arecibo radio telescopes is described in Sect. 2, the observations and the data reduction are presented in Sect. 3, and the results in Sect. 4. A discussion of the results is given in Sect. 5 and the conclusions are presented in Sect. 6.
2 Sample selection
2.1 Nançay sample
At Nançay, we surveyed the ZoA within






















Using a 2002 pre-release version of the 2MASS working database, for Nançay we selected the 2MASS sources that
were classified as galaxies with the largest angular radii (>
at 20 mag arcsec-2 in the
-band)
within the region to be covered at Nançay (
3600 deg2). This led to over 8000 galaxies with
extinction-corrected
.
We selected the strongest candidate galaxies from among these based on
the classification scheme used to identify galaxies among the extended
sources in the 2MASS catalogue (Jarrett et al. 2000). This automated
classification scheme used training sets of known objects to assign a
probability of a source being a galaxy or other extended source based on
shape, symmetry, colour and other catalogued parameters.
We then chose the largest from among these sources, leaving 1448.
We searched the NASA/IPAC extragalactic database (NED) for any matching sources within 3
of the
positions of these sources and found that 581 had been previously classified as galaxies, and 278 had
published redshifts. Eliminating all sources with known redshifts, we selected the 300 largest remaining
sources, 104 of which were previously classified as galaxies.
The final selection of the 132 sources that were actually observed was made based on the availability
of telescope time, in which we aimed to observed the largest galaxies first.
Note that for a sample chosen at high latitudes with
matching 2MASS criteria, 96% of the sources were previously catalogued in NED and all of these had measured
redshifts.
The following 16 2MASS working database sources which we observed at Nançay did not make it into the final 2MASS source catalogue - none of them were detected: 002552+670925, 004129+600437, 022135+642636, 165630-324741, 171245-341722, 173137-361819, 173535-141928, 174604-253240, 180835-231312, 181804-044820, 181856-182017, 190102-140747, 201300+354137, 2105131+493946, 2105142+493950, and 234810+610154.
2.2 Arecibo sample
At Arecibo, we surveyed the ZoA galaxies (












In this zone, we observed 81 2MASS mostly low surface brightness sources, selected on a mean central
surface brightness in the inner 5
radius of
mag arcsec-2, in the same manner as the
2MASS LSB objects selected outside the ZoA by Monnier Ragaigne et al.
(2003a).
All but one of 2MASS working database sources observed at Arecibo made it into final version of the 2MASS extended source catalog; the one source that did not (0639299+170558) was not detected.
It is worthwhile pointing out that most of the selected galaxies are on average much fainter and smaller than the Nançay ZoA sample (see Fig. 5 in Sect. 5) despite these galaxies being mostly in an area of low extinction and star density (roughly the Galactic anti-centre). The reason for this is that the majority of the brighter galaxies had been identified before by Pantoja et al. (1994) by visual examination of POSS E prints in their efforts to search for optical galaxy candidates to trace the possible continuity across the Galactic plane of the southwestern spur of the Perseus-Pisces complex. They had then used Arecibo for follow-up H I observations of the larger galaxies (Pantoja et al. 1997).
3 Observations and data reduction
All radial velocities in this paper, both H I and optical, are heliocentric and calculated according to the conventional optical definition (V=c(


3.1 Nançay observations and data reduction
The Nançay decimetric radio telescope, a meridian transit-type instrument of the Kraus/Ohio State design, consists of a fixed spherical mirror (300 m long and 35 m high), a tiltable flat mirror (








The initial observations were made in the period June - December 2002, using a total of about 350 h
of telescope time. A number of follow-up observations were made to check tentative detections in
late 2007 and in late 2008. We obtained our observations in total power (position-switching) mode using
consecutive pairs of 40 s ON and 40 s OFF-source integrations. OFF-source integrations were
taken at a position about 20
E of the target position. Different autocorrelator modes were used for
the observation of sources with previously known radial velocities and for velocity searches of objects
of unknown redshift.
The autocorrelator was divided into one pair of cross-polarized receiver banks, each with
4096 channels
and a 50 MHz bandpass, resulting in a channel spacing of 2.6 km s-1. The centre frequencies of the 2 banks
were usually tuned to 5600 km s-1, for a velocity search in the 325 to 11 825 km s-1 range; for 8 undetected galaxies, which are flagged in Table 2, the centre frequency was tuned to 5000 km s-1, resulting
in a
-275 to 11 225 km s-1 search range.
Flux calibration, i.e., the declination-dependent conversion of observed system temperatures to
flux densities in mJy, is
determined for the Nançay telescope through regular measurements of a cold load calibrator and periodic
monitoring of strong continuum sources by the Nançay staff. Standard calibration procedures include
correction for the above-mentioned declination-dependent gain variations of the telescope (e.g.,
Fouqué et al. 1990). We also observed a number of calibrator galaxies throughout our observing runs,
whose integrated line fluxes are on average
times our values (Monnier Ragaigne et al. 2003b).
The first steps in the data reduction were made using software developed by the Nançay staff (NAPS, SIR program packages). With this software we averaged the two receiver polarizations and converted the flux densities to mJy. Further data analysis was performed using Supermongo routines developed by one of us (SES). With these we subtracted baselines (generally third order polynomials were fitted), excluding those velocity ranges with H I line emission or radio frequency interference (RFI). Once the baselines were subtracted, the radial velocities were corrected to the heliocentric system. The central line velocity, line widths at, respectively, the 50% and 20% level of peak maximum (Lewis 1983), the integrated flux of the H I profiles, as well as the rms noise of the spectra were determined. All data were boxcar smoothed to a velocity resolution of 15.7 km s-1 for further analysis.
In order to reduce the effect of radio frequency interference (RFI) in our observations, we used an off-line RFI mitigation program, which is part of the standard Nançay NAPS software package, see Sect. 3.3.
3.2 Arecibo observations and data reduction
We observed a sample (see Sect. 2.2) of 81 LSB galaxies in the ZoA from a pre-release version of the 2MASS working database using the refurbished 305-m Arecibo Gregorian radio telescope in November 2000 and January 2001, for a total of about 30 h observing time. Data were taken with the L-Band Narrow receiver using nine-level sampling with two of the 2048 lag subcorrelators set to each polarization channel. All observations were taken using the position-switching technique, with the blank sky (or OFF) observation taken for the same length of time, and over the same portion of the Arecibo dish (as defined by the azimuth and zenith angles) as was used for the on-source (ON) observation. Each ON+OFF pair was followed by a 10 s ON+OFF observation of a well calibrated, uncorrelated noise diode. The observing strategy used was as follows: first, a minimum of one 3 min ON/OFF pair was taken of each galaxy, followed by a 10 s ON/OFF calibration pair. If a galaxy was not detected, one or more additional 3 min ON/OFF pairs were taken of the object, if it was deemed of sufficient interest (e.g., large diameter, known optical velocity).
The 4 subcorrelators were set to 25MHz bandpasses, and both
subcorrelators with the same polarization were set to overlap by
5MHz. This allowed a wide velocity search while ensuring that the
overlapping region of the two boards was adequately covered. Two
different velocity searches were made - first in the velocity range
-500 to 11 000 km s-1 and subsequently in the range 9500 to 21 000 km s-1
(assuming the galaxy was not detected in the lower velocity range and observing time permitting).
The instrument's HPBW at 21 cm is
and the pointing accuracy is about 15
.
Table 1: Nançay: observational data.
Table 2: Nançay: derived data.
Using standard data reduction software available at Arecibo, the two polarizations were averaged, and corrections were applied for the variations in the gain and system temperature of the telescope with zenith angle and azimuth using the most recent calibration data available at the telescope. Further data analysis was performed as mentioned above for the Nançay data. A baseline of order zero was fitted to the data, excluding those velocity ranges with H I line emission or radio frequency interference. Once the baselines were subtracted, the velocities were corrected to the heliocentric system, and the central line velocity, line widths at, respectively, the 50% and 20% level of peak maximum (Lewis 1983), the integrated flux, as well as the rms noise of the spectra were determined. All data were boxcar smoothed to a velocity resolution of 14.3 km s-1 for analysis.
The stability of the chain of reception of the Arecibo telescope is shown by the observations we
made of strong continuum sources and of a calibration galaxy with a strong line signal: the latter
showed a 6% standard deviation in its integrated line flux.
3.3 Radio frequency interference (RFI)
As a consequence of their high sensitivity, radio astronomy telescopes are vulnerable to radio frequency interference (RFI), with signal strengths usually greatly exceeding those of the weak observed celestial radio sources. Broad-band RFI raises the noise level of the observations, while narrow-band RFI may mimic spectral lines like the H I lines from galaxies that are being searched for in the present study. Besides external RFI, interference signals generated within the radio observatory, including the telescope system itself, may degrade the quality of the observations.At Nançay, where the renovated telescope had only recently been put back into operation at the time of the observations, persistent internal RFI occurred in the 3600-3800 and 4600-4900 km s-1 range and external RFI often occurred around 8300, 9000 and 10 500 km s-1. The external RFI can be highly variable in time, and some occur in one polarisation only.
At Arecibo an internal RFI source that wandered in frequency throughout the observed band, occurred regularly throughout the observing runs, besides the usual external RFI around 8300 and 15 000 km s-1.
RFI signals with strengths that make the detection of faint H I line signals impossible in certain radial velocity ranges were present during a significant fraction of the observations, both at Nançay and at Arecibo. At Arecibo, besides a hardware radar-blanker, no software was available to identify and mitigate RFI signals. At Nançay, we used an off-line RFI mitigation program, as described in Monnier Ragaigne et al. (2003b).
4 Results
Nançay observations of the following 12 2MASX sources could not be used due to extremely high rms noise levels (>15 mJy), most likely caused by strong nearby continuum sources: 04124692+3835153, 04350092+5939419, 07392356-3221214, J08204513-3616164, 16171926-3740403, 16434955-3705384, 17504702-3116296, 18223005-0232233, 18340392-2524398, 20423202+4256315, 23044546+6004370, and 23045989+6014030. We completely exclude these objects in the following tables, plots an discussions. Apart from these sources, the Nançay rms typically ranges from about 2-4 mJy for the detections with a few outliers, whereas the non-detections spread more widely between 2 and 8 mJy (see Tables 1 and 3). For the Arecibo observations, detections and non-detections all have quite low rms, hovering narrowly around 1 mJy (see Tables 4 and 6).
![]() |
Figure 2:
Nançay 21-cm H I line spectra of the detected galaxies (see Table 1). Velocity
resolution is 15.7 km s-1. For object N13 two spectra are shown - the solid line represents the data taken
in the direction of the target source, whereas the dashed line shows the data taken towards
the galaxy 10
|
Open with DEXTER |
Furthermore, we have not included the results for the 16 2MASS sources from versions of the working database sources that did not make it into the final 2MASS source catalogue (see Sect. 2.1), none of which were detected in H I.
The resulting data are presented in the following figures and two sets of 3 tables for the Nançay and
Arecibo data. The H I spectra of the objects detected at Nançay and at Arecibo are shown in
Figs. 2 and 3, respectively and the composite 2MASS
images of all detected sources are
displayed in Fig. 4. The H I parameters of the detected galaxies obtained with the Nançay
and Arecibo instruments are given in Table 1 and Table 4, respectively, including the main 2MASS
parameters, and where available also the optical magnitudes and diameters. The latter were taken
from the HyperLeda database (Paturel et al. 2003a), or retrieved from NED (then noted in parentheses).
This is followed by Tables 2 and 5 which give the derived global properties of the galaxies.
The non-detections are listed in Tables 3 and 6 with their main 2MASS parameters, Galactic extinction
AB, and H I rms noise levels.
![]() |
Figure 3: Arecibo 21-cm H I line spectra of detected galaxies (see Table 4). Velocity resolution is 14.3 km s-1. |
Open with DEXTER |
Table 3: Arecibo: observational data.
The 2MASS data for these sources have all been updated to the final release values. In a number of cases the working database values differed significantly, particularly because ellipse fits to galaxies can be quite unstable in the presence of multiple confusing foreground stars.
The global H I line parameters listed in the tables are directly measured values; no corrections
have been applied to them for, e.g., instrumental resolution. Uncertainties
in
and
in
can be determined following Schneider et al. (1986, 1990), as, respectively
![]() |
(1) |
![]() |
(2) |
where R is the instrumental resolution in km s-1 (see Sect. 3), X is the signal-to-noise ratio of a spectrum, which we define as the ratio of the peak flux density



Description of all parameters listed in the tables, in alphabetical order:
- (1)
- 2MASX J is the entry number of a source in the final 2MASS Extended Source Catalog, corresponding to the right ascension and declination of the source centre in (J2000.0) coordinates;
- (2)
- AB is the Galactic B-band extinction in this direction in the Milky Way as estimated by Schlegel et al. (1998);
- (3)
- b/a is the infrared axis ratio determined from an ellipse fit to
the co-addition of the J-, H-, and
-band images;
- (4)
- BTc is the total apparent B-band magnitude reduced to the RC3 system (de Vaucouleurs et al. 1991) and corrected for Galactic extinction, inclination and redshift effects (see Paturel et al. 1997, and references therein);
- (5)
- D=V0/H0 is the galaxy's distance (in Mpc), where V0 is its radial velocity (in km s-1) corrected to the Galactic Standard of Rest and for infall towards various galaxy clusters in the local Universe, following Tonry et al. (2000), and a Hubble constant H0=75 km s-1 Mpc-1;
- (6)
- D25 is the diameter (in arcmin) at a visual surface brightness of approximately 25 mag arcsec-2;
- (7)
-
is the integrated line flux (in Jy km s-1);
- (8)
- H-K and J-K are the infrared colours within the rK20 isophotal aperture. Note that these colours are not corrected for extinction, hence reddened depending on the foreground dust column density by the amounts of fH-K=0.05AB for H-K and fJ-K=0.12AB for J-K respectively;
- (9)
- K20 is the total
-band magnitude measured within the rK20 isophotal aperture;
- (10)
- kJ-K is the k-correction to the J-K colour;
- (11)
- l and b are, respectively, the Galactic longitude and latitude of the source centre (in degrees);
- (12)
- LB is the B-band luminosity corrected for Galactic and internal extinction in solar units,
for an assumed solar absolute magnitude of
(Allen 1973);
- (13)
- LK is the
band luminosity of the galaxy in solar luminosities within the rK20 isophotal aperture, for an assumed solar absolute magnitude of
(Colina & Bohlin 1997);
- (14)
-
is the ratio of the combined H I and stellar baryonic mass as a fraction of the total dynamical mass, where
LK + 1.4
(McGaugh et al. 2000 or 2003);
- (15)
-
is the dynamical mass (in
) estimated from the rotation speed and the
-band radius,
;
- (16)
-
is the total H I mass (in
),
= 2.356 105 D2
;
- (17)
-
is the ratio of the total H I mass to the
-band luminosity in solar units;
- (18)
- No gives the source number used in Figs. 2-4;
- (19)
- Other Name is the entry number in another major galaxy catalogue;
- (20)
- PGC No is the entry number in the Principal Galaxy Catalogue (Paturel et al. 1989);
- (21)
-
rK20 is the radius (in arcsec) at a surface
brightness of 20 mag arcsec-2 in the
band (in arcsec in Tables 1 and 4, in kpc in Tables 2 and 5);
- (22)
- rms is the rms noise level or
in the H I spectrum (in mJy) - if two numbers are given, the first is for the low-velocity search and the second for the high-velocity one (see Sect. 2.2);
- (23)
-
is the peak flux density of the line (in mJy);
- (24)
- V50 is the heliocentric central radial velocity of a line profile (in km s-1), in the optical convention, taken as the average of the high and low velocity edges of the H I profile, measured at 50% of peak flux density;
- (25)
-
is the rotation speed corrected for inclination i;
for sin(i) < 0.2, for galaxies with higher inclinations we assumed
;
- (26)
- W50 and W20 are the profile's velocity widths (in km s-1) at 50% and 20% of peak maximum, respectively.
![]() |
Figure 4:
Composite 2MASS composite
|
Open with DEXTER |
4.1 Comparison with published H I data
In the literature we found the following 10 H I detections of sources we observed (see also
Sect. 4.1 and Table 7): 4 made at Arecibo (A01, A19, A21 and A23), 1 at Effelsberg (N12),
1 at Nançay (N10), and 4 at Parkes (A24, N11, N17 and N18). Excluding the Effelsberg
data for N12, which are affected by RFI, and the Parkes data for A24, which appears to have been
resolved by the Arecibo beam, we find a good overall agreement between the global profile parameters
measured by us and taken from the literature: the mean of the absolute values of the differences is
km s-1 in
and
km s-1 in W50, and our
fluxes are on average
times the literature values.
Table 4: Arecibo: derived data.
4.2 Notes on individual galaxies
In order to identify galaxies within the telescope beams that might give rise to confusion with the H I profile of the target galaxy, we inspected 2MASS and DSS images centred on the position of each clearly or marginally detected source, over an area of


2MASX J04212943+3656572 (=N06): it appears also to have been detected at Nançay by Theureau et al. (1998) while pointing towards the nearby galaxy NGC 3016, which lies on the Nançay HPBW edge. NGC 3016 has a much higher redshift of 5900 km s-1 (Pantoja et al. 1997; Huchra et al. 1983). The galaxy NGC 3019, which also lies on the Nançay HPBW edge, has a similarly high redshift of 5664 km s-1 (Fisher et al. 1995; Pantoja et al. 1997; Takata et al. 1994; Theureau et al. 1998).
2MASX J04215207+3607373 (=UGC 3021): classified as an elliptical galaxy, and therefore not
expected to be gas-rich. Its optical velocity is
km s-1 (Huchra et al. 1983).
It was not detected by us at Nançay with an rms of 2.8 mJy, nor at Arecibo by Pantoja et al.
(1997), whose mean rms is 1.3 mJy. It has a possible companion superimposed on it, hence the
two PGC entries.
2MASX J04514426+3856227 (=N10): its optical redshift of
km s-1 (Saunders et al. 2000)
is in agreement with our Nançay detection and that of Paturel et al. (2003b).
2MASX J05315137+1517480 (=A08): an optical velocity of
km s-1 (Tully 2002,
private communication) was listed in Hyperleda after our Arecibo detection was made at
5812 km s-1.
2MASX J06301575+1646422 (=A24): detected in H I at Parkes as HIZOA J0630+16 (Donley et al. 2005)
with a three times higher line flux than our Arecibo detection. This indicates that the source has a
quite extended LSB disc, which has to be much larger than its 1
extent on the composite 2MASS
-band image, given the respective Arecibo and Parkes HPBW of
and
.
2MASX J07300453-1833166: this source appears to be part of a Galactic H II region, together with other nearby 2MASS sources and object ESO559-N015.
2MASX J08080461-1452387 (=N12): detected in H I at Effelsberg by Huchtmeier et al. (2005) at 6679 km s-1, which is significantly higher than our value of 6575 km s-1. The published spectrum shows a narrow peak at 7050 km s-1, however, which does not occur in our spectrum and thus appears due to RFI. We re-estimate the Effelsberg profile parameters as listed in Table 7, which are in agreement with ours.
08544150-3248590 (near N15; =ESO 371-27): after our Nançay detection was made at 1302 km s-1 (consistent with the Nançay spectrum of Chamaraux et al. 1999, taking into account beam attenuation) an optical redshift of 2198 km s-1 was published (Wegner et al. 2003), which shows that we actually detected another galaxy within the beam, ESO 371-27, which was also detected at Parkes (Doyle et al. 2005). All H I redshifts of ESO 371-27 are consistent with the optical value of 1313 km s-1 (Karachentseva & Karachentsev 2000). As the Parkes line flux is significantly higher we have used it for calculating the total H I mass. The detected galaxy is very LSB and it does not have an entry in the 2MASS catalogue.
2MASX J18153013-0253481 (=N18): Inspecting the on-line Parkes H I spectrum of HIPASS J1815-02 (Meyer et al. 2004) we found that the HIPASS redshift of 1664 km s-1 listed in NED is actually that of one of the profile's two peaks; the correct value is 1788 km s-1.
2MASX J21135161+4255323 (=N22): Although the targeted source (21135100+4257568) did not make
it into the final 2MASS catalogue we made a clear detection towards this position at Nançay. We have
assumed this to be of 2MASX J21135161+4255323, located
towards the south.
4.3 Unusual galaxies
2MASX J05422061+2448359 (=A12): Curiously, the galaxy that is nearest (v=521 km s-1) and lowest in H I mass (log



Another obvious possibility is confusion with a red star. Apart from the entry in 2MASX, the 2MASS
point source catalogue lists two very nearby stars at 1
and 5
distance respectively,
where the nearer stellar counterpart might be one and the same object. The source centred on the
targeted 2MASS position actually looks more like a point source with more typical colours of a star
(strong in H), whereas the slightly more offset 2MASS source looks more like a fuzzy reddened
galaxy candidate (see Fig. 4; slightly to the NW of the central source:
2MASS 05422020+24483880).
In either case the colour of the extended object will be substantially contaminated by the nearby star.
A third possibility is that neither of these objects is the counterpart of what must be a highly obscured late-type spiral galaxy (narrow Gaussian profile, low H I mass), but that the H I detection originates from the nearby completely obscured infrared source IRAS 05393+2447 - or even some other invisible galaxy.
2MASX J08170147-3410277 (=N13): This thin edge-on spiral galaxy has a radial velocity (10 369 km s-1),
H I mass (
), and inclination-corrected rotation velocity (314 km s-1) similar to
that of the very H I-massive disc galaxy HIZOAJ0836-43, discovered by Donley et al. (2006, V=10 689 km s-1,
=
,
and
km s-1). The latter has about twice the estimated total
dynamical mass of N13 (1.4 vs.
), comparable to that of the most massive known
disc galaxies such as giant LSB galaxy Malin 1. It is, however, a NIR luminous star-bursting galaxy
(Cluver et al. 2008) with quite distinct properties from giant LSBs.
Because such high H I mass galaxies are (a) extremely rare (they are only being formed now and
their properties poorly known) and (b) our clear Nançay detection with a 9 peak
flux density of 27 mJy could not be found back in the deep Parkes ZoA survey data cube ZOA252
(http://www.atnf.csiro.au/research/multibeam/release/), we looked at this
galaxy and detection in further detail. We first looked for possible companions which might have
contributed to the broad signal by inspecting all bands of the digitized sky survey within the
Nançay beam as the extinction is relatively low (
mag). We found a previously uncatalogued
galaxy of similar surface brightness about 10
south of the target, at
16
58
6,
20
.
It appears on the B, R
as well as the IR images as a smaller (roughly half an arc minute in diameter) face-on spiral
with a smallish bulge.
With an angular N-S separation of 10
corresponding to 0.45 times the instrument's HPBW, observations
towards both objects allow us in principle to disentangle their H I profiles, if their angular diameters
are sufficiently small. We therefore (re)-observed both the sources N13 North and N13 South, to a
similar low noise levels (see Fig. 2). The velocity range and central velocity of both profiles are the same,
whereas the profile towards the southern object has 76% of the H I mass measured towards the northern
one and its H I is mainly concentrated in the high-velocity peak of the double horned profile.
This appears to indicate the presence of a single, relatively large source towards N13 North whose
receding half is towards the south.
We attempted to reconstruct the spectra of the hypothetical H I sources N13 north and south, assuming that their sizes are significantly smaller than the telescope beam. As they are separated by about half a HPBW, this would imply that an observed spectrum is due to the target plus half of the emission of the other galaxy. This exercise showed that N13 South is not a significant source of H I in itself.
Whereas our provisional conclusion is that N13 appears to be an extended, very massive H I galaxy, it is clear that only ATCA imaging observations will allow a firm conclusion whether it has a very extended lopsided H I disc and belongs to the class of extremely massive spiral galaxies.
5 Discussion
5.1 Detection rate
The detection rate is quite low. Excluding the 16 working database sources that did not make it into the final 2MASS extended source catalog (Sect. 2.1), only 22 of the 116 observed sources (19%) were detected at Nançay, excluding the detections of spurious, untargeted galaxy in the beams of N15 and N22. This value is only a bit higher (21%) if we exclude the 12 strongly continuum perturbed spectra (Sect. 4) - generally a problem at very low latitudes. The on average 3.5 times higher sensitivity Arecibo observations resulted in a detection rate of 36% (29/81), only 1.9 times higher (36/19) than that at Nançay. Criteria for improving the selection of likely 2MASX candidate ZoA galaxies are discussed in Sect. 5.2 and 6. ![]() |
Figure 5:
Plots of the extinction-corrected total |
Open with DEXTER |
![]() |
Figure 6: Plots of the observed ( top) and extinction-corrected ( bottom) colours (J-K) versus (H-K), for detections ( left panels) and non-detections ( right panels). Objects observed at Nançay are shown in light blue and those observed at Arecibo in dark blue. |
Open with DEXTER |
There are various reasons - partly different for the Nançay and Arecibo observations - for the low detection rate. For both samples, no morphological type criterion was introduced when selecting the target objects, i.e. the sample includes both red gas-poor galaxies as well as blue gas-rich ones. Being NIR selected, the bias against the more blueish gas-rich galaxies is quite strong, stronger than, for instance, for optical selected samples. This bias is even more extreme for the relatively shallow and low-resolution 2MASS survey (as discussed in Sect. 1), which is hardly sensitive to LSB galaxies. It is exacerbated for ZoA galaxy candidates because of the increasing loss of low-surface brightness features and the selective reddening, which results in an even stronger bias towards redder, higher surface brightness early type galaxies or bulges of spirals. Given that optical spectroscopy of these optically heavily or completely obscured 2MASS galaxies is hardly possible, it should be noted that despite this relatively low detection rate, H I observations of galaxy candidates still remain the most efficient tool in mapping large-scale structures across the ZoA.
Some of the properties of the two galaxy samples (Nançay and Arecibo) and the differences between them, as well as between detections and non-detections are apparent from Fig. 5 which shows plots of the extinction-corrected K020 magnitude versus optical extinction AB (top panel) and of radius (at rK20) versus AB (bottom panel) for both detections (left panels) and non-detections (right panels). The Nançay sample is indicated in light blue and the Arecibo one in dark blue.
It is obvious that the Arecibo galaxy sample contains a fainter subset of galaxies compared to the
Nançay sample, i.e. roughly ranging from 12 - 14 mag compared to 6 - 11 mag, as well as smaller
galaxies (
versus
). This is due to the fact that many of the larger
galaxies in the low extinction Arecibo area were already identified optically and observed with
Arecibo by Pantoja et al. (1994, 1997). Although the Nançay galaxies are considerably brighter (both
observed and extinction-corrected), they are traced deeper into the Galactic dust layer. The lower
detection rate of the brighter Nançay galaxies is purely the result of the lower sensitivity.
Overall there seems no marked difference between the locus of points for detections and non-detections, substantiating once more that H I observations are unaffected by dust-extinction. However, two trends seem to stand out: for extinction values AB above about 5 mag all 2MASX Arecibo sources are undetected. This implies that these apparently small, highly obscured non-detections must be very distant early-type galaxies.
Secondly, it seems surprising that none of the brightest Nançay objects (extinction-corrected) were detected. This must be due to confusion in 2MASX with Galactic sources or misclassification of galaxies due to blending of sources. To verify this claim, and learn from this pilot project, we investigated the brightest sources individually.
The two brightest sources (
,
;
of which the brighter lies beyond the boundaries of
Fig. 5) are globular clusters with the first one having colours inconsistent with a galaxy (see
also Fig. 6 in the next section) and the second being borderline. But the globular cluster
morphology is obvious enough in all optical and NIR wavebands.
The
galaxy in the list of decreasing extinction-corrected brightness
(with 6
82) is
also an unlikely galaxy. It has the extreme colour of
,
and is clearly
heavily contaminated, in particularly in the H-band, by a bright nearby star. The remaining feature
does not look like a galaxy candidate.
The
galaxy candidate (6
96) is a more difficult case. The extinction-corrected colours are
compatible with this being a real galaxy (0
29, 0
91) and it certainly has the appearance of a
galaxy, looking like a spiral with a clear bulge and LSB disc - or a central bright star with some
nebulosity around it. The optical does not provide clarification either. However, if it were a large
spiral galaxy (which then should have
)
it should be visible on the blue (IIIaJ) and
red sky survey plate at an extinction of ``only''
.
There is no evidence for that at all
on the respective optical sky survey plates. Hence we doubt it to be real.
The
object (
)
is clearly disqualified based on extinction-corrected colours
alone (0
23, -0
06). It is also improbable given the thick dust layer (AB=24
69)
through which it has been observed. However, it looks like a possible galaxy on the 2MASX image
with a bright centre and LSB disc, despite its blue
colour. Here, the optical image
provides additional help. It shows the target to be a nebular object with a star at its centre.
Interestingly this is also a strong radio continuum source.
The
galaxy in the list (with 7
05) is a very obvious and bright galaxy at only intermediate
extinction levels in the Puppis area. It is catalogued as ESO 430-G028 in the ESO Uppsala and ESO-LV
catalogues (Lauberts 1982; Lauberts & Valentijn 1989). But being an
S0 galaxy, it is clear why it was
not detected with our H I observation. In fact, it does not even have a published redshift yet.
The colours (-0
19; -0
49)and high extinction level extinction
(
)
of the
galaxy (
)
make it completely unrealistic for this
to be a galaxy. Again visual inspection of both the NIR and optical image confirms this. The object
looks like a star forming region with resolved individual stars and some reddish nebulosity around
it in 2MASS whereas the optical shows none of these resolved stars, only a few stars (probably
foreground) on a nebula (probably a reflection nebula). This also is a strong radio source.
The subsequent objects further down on this magnitude list (
)
are mostly definite
galaxies, with only a minor fraction of uncertain or unlikely galaxies. The majority of these unlikely
galaxies have extinction-corrected colours that are incompatible with these being galaxies obscured
- and reddened - by the Milky Way, as they are quite blue.
So the relative lack of detections of intrinsically bright 2MASX objects in our sample can be understood.
5.2 Near-infrared colours of the galaxy sample
Figure 6 displays the observed and extinction-corrected H-K vs. J-K colour-colour diagram for the 2MASS galaxy sample. The cross gives an indication of the mean colours for unobscured galaxies as given in Jarrett et al. (2003), namely



A comparison between the four panels allows some interesting observations. The colour plots uncorrected for extinction (top) are, as expected, quite similar as the extreme blueish objects were eliminated from the observing list, but no discrimination against reddened objects was made because of the foreground Galactic dust reddening. When correcting the colours for extinction, the detected galaxies (left bottom panel) fall quite nicely within the expected colour range for galaxies. The data points are, however, not distributed in a Gaussian cloud but rather in a more elongated linear distribution along the line of reddening. This can be explained by an over- or underestimate in the adopted extinction corrections. The likelihood for an overestimate is larger (independently confirmed in Sect. 5.5.) given that a larger fraction of the galaxies lie above the mean of unobscured galaxies as found in Jarrett et al. (2003) in the extinction-corrected colour-colour diagram.
This also holds for a large fraction of the non-detections. However, there are over a dozen extremely
blue objects (three blue objects fall beyond the axes limits displayed in Fig. 6, with
the most extreme having colours of
and
,
and one with extreme
red
colours). These objects clearly cannot be extragalactic.
The two most extreme sources (2MASX J07300453-1833166 and 2MASX J07300594-183254) lie at extinction
levels of
mag (according to the Schlegel et al. 1998 maps), which even in the
band implies over 7 mag of extinction.
Indeed, visual inspection of the NIR images as well as the optical images, find these two objects
to be stars within H II regions, i.e. point-like objects with some fuzziness around them that emit
strongly in the H-band. The same holds for most of the other objects. They are either stars that
are located in or associated with an H II region, or some filamentary Galactic nebulosity (e.g. 2MASX J23352762+6452140) generally visible in both the optical and infrared, or the image is heavily
contaminated by a very bright nearby star, and the resulting galaxy classification highly uncertain
(e.g. 2MASX J20491597+5119089 with colours of
and
).
Indeed, the galaxy with the bluest colours that was actually detected in H I (N17; with 0
07
and 0
59) is the first of the targeted objects in a list of increasing (J-K)0 colour that
actually has the appearance of a galaxy on NIR and optical images, and its properties. This
observation is independently confirmed by Jarrett who visually examined all 2MASX sources within
10
of the Galactic plane (priv. comm.) and classified most of these objects bluer than N17 as non-galaxies.
The lesson learned from these results indicate that the H I detection rate of 2MASS-selected
ZoA galaxies can be significantly improved if, in addition to the exclusion of galaxy candidates
with observed blue NIR colours J-K and H-K, 2MASX objects are also excluded that have
extinction-corrected colours
and
.
The easiest way to apply such corrections is by systematically using the DIRBE extinction maps. Although we do find (see Sect. 5.5) - like many others (Schröder et al. 2007, and references therein; Cluver et al. 2008) - that when taking the DIRBE extinction measures at face value in southern ZOA studies, these seem overestimated by about 15% to 50% (Nagayama et al. 2004; Schröder et al. 2007; Tagg 2008; Cluver 2008).
This overestimate will, however, have only a minimal effect on the extinction-corrected colours,
as the selective reddening of
fJ-K=0.12AB for J-K and
fH-K=0.05AB for H-K
will reduce the colours by a relatively low amount. For instance, a reduction by an intermediate
overestimate of 30% of the DIRBE extinction values and an intermediate to high ZoA dust column
density of
results in a decrease of the extinction-corrected colour by the relative
modest amount of
and
.
If such an extinction-corrected colour limitation is then followed by visual examination of the
individual and combined
images, partly in combination with optical images to help eliminate
Galactic objects, cirrus, filaments and blended images, then a fairly high H I detection rate should
be guaranteed given that spiral galaxies are generally more common than early type galaxies - even
if we cannot discriminate against morphological type with NIR colours.
![]() |
Figure 7:
Histograms of the distribution of the radial velocity, the total H I mass
|
Open with DEXTER |
5.3 Global properties of the detected galaxies
In the following, we have a brief look at the distribution of the global properties of the detected galaxies such as radial velocity,


The velocity distribution of the Nançay detections shows galaxies out to about 7000 km s-1, but beyond that
its efficiency drops quite rapidly toward the velocity search limit of
km s-1. This
is very similar to the systematic southern Parkes HIPASS ZoA survey (Kraan-Korteweg et al. 2005, see
their Fig. 2), which has similar instantaneous velocity coverage with a slightly lower sensitivity limit
(rms = 6 mJy), except for the prominent peaks in their survey due to the crossing of the Hydra/Antlia
filament and Great Attractor Wall (centred at about 3000 and 5000 km s-1 respectively). This suggests that
a systematic H I follow-up of 2MASX ZoA objects - with the current setup of the Nançay pilot project
observations - would be quite complementary to the southern ZoA efforts. Apart from the systematic
(``blind'') ZoA ALFA survey undertaken for the declination range visible with the Arecibo telescope
(e.g. Henning et al. 2008; Springob 2008), no such efforts are currently being undertaken for the northern ZoA.
The present Arecibo detections have a higher mean velocity Peak - the majority lie between 5000 10 000 km s-1
with a handful of galaxies up to 18 000 km s-1. The fact that very few galaxies have been detected at
low velocities is - as mentioned in Sect. 5.1 - due to the work of Pantoja et al. (1997) who
have already observed most of the (optically) larger galaxies at Arecibo in their efforts to map
the southwestern spur of the Perseus-Pisces complex across the ZoA. Their sample peaked around the
distance of this large-scale structure, namely 4250-8000 km s-1. For comparison the detection rate of
their nearer, optically visible, and partly classifiable into rough morphological type, is 53%
for the 369 galaxy candidates targeted for observation, compared to our 35% of 2MASX selected Arecibo
galaxies without previous redshift information. Their average noise was mJy, comparable to
our observations.
Both (our work and Pantoja's) are considerably deeper than the ALFA precursor observations of 5-6 mJy rms sensitivity. Then again, the multibeam ALFA blind survey observations are powerful in that they do not require any previous identification of optical or NIR counterparts. They will therefore be much more efficient for nearby gas rich dwarfs at the highest extinction levels. In that sense, the Arecibo data presented here are complementary to the ALFA survey, at least for the Galactic anti-centre part of the ZoA visible from Arecibo (with relatively low extinction and star density).
The H I mass distribution is without further surprises. The Arecibo data find on average more
H I-massive galaxies compared to Nançay, which is the effect of the galaxies being more distant on
average. The overall H I-mass distribution is quite similar to the (as yet mostly unpublished) deep
Parkes H I ZoA surveys (RCKK for the ZoA team; see also Donley et al. 2005, for the northern extension;
Shafi 2008, for the Galactic Bulge extension) with the majority of galaxies lying in the range of
9-10.5 log
(
)
with a few outliers down to lower masses of a few times 107
,
and
two above that range (>
). Both the faintest and most massive galaxy are
peculiar and discussed in further detail in Sect. 4.3.
The -band luminosity distribution is not dissimilar to the H I-mass distribution except for an
overall shift of one dex in the logarithmic solar units scale. This implies that the overall H I-mass to
-band light ratio has a mean of about 0.1, which corresponds closely to other surveys.
The more nearby Nançay objects have a slightly lower
compared to the Arecibo observations, due to the
slightly different selection criteria, with the more distant, small and compact Arecibo galaxies more
likely to be high surface brightness massive spirals.
The estimated dynamical masses are smaller than the combined stellar and gas masses for many of the galaxies.
There are several biases contributing to this problem that stem from the location of these galaxies in
the ZoA. Extinction effects are probably not the main cause of this problem: although the isophotal radii
of the galaxies are underestimated, so too are the total -band luminosities. In fact, by pushing the
effective isophotal surface brightness fit to a higher level (like for a less obscured galaxy), the axis
ratio would be measured closer to the bulge and likely to be rounder than if it were measured farther out
in the disc (e.g. Cameron 1990). After correcting for inclination effects, this would generally lead to
an overestimate of the dynamical mass.
The more likely explanation therefore is that the stellar confusion is the culprit. Unidentified faint
stars, which are plentiful at these low latitudes, can add to the -band light and distort the shape
of the galaxy isophotes. The latter effect would diminish the inclination correction to the rotation
velocities, which has a strong effect on our dynamical mass indicator.
5.4 Extinction model tests
The galaxies detected in this study mostly have H I masses in the range of 109 to 1010




![]() |
Figure 8: Extinction-corrected (J-K)0 colours of the H I detected galaxies as a function of the Galactic foreground B-band extinction correction factor in their direction, AB, as estimated by Schlegel et al. (1998). |
Open with DEXTER |
![]() |
Figure 9: Results of the search for the adjustments to the Galactic B-band extinction correction factor, AB, as estimated by Schlegel et al. (1998), that would give the least scatter about the expected mean J-K colour of the H I detected galaxies. The galaxies were divided into 3 bins, with AB extinction correction factors between 0-2 mag, 2-6 mag, and 6-12 mag. |
Open with DEXTER |
Since our galaxies should intrinsically have typical J-K colours of about 0
9, we performed the
following test. We selected the galaxies with AB extinctions in the ranges of 0-2, 2-6, and 6-12 mag,
respectively, and searched for the value of the adjustment to the Schlegel et al. values that would give
the least scatter about the expected mean colour. The results of this test are shown in Fig. 9
where the curves show that the scatter of the colour is minimized at approximately the Schlegel et al. (1998)
extinction values for moderate extinction - which generally are at
,
where the Schlegel et al.
values are indeed said to still be valid, whereas in regions of moderate extinction this reduces to 86% of
the Schlegel et al. values, and to about 69% for our most heavily-extincted galaxies. This falls well within
the range of other ZoA studies. As previous studies were mostly restricted to the southern sky, it is reassuring
to notice that this effect for our mostly northern sample seems to be of the same order in the southern sky.
This trend suggests that either the Schlegel et al. (1998) extinctions are overestimated in the most-highly extincted regions or that the relative infrared extinction is less in these regions.
5.5 Indications of large-scale structures connections
Although this is a pilot project, and the number of newly measured redshifts of obscured galaxies is relatively small (N=51), we nevertheless had a look at their distribution in redshift space to see what kind of structures they trace across the ZoA or what new large-scale features they might hint at. The locations of these new H I detections are shown in Fig. 10. Their positions on the sky are plotted as square symbols in four radial velocity slices (or shells) of 3000 km s-1 width for the velocity range 0-12 000 km s-1 (the 3 even higher velocity detections are not displayed). They are superimposed on the distribution of galaxies with previously measured redshifts, as obtained from HyperLeda. The colour coding refers to the different redshift ranges within a slice, with red marking the nearest, dark blue the middle and the fainter cyan the most distant


![]() |
Figure 10:
Plots showing our H I detections (black squares) superimposed on known large-scale
structures (as available in HyperLeda) in four radial velocity intervals (0-3000, 3000-6000,
6000-9000 and 9000-12 000 km s-1) in Galactic coordinates. Within each 3000 km s-1 slice in radial
velocity, the galaxies in the nearest 1000 km s-1 wide bin are shown in red, the middle 1000 km s-1
in dark blue, and the furthest 1000 km s-1 in light blue. The dark lines indicate declinations of
|
Open with DEXTER |
The magenta boxes outline the survey coverage of the various deep ZoA H I-surveys (rms = 6 mJy) centred on the southern Galactic plane undertaken with the multi-beam receiver of the Parkes 64m radio telescope (see Kraan-Korteweg et al. 2008, for survey details). The displayed data points also include the more shallow HIPASS survey results (Meyer et al. 2004; Wong et al. 2006). Note again that there is little overlap between our survey and this southern one, partly because of our selection of northern telescopes, and partly because 2MASS does not uncover galaxies in the high star-density around the wider Galactic bulge area (Fig. 9 in Kraan-Korteweg 2005).
Overall, the new detections are mostly seen to follow the filamentary structures. We will comment on the detection slice by slice by comparing them to known structures, the northern, very shallow (40 mJy, V < 4000 km s-1) H I Dwingeloo obscured galaxy survey (DOGS; e.g. Henning et al. 2000a), the 2MASX redshift survey (2MRS) Wiener filter density field reconstructions (2M-WF, Erdogdu et al. 2006b), as well as the most recent 2MASX photometric redshift slices (2MX-LSS; Jarrett 2008 - see http://web.ipac.caltech.edu/staff/jarrett/lss/index.html).
Top panel (0 km s-1 <
< 3000 km s-1): the galaxies around
240
clearly
form part of the well-established nearby Puppis filament, whereas the detections clumped around
180
-200
are more of a mystery. They seem to lie in a fairly underdense region. Interestingly
enough, the 2M-WF reconstruction finds a clear unknown overdensity (called C5) in this region which
is also notable in the respective 2M-LSS slice. This region and overdensity might be worthwhile
pursuing further. No new galaxies were detected at the super-Galactic plane (SGP) crossing (at
140
)
though some are visible in the next panel. The two galaxies at about 30
must
form part of a nearby filamentary structure identified for the first time in the Parkes multibeam
Galactic bulge extension survey (Kraan-Korteweg et al. 2008;
Shafi 2008) that protrudes into the
local void.
Second panel (3000 km s-1 <
< 6000 km s-1): the two detections at both
and
are clearly connected with the Perseus-Pisces (PP) complex. The connection
across the ZoA leading from Perseus to A569 is very strong in the 2M-WF reconstructions as well
as evident in the 2M-SS slice, more clearly so in the heavily smoothed display. The two other
galaxies belong to the south-western spur of the PP complex, which was also found by 2M-WF and
2M-LSS. Both the detections at
slightly below the plane) and
(above
the plane) show a prominent signal in 2M-WF (marked as OR for Orion and CAM for Cameleopardis there).
While visible in 2M-LSS it is less obvious there, though more so in the smoothed version.
Third panel (6000 km s-1 <
< 9000 km s-1): two clouds of detections can be attributed
to known, or rather suspected, structures. The two galaxies at
(below the plane)
also seem part of Orion (OR), as this has an even stronger signal in the reconstructed density
field in 2M-WF for this redshift range. The other galaxies seem to follow the main PP-chain.
Bottom panel (9000 km s-1 <
< 12 000 km s-1): six of the 7 detections are remarkably
aligned and seem to suggest some kind of filament - or the far end of the sheet-like PP-chain?
Such a feature is not recovered in the 2M-WF, however, though it is evident in the 2M-LSS slices.
It might be worthwhile to observe more galaxies in this velocity range to verify whether this
truly is a previously unknown filament.
6 Conclusions
To complement ongoing ``all-sky'' redshift surveys to map extragalactic large-scale structures in the nearby Universe and improve our understanding of its dynamics and observed flow fields, we undertook a pilot project to obtain H I observations of about 200 optically obscured or invisible galaxy candidates behind the Milky Way (
Apart from excluding extremely blue objects, no further selection criteria were applied, as near-infrared galaxy colours show hardly any dependence on morphology. Furthermore, the NIR galaxy colours are affected quite strongly by the varying Galactic dust column density through which the galaxies are viewed.
The overall detection rate of the 185 observed 2MASX galaxies whose spectra were not affected by nearby continuum sources is quite low: 24% and 35% for the Nançay and Arecibo samples respectively This detection rate is lower than for H I-follow-ups of optically selected galaxies, even in the ZoA. For instance, a similar H I survey of optically selected ZoA galaxies (Kraan-Korteweg et al. 2002) reached a 44% detection rate (though a pre-selection on morphological type favouring spiral galaxies was made).
Despite this relatively low detection rate, it should be noted that other means of obtaining redshifts (optical spectroscopy) for galaxies hidden by the Milky Way remain extremely difficult due to their reduced surface brightness. H I observations of galaxy candidates still remain the most efficient tool in mapping large-scale structures across the ZoA.
In addition, this pilot survey taught us that a significant number of the non-detections could have easily been excluded from the observing list by:
- (a)
- examination of the composite
images of the extinction-corrected brightest sources (now easily available), in combination with the higher-resolution optical SDSS images (when available - which is rarely the case for the present sample). Most of the extended objects with
or
could readily be dismissed as galaxy candidates in that way (see Fig. 5 and Sect. 5.1);
- (b)
- considering extinction-corrected colour limits. All objects that were bluer than
and
(Fig. 6) were found to be Galactic objects, mostly H II regions or filamentary structures associated with Galactic objects (Sect. 5.5).
The Schlegel et al. (1998) Galactic extinction values serve as a good first proxy for these tests,
even though they are not calibrated at the lowest Galactic latitudes (
). An extinction
overestimate will have a minimal effect on the verification procedures, or even on the extinction-corrected
colours delimitations as the selective reddening will reduce the colours by only a relatively low amount
of 0.12AB and 0.05AB for J-K and H-K respectively.
The NIR colours of the detected galaxies were actually used to assess the accuracy of the DIRBE
extinction values at low latitudes if taken at face value. The results confirm that the values from
Schlegel et al. (1998) are valid for latitudes above
,
whereas in regions of moderate
extinction this reduces to 86%, and to 69% for our most heavily-extincted galaxies.
It is reassuring that these values (from our mostly northern sky sample) seems in good agreement with
the previous mostly southern sky derivations (e.g., Schroeder et al. 2007, and references therein).
Overall, the properties of the detected galaxies match those of other surveys. The sample is too sparse yet to give an improved insight into suspected or unknown large-scale structures behind the Milky Way. However, the detections already indicate (see Fig. 10) that a further probing of the galaxy distribution will quantify filaments, clusters and also voids in this part of the ZoA.
Hence our H I detection rate of 2MASS-selected ZoA galaxies can be significantly improved,
if the above mentioned image examinations and extinction-corrected limits are employed.
Such a systematic survey would actually be a worthy pursuit, as it would be complementary to the
ongoing ``blind'' deep ZoA H I Parkes multi-beam survey (at
;
of similar velocity
coverage though slightly lower sensitivity than the Nançay pilot project), if done for the ZoA
in the latitude range of
accessible to Nançay, and for the northern ZoA
at
in the areas not covered by the Arecibo ALFA surveys (
).
We intend to pursue such a survey at Nançay.
Acknowledgements
This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation. We also wish to thank the Arecibo Observatory which is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation. This research has made use of the HyperLeda database (http://leda.univ-lyon1.fr), the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and the Aladin database, operated at CDS, Strasbourg, France. We acknowledge financial support from CNRS/NSF collaboration grant No. 10637 and from the ASTE of CNRS/INSU. RCKK wishes to thank the South African National Research Foundation for support.
References
- Allen, C. W. 1973, Astrophysical Quantities (London: Athlone Press) (In the text)
- Andreon, S. 2002, A&A, 382, 495 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Cameron, L. M. 1990, A&A, 233, 16 [NASA ADS] (In the text)
- Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, 345, 245 [NASA ADS] [CrossRef] (In the text)
- Cluver, M. E. 2009, Ph.D. Thesis, University of Cape Town (In the text)
- Cluver, M. E., Jarrett., T. H. Appleton, P. N., et al. 2008, ApJ, 686, 17L [NASA ADS] [CrossRef] (In the text)
- Chamaraux, P., Masnoux, J. L., Kazès, I., et al. 1999, MNRAS, 307, 236 [NASA ADS] [CrossRef] (In the text)
- Colina, L., & Bohlin, R. 1997, AJ, 113, 1138 [NASA ADS] [CrossRef] (In the text)
- de Vaucouleurs, G., de Vaucouleurs, A., Corwin, H. G., et al. 1991, The Third Reference Catalogue of Bright Galaxies (New York: Springer-Verlag) (RC3) (In the text)
- Donley, J. L., Staveley-Smith, J. L., Kraan-Korteweg, R. C., et al. 2005, AJ, 129, 220 [NASA ADS] [CrossRef] (In the text)
- Donley, J. L., Koribalski, B. S., Staveley-Smith, J. L., et al. 2006, MNRAS, 369, 1741 [NASA ADS] [CrossRef] (In the text)
- Erdogdu, P., Huchra, J. P., Lahav, O., et al. 2006a, MNRAS, 368, 1515 [NASA ADS] [CrossRef] (In the text)
- Erdogdu, P., Lahav, O., Huchra, J. P., et al. 2006b, MNRAS, 373, 45 [NASA ADS] [CrossRef]
- Fisher, J. R., Huchra, J. P., Strauss, M. A., et al. 1995, ApJS, 100, 69 [NASA ADS] [CrossRef] (In the text)
- Fouqué, P., Bottinelli, L., Durand, N., Gouguenheim, L., & Paturel, G. 1990, A&AS, 86, 473 [NASA ADS] (In the text)
- Henning, P. A., Rivers, A. J., & Staveley-Smith, L. 2000a, in Mapping the Hidden Universe, ed. R. C. Kraan-Korteweg, P. A. Henning, & H. Andernach, ASP Conf. Ser., 218, 61 (In the text)
- Henning, P. A., Staveley-Smith, L., Ekers, R. D., et al. 2000b, AJ, 119, 2686 [NASA ADS] [CrossRef]
- Henning, P. A., Springob, C. M., Day, F., et al. 2008, in The Evolution of Galaxies through the H I window, ed. R. Minchin, AIP Conf. Proc., 1035, 246 (In the text)
- Roberts, M. S., & Haynes, M. P. 1994, ARA&A, 32, 115 [NASA ADS] [CrossRef] (In the text)
- Huchra, J., Davis, M., Tonry, J., & Latham, D. 1983, ApJS, 52, 89 [NASA ADS] [CrossRef] (In the text)
- Huchra, J., Jarrett, T., Skrutskie, M., et al. 2005, in Nearby Large-Scale Structures and the zone of avoidance, ed. A. P. Fairall, & P. A. Woudt, ASP Conf. Ser., 329, 135 (In the text)
- Huchtmeier, W. K., Karachentsev, I. D., Karachentseva, V. E., et al. 2005, A&A, 435, 459 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Jarrett, T. H., Chester, T., Cutri, R., et al. 2000, AJ, 120, 298 [NASA ADS] [CrossRef] (In the text)
- Jarrett, T. H., Chester, T., Cutri, R., et al. 2003, AJ, 125, 525 [NASA ADS] [CrossRef] (In the text)
- Karachentseva, V. E., & Karachentsev, I. D. 2000, A&AS, 146, 359 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Kirby, E. M., Jerjen, H., Ryder, S. D., & Driver, S. 2008, AJ, 136, 1866 [NASA ADS] [CrossRef] (In the text)
- Kraan-Korteweg, R. C. 2005, RvMA, 18, 48 [NASA ADS] (In the text)
- Kraan-Korteweg, R. C., & Lahev, O. 2000, ARA&A, 10, 211 (In the text)
- Kraan-Korteweg, R. C., Henning, P. A., & Schröder, A. C. 2002, A&A, 391, 887 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Kraan-Korteweg, R. C., Staveley-Smith, L., Donley, J., Koribalski, B., & Henning, P. A. 2005, in Maps of the Cosmos, ed. M. Colless, L. Staveley-Smith, & R. Stathakis, ASP Conf. Ser., 216, 203
- Kraan-Korteweg, R. C., Shafi, N., Koribalski, B., et al. 2008, in Galaxies in the Local Volume, ed. B. Koribalski, & H. Jerjen, ApSS, 13 (In the text)
- Lauberts, A. 1982, The ESO/Uppsala Survey of the ESO (B) (ESO, Garching) (In the text)
- Lauberts, A., & Valentijn, E. A. 1989, The surface photometry catalogue of the ESO-Uppsala galaxies (ESO, Garching) (In the text)
- Lewis, B. M. 1983, AJ, 88, 962 [NASA ADS] [CrossRef] (In the text)
- Loeb, A., & Narayan, R. 2008, MNRAS 386, 2221L (In the text)
- Masters, K. L., Springob, C. M., & Huchra, J. P. 2008, AJ, 135, 1738 [NASA ADS] [CrossRef] (In the text)
- McGaugh, S. S., Schombert, J. M., Bothun, G. D., & de Blok, W. J. G. 2000, ApJ, 533, L99 [NASA ADS] [CrossRef] (In the text)
- Matthews, L. D., & van Driel, W. 2000, A&A, 143, 421 [NASA ADS] [CrossRef] (In the text)
- Meyer, M. J., Zwaan, M. A., Webster, R. L., et al. 2004, MNRAS, 350, 1195 [NASA ADS] [CrossRef] (In the text)
- Monnier Ragaigne, D., van Driel, W., Schneider, S. E., Jarrett, T. H., & Balkowski, C. 2003a, A&A, 405, 99 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Monnier Ragaigne, D., van Driel, W., Schneider, S. E., Balkowski, C., & Jarrett, T. H. 2003b, A&A, 408, 465 [NASA ADS] [CrossRef] [EDP Sciences]
- Nagayama, T., Woudt, P. A., Nagashima, C., et al. 2004, MNRAS, 354, 980 [NASA ADS] [CrossRef] (In the text)
- Pantoja, C. A., Altschuler, D. R., Giovanardi, C., & Giovanelli, R. 1994, in Unveiling Large-Scale Structures Behind the Milky Way, ed. C. Balkowski, & R. C. Kraan-Korteweg, ASP Conf. Ser., 67, 143 (In the text)
- Pantoja, C. A., Altschuler, D. R., Giovanardi, C., & Giovanelli, R. 1997, AJ, 113, 905 [NASA ADS] [CrossRef] (In the text)
- Paturel, G., Fouqué, P., Bottinelli, L., & Gouguenheim, L. 1989, A&AS, 80, 299 [NASA ADS] (PGC) (In the text)
- Paturel, G., Bottinelli, L., Di Nella, H., et al. 1997, A&AS, 124, 109 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Paturel, G., Petit, C., Prugniel, P., et al. 2003a, A&A, 412, 45 [NASA ADS] [CrossRef] [EDP Sciences] (In the text)
- Paturel, G., Theureau, G., Bottinelli, L., et al. 2003b, A&A, 412, 57 [NASA ADS] [CrossRef] [EDP Sciences]
- Saunders, W., Sutherland, W. J., & Maddox, S. J. 2000, MNRAS, 317, 55 [NASA ADS] [CrossRef] (In the text)
- Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525 [NASA ADS] [CrossRef] (In the text)
- Schneider, S. E., Helou, G., Salpeter, E. E., & Terzian, Y. 1986, AJ, 92, 742 [NASA ADS] [CrossRef] (In the text)
- Schneider, S. E., Thuan T. X., Magri, C., & Wadiak, J. E. 1990, ApJS, 72, 245 [NASA ADS] [CrossRef] (In the text)
- Schröder, A. C., Mamon, G. A., Kraan-Korteweg, R. C., & Woudt, P. A. 2007, A&A, 466,481 (In the text)
- Shafi, N. B. 2008, MSc Thesis, University of Cape Town (In the text)
- Skrutskie, M. F., Cutri, R. M., Stiening, R., et al. 2006, AJ, 131, 1163 [NASA ADS] [CrossRef] (In the text)
- Springob, C. M., Henning, P. A., Catinella, B., et al. 2008, in Dark Galaxies and Lost Baryons, ed. J. I. Davies, & M. J. Disney, IAU Symp., 244, 383 (In the text)
- Tagg, J. 2008, MSc Thesis, University of Cape Town (In the text)
- Takata, T., Yamada, T., Saito, M., Chamaraux, P., & Kazès, I. 1994, A&AS, 104, 529 [NASA ADS] (In the text)
- Theureau, G., Bottinelli, L., Coudreau-Durand, N., et al. 1998, A&AS, 130,333 (In the text)
- Tonry, J. L., Blakeslee, J. P., Ajhar, E. A., & Dressler, A. 2000, ApJ, 530, 625 [NASA ADS] [CrossRef] (In the text)
- van Driel, W., Pezzani, J., & Gérard, E. 1997, in High-sensitivity Radio Astronomy, ed. N. Jackson, & R. J. Davies (Cambridge: Cambridge Univ. Press) 229 (In the text)
- Wegner, G., Bernardi, M., Willmer, C. N. A., et al. 2003, AJ, 126, 2268 [NASA ADS] [CrossRef] (In the text)
- West, R. M., Surdej, J., Schuster, H.-E., et al. 1981, A&AS, 46, 57 [NASA ADS]
- Wong, O. I., Ryan-Weber, E. V., Garcia-Appado, D. A., et al. 2006, MNRAS, 371, 1855 [NASA ADS] [CrossRef] (In the text)
Footnotes
- ... avoidance
- Tables 1, 2, 4 and 5 are also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/505/29
- ...
- Tables 3, 6, 7, and spectra in FITS format are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/505/29
All Tables
Table 1: Nançay: observational data.
Table 2: Nançay: derived data.
Table 3: Arecibo: observational data.
Table 4: Arecibo: derived data.
All Figures
![]() |
Figure 1:
All-sky plot of bright (
|
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Nançay 21-cm H I line spectra of the detected galaxies (see Table 1). Velocity
resolution is 15.7 km s-1. For object N13 two spectra are shown - the solid line represents the data taken
in the direction of the target source, whereas the dashed line shows the data taken towards
the galaxy 10
|
Open with DEXTER | |
In the text |
![]() |
Figure 3: Arecibo 21-cm H I line spectra of detected galaxies (see Table 4). Velocity resolution is 14.3 km s-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Composite 2MASS composite
|
Open with DEXTER | |
In the text |
![]() |
Figure 5:
Plots of the extinction-corrected total |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Plots of the observed ( top) and extinction-corrected ( bottom) colours (J-K) versus (H-K), for detections ( left panels) and non-detections ( right panels). Objects observed at Nançay are shown in light blue and those observed at Arecibo in dark blue. |
Open with DEXTER | |
In the text |
![]() |
Figure 7:
Histograms of the distribution of the radial velocity, the total H I mass
|
Open with DEXTER | |
In the text |
![]() |
Figure 8: Extinction-corrected (J-K)0 colours of the H I detected galaxies as a function of the Galactic foreground B-band extinction correction factor in their direction, AB, as estimated by Schlegel et al. (1998). |
Open with DEXTER | |
In the text |
![]() |
Figure 9: Results of the search for the adjustments to the Galactic B-band extinction correction factor, AB, as estimated by Schlegel et al. (1998), that would give the least scatter about the expected mean J-K colour of the H I detected galaxies. The galaxies were divided into 3 bins, with AB extinction correction factors between 0-2 mag, 2-6 mag, and 6-12 mag. |
Open with DEXTER | |
In the text |
![]() |
Figure 10:
Plots showing our H I detections (black squares) superimposed on known large-scale
structures (as available in HyperLeda) in four radial velocity intervals (0-3000, 3000-6000,
6000-9000 and 9000-12 000 km s-1) in Galactic coordinates. Within each 3000 km s-1 slice in radial
velocity, the galaxies in the nearest 1000 km s-1 wide bin are shown in red, the middle 1000 km s-1
in dark blue, and the furthest 1000 km s-1 in light blue. The dark lines indicate declinations of
|
Open with DEXTER | |
In the text |
Copyright ESO 2009
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.