Gopal-Krishna1 - C. Ledoux2 - J. Melnick2 - E. Giraud3 - V. Kulkarni1 - B. Altieri4
1 - National Centre for Radio Astrophysics, TIFR, Post Bag 3, Ganesh Khind, Pune 411 007, India
2 -
European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago, Chile
3 -
GAM, Univ. Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex, France
4 -
European Space Agency, Villafranca del Castillo, Apartado 50727, 28080 Madrid, Spain
Received 12 January 2005 / Accepted 22 February 2005
Abstract
We present the results of radio (VLA) and optical (ESO/La Silla) imaging of
a sample of 52 radio sources having an ultra-steep radio spectrum with mostly steeper than -1.1 at decimetre wavelengths (median
).
Radio-optical overlays are presented to an astrometric accuracy of
.
For 41 of the sources, radio spectral indices are newly
determined using unpublished observations made with the 100-m Effelsberg
radio telescope. For 14 of the sources identified with relatively
brighter optical counterparts, spectroscopic observations were also carried
out at La Silla and their redshifts are found to lie in the range 0.4 to 2.6. These observations have revealed three distant clusters of galaxies
with redshifts of 0.55, 0.75 and 0.79, and we suggest that, together with an
ultra-steep radio spectrum and relaxed radio morphology, the presence of a
LINER spectrum in the optical can be used as a powerful indicator of rich
clusters of galaxies. Additional candidates of this type in our sample are
pointed out. Also, sources exhibiting particularly
interesting radio-optical morphological relationships are highlighted. We
further note the presence of six sources in our sample for which the optical
counterpart (either detected or undetected) is fainter than
and the
radio extent is small (<
). These ultra-steep spectrum radio sources
are good signposts for discovering massive galaxies out to very
large redshifts.
Key words: cosmology: observations - galaxies: active - cooling flows - galaxies: clusters: general - quasars: general - radio continuum: galaxies
The discovery of a correlation between the output of radio galaxies in optical emission lines and the radio band (e.g., McCarthy 1993; Rawlings & Saunders 1991) led to the recognition of using radio source samples as a means to detect galaxies located at very large distances. Indeed, radio surveys backed up with optical spectroscopy provided the first detections of objects beyond a redshift of two (e.g., Spinrad 1986). A major technical breakthrough came with the demonstration of a statistical correlation between radio spectral index and distance (McCarthy et al. 1987; Chambers et al. 1987), as already hinted in some earlier studies (e.g., Blumenthal & Miley 1979; Gopal-Krishna & Steppe 1981; Gopal-Krishna et al. 1980; Tielens et al. 1979). This correlation is now understood to be largely the result of a radio K-correction operating on the integrated spectrum which usually has a downward curvature arising from radiative losses (Laing & Peacock 1980). Consequently, the radio spectrum in a given frequency range appears increasingly steeper with redshift. The most distant radio galaxy found so far using the clue of ultra-steep radio spectrum has a redshift z=5.2 (Venemans et al. 2004).
The spectacular success of using ultra-steep spectrum radio sources (hereafter USSRS) to discover high-z galaxies, from mid-80s to early 90s, later faced a stiff competition from a purely optical technique which involved multi-color photometry (Steidel et al. 1996). Nonetheless, the radio technique continues to offer at least two unique advantages. Firstly, the high-z galaxies thus found belong to the most massive stellar systems existing at their redshifts; this is particularly relevant for cosmological theories of structure formation (e.g., De Breuck et al. 2002; Lilly & Longair 1984). Secondly, an eventual discovery of radio sources beyond z=6 would provide a direct probe of the "re-ionization era'', via H I spectroscopy (e.g., Djorgovski et al. 2001). In this context, finding USSRS whose optical counterparts remain undetected down to fairly deep magnitudes ("empty fields'') is particularly important. Moreover, from the astrophysical perspective, it is also of great interest to detect optical emission associated with the lobes and hot spots of radio galaxies at high redshifts where inverse Compton losses against the intense cosmic microwave background would severely limit the range of the electrons capable of optical synchrotron radiation (e.g., Gopal-Krishna et al. 2001; Brunetti et al. 2003).
In this paper, we present radio and optical imaging data for a sample of 52 USSRS (Sects. 2 and 3). Redshift measurements for the brighter subset of 14 sources are also presented. The radio maps are based on our Very Large Array (VLA) observations while the optical data were obtained using the telescopes of the European Southern Observatory (ESO) at La Silla.
The present sample of 52 USSRS with
(but mostly <-1.1;
see Table 1) is an assortment derived from the following
four data-sets:
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Figure 1: Radio-optical overlays. The insets show the optical counterparts without radio contour. The radio contours are based on VLA snapshots taken at 4.8 GHz. |
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For all sources in our sample, except those few for which VLA maps at 4.8 GHz (A-array) were available in Bennett et al. (1986), we took five to ten minutes snapshots using the VLA at 4.8 GHz. The majority of these observations were made in the BnC hybrid configuration while the DnC array was used in the remaining cases. The data restoration was done using the AIPS package (details will be given in Kulkarni et al. 2005, in prep.). The derived radio images are shown as overlays in Fig. 1 while the main structural parameters are given in Table 1.
The optical observations were taken during several observing runs between
1989 and 1996 using telescopes at La Silla. The bulk of the data was
obtained in the period 1989-1991 using EFOSC 1 at the La Silla 3.6 m telescope.
Standard Bessel filters were used
at all observing runs, and both imaging and spectroscopy were performed using
EFOSC-type instruments. The characteristics of these instruments is summarized
in Tables 2 and 3, that condense the
relevant information from successive generations of User's Manuals. The data
reduction was performed using MIDAS, the data reduction and analysis
software developed at ESO. Most observations were taken under reasonably
good seeing conditions (at least by the standards of those days) ranging
between one and
,
with little or no moon illumination. However, the
observations of the sources 0001+058, 2232-062, 2236-039 and
2245-037 were made on a bright moonlit night with rather poor seeing
conditions (
). Consequently, the detection limit attained
for these fields is only around
.
The optical and radio images are
shown in Fig. 1 when not already published elsewhere.
We emphasize that, by itself, the astrometric error class can be treated as the
principal reliability indicator of optical identification only provided
the radio source is compact (smaller than
), or it contains
a compact central radio component. For extended radio sources larger
than
and lacking a central component, offsets of up to
25% of the size of the radio source, between the optical
identification and the radio centroid, can be accepted.
Direct imaging observations were not always obtained under photometric conditions, although, due to the faintness of the sources, rather good transparency was always required. Not all observations, therefore, were photometrically calibrated. On photometric nights, calibrations were done using several stars from the Landolt (1992) CCD fields. Given the excellent photometric stability of the EFOSC instruments, we were able to determine the magnitudes of the objects observed under non-photometric conditions using the standard zero points given in the User's Manuals. Due to the unknown atmospheric extinction, however, these magnitudes carry rather large observational errors (in some cases even up to 0.5 mag).
To determine the magnitudes of the optical counterparts of the radio sources,
we followed two procedures. For most objects, we carried out circular aperture
photometry with adequate aperture size (using the MIDAS software) whereas for
objects located in crowded/confused regions we additionally employed the
SExtractor algorithm (Bertin & Arnouts 1996) to deblend the different
close-by objects. Each of these procedures gives a magnitude estimate with
an associated error. These values are reflected in the estimates given in
Table 1. It should be stressed that additional uncertainties can
be expected due to systematic effects, mainly non-photometric conditions
(see above). For blank fields, a lower limit to the magnitude was
determined by circular aperture photometry taking the radius to be equal to
the mean FWHM of several stars on the same CCD frame.
Whenever optical identification could be established and weather
conditions allowed, we used the available telescope time to obtain spectra of
the sources using the identification images to place the slit on the
objects. Otherwise, we performed the source identifications off-line, and
carried out spectroscopy subsequently on any available nights. In spite
of considerable efforts, however, the number of sources for which we were
able to obtain redshifts remains rather small and for several sources the
spectroscopy did not yield redshift (these sources are marked with an asterisk
in the last column of Table 1). Plausibly, this is due to the fact
that, due to the high read-out noise and low blue sensitivity of the CCDs of
those days, our ability to detect Ly emission in the range
1.8<z<2.3 was rather limited, while powerful radio sources tend to cluster at
redshifts between one and two (see, e.g., Condon 2003).
The optical identification is a fuzzy object close to the detection limit
situated between the two radio lobes. An object of R=22.7
0.3 is
located about
to the SW of the western lobe. Another close-by, much
fainter object (R=23.2
0.4) is seen about
North of the
eastern lobe.
It cannot be entirely excluded that the optical identification of this radio source is not located on a bad CCD column. For this reason, the source remains undetected down to R=23.
This is a clear empty field since no optical object is visible within a
radius of
from the radio source.
The optical field without radio contours is shown in the inset of the overlay
in Fig. 1. A faint object with R=21.9
0.1 seen at the
centre of the inset, which coincides with the mid-point of the two radio
lobes, is a likely identification. Another candidate is the brighter
object with R=20.8
0.1 situated
NW of the radio centre.
This radio source is located in a clear empty field.
The R-band overlay image shown in Fig. 1 has been taken
with EFOSC 2 at the NTT which has a higher resolution than the B-band image
used for the photometry (see Table 1). The optical identification,
more clearly seen in the inset of the overlay, is a QSO with redshift z=2.53based on prominent broad Ly
and C IV lines.
The hint of a wide-angle-tail radio morphology signifies the presence of a
cluster. Interestingly, the optical counterpart is a QSO with redshift z=1.12based on several emission lines visible in our spectra. About
to the
SW of the quasar, a foreground elliptical galaxy is seen for which we find a
redshift z=0.27. The R-band image shown in Fig. 1 has
been taken with EFOSC 2 at the NTT which has a higher resolution than
the R-band image used for the photometry.
The R-band overlay image shown in Fig. 1 resulted from a 30 min
exposure taken with EFOSC 2 at the NTT which has a higher resolution than
the R-band image used for the photometry (see Table 1). The
optical ID lies closer to the eastern lobe of the radio source. It has a
fuzzy appearance, roughly extended along the radio axis. It has a northward
extension of
,
which is also seen on the EFOSC 1 image used for the
photometry. It could possibly be a chain of fainter galaxies, or even a tidal
tail resulting from a merger event.
Our results for this source are published in Giraud et al. (1996a). It is
identified with the dominant galaxy of a z=0.79 cluster. A remarkable
optical extended emission-line region (EELR) of low excitation and size,
100 kpc, was found associated with the radio galaxy. The EELR bears a
close morphological relationship to the radio lobes which
exhibit Z-shaped symmetry. The EELR appears to consists of three cones associated with the radio galaxy and a neighbouring galaxy also located
at the same redshift and connected to the former by a long stellar filament.
The optical counterpart is clearly extended along the two radio lobes (see
inset in Fig. 1). Our low-dispersion spectra yielded a
redshift z=1.12 based on three (narrow?) emission
lines (C III] 1909, Mg II
2800,
[O II]
3727).
An almost point-like object lies at the position of this barely resolved radio source.
Only a pointing optical image (3 min exposure) is available for this small diameter radio source which remains unidentified down to R=22.7. A deeper image is needed.
The optical counterpart is a diffuse object with two peaks separated by
along the radio
axis.
The optical counterpart of this marginally resolved radio source is not visible in our 3 min pointing image. A deeper image is needed for this source.
A galaxy of R=22.3
0.2 which is seen at the centre of the overlay
is situated roughly midway between the two widely separated radio "lobes''. This
is a possible identification. However, since the southern lobe is
itself resolved into two components, it is possible that the two lobes are in
fact independent radio sources. In that event, their optical counterparts
are undetected down to R=23.7.
This barely resolved radio source is a clear empty field located towards
the Galactic anticentre (
).
Our results for this source are published in Gopal-Krishna et al. (1995).
A giant cloud of Ly emission at z=2.468 extended over
100 kpc
is associated with the southern radio lobe. The equivalent width
of Ly
is exceptionally large (
1000 Å in the rest-frame)
and the line profile indicates expansion at a velocity of
550 km s-1, probably driven by the radio lobe from within. The Ly
emission shows a sharp cut-off near the radio galaxy indicative of a dusty disc around the galaxy, oriented roughly perpendicular to the
radio axis.
The only optical object detected between the two radio lobes is a galaxy at
,
(J 2000).
The optical counterpart of this small diameter (
)
radio source is
below the detection limit in our image, R>23.3. There is a bright object
about
West of the radio position.
Since our VLA map shows the size of this source to be
,
its flux
density given in the Molonglo Reference Catalogue at 408 MHz (MRC does not give
integrated flux) might be significantly underestimated. Hence we have computed
the spectral index using the flux densities estimated from the NVSS map (353 mJy at 1.4 GHz) and the Parkes survey (87 mJy at 4.85 GHz; Griffith et al. 1995). We further note that our VLA map shows a prominent radio core in this triple source, which contributes
9 mJy at 4.85 GHz.
The optical identification is a bright, narrow emission-line radio
galaxy (NELG) at a redshift z=0.618. A close inspection of the image
shows that the optical counterpart could be a tight chain of three objects. The
southern radio hot spot coincides with a point-like optical object with
R=19.7
0.1.
The optical identification lies closer to the northern radio lobe and is
seen more clearly near the centre of the inset. After 4 h of integration,
the optical spectrum shows two narrow faint emission lines that we tentatively
identify with Mg II 2800 and [O II]
3727 at
z=0.71. Note also that a faint optical object is coincident with the outer
edge of the southern radio lobe.
The optical ID is a galaxy that coincides with the central radio component
and is elongated along the radio axis. A relatively bright star is
located
to the SE of the galaxy.
The optical ID of this radio source is a point-like object with
R=22.0
0.1.
The best candidate for optical identification is a R=22.6
0.1galaxy situated at
,
(J 2000). It could be the dominant
member of a distant cluster. Our 2 h spectrum of this galaxy shows a
strong [O II]
3727 emission line and a rich Balmer absorption
spectrum, reminiscent of other cluster sources (Melnick et al. 1997).
Unfortunately, our B 300 spectra cuts off just where we would expect to see
the [O III] emission lines at z=0.42, so we cannot be certain of
the LINER nature of this source.
The identified optical structure consists of
a R=23.5
0.3 point-like object and a faint,
long wisp to SW coincident with the southern radio lobe and elongated along the radio axis.
The entire structure has
.
The radio structure is poorly resolved in our low-resolution VLA map. The only
object within
of the radio peak is a
fuzzy object
situated at
,
(J 2000).
The likely optical counterpart is a point-like object located
NW
of a bright star.
The optical counterpart of this slightly resolved (
)
radio
source is a R=22.8
0.4 object. A fainter object is also seen
NE of it.
The central radio component lies within
of a pair of optical
objects which are themselves separated by
and have an integrated
R=22.7
0.3. One of these objects is the likely optical ID.
An object of R=24.4
0.2 is seen
NE of the central component
of this extended radio source. The astrometry has been independently
checked using a 10 min exposure taken with EFOSC 1 at the ESO 3.6 m telescope.
This image is less deep but includes five reference stars and confirms
the astrometry as shown in the overlay.
The likely optical ID is close to the north-eastern radio lobe and appears to be extended.
An optical object is seen about
offset from the radio peak
towards NE. Within the astrometric uncertainties for this case, this object
is the likely identification. Faint and narrow emission lines of
[C II]
2326 and Mg II
2800 are visible in
our 2 h spectrum of this object indicating that the radio source is probably
a NELG at a redshift z=0.98.
Our results for this source are published in Gopal-Krishna et al. (1992).
An EELR of size 100 kpc and z=0.477 is associated with this
double radio source. Its [O II]
3727 emission line has a
very large rest-frame equivalent width (350 Å), consistent with the high
radio luminosity. Intriguingly, we found that its optical spectrum is
ultra-soft,
with [O II]
3727/[O III]
5007 = 10, nearly 30 times the typical value for distant 3CR radio galaxies (van Breugel & McCarthy 1990). Together with Hydra A, this source thus provides an outstanding example of extremely low-ionization
optical emission-line spectra being associated with powerful radio galaxies
(Melnick et al. 1997). Unfortunately, the object is seen very close to
a bright star, so it is difficult to say whether it is a cluster source like
Hydra A.
Deep multi-color optical observations of this source have been reported by us (Giraud et al. 1996b). The source was identified with a z=0.753 elliptical galaxy in the process of formation by accreting material from neighbouring galaxies, all members of a group. The radio source has a central core and two relatively relaxed lobes consistent with its being in a cluster.
The peak of this unresolved radio source lies just
from a pair
of bright stars. No optical identification is visible above a detection limit
of
.
The radio flux measurement at 2.7 GHz (Effelsberg) contains
a significant contribution from a
double radio source
located
to the NW of 1509-158. Due to this, the true spectral
index of this source is probably even steeper than the -1.24 value given in
Table 1. The parameters of the confusing source, estimated from
our VLA observations, are
,
(J 2000), with a flux density equal to
36 mJy at 5 GHz (Kulkarni et al. 2005, in prep.).
The radio map of this triple source is taken from the VLA 5 GHz observations by Bennett et al. (1986). The redshift of the optical counterpart, z=0.93, was derived from a 3 h low-dispersion (B 1000) spectrum taken with EFOSC 1. The spectrum shows prominent, probably narrow, Mg II and [O II] emission lines. The presence of H+K in absorption and the lack of strong [O III] emission lines indicate that this object may be a LINER.
The optical counterpart of this unresolved radio source is quite bright and point-like. The blue spectrum of this object obtained from 2 h integration with EFOSC 1 is typical of an elliptical galaxy at z=0.31. The lack of emission lines and the compactness of the object, however, are rather puzzling.
Although this object has a large radio size, its optical identification remains
uncertain. The nearest object to the radio centre is a R=21.6
0.2point-like object, which we suggest as the possible identification. However, it
is offset by
to the South of the faint radio peak seen between the
two radio lobes. In addition to the R-band photometry, we also took a 15 min
V-band exposure using EFOSC 1 from which we get V=21.2
0.1 and V-R=-0.4assuming that the object did not vary over the 11 months time interval between
the two observations. If a higher resolution radio map confirms the faint,
central peak to be the radio nucleus then its separation from the
suggested optical ID would be four times the rms astrometric error. Any other
optical ID would have to be below the detection limit of the image (
for a point source).
The optical counterpart of this radio source is a point-like object.
The source lies in a crowded field close to the Galactic plane
(latitude
). The only detected object within
is a
R=25.5
0.4 object located about
North of the radio peak.
This is the likely optical ID for this unresolved radio source.
This compact steep spectrum radio source lies in a clear empty field.
This extended radio source lies in a crowded field close to the Galactic plane
(latitude
). The inset shows five bright objects near
the mid-point between the two radio lobes. Of these, the western-most object,
which is slightly extended, is the most likely optical identification.
The optical counterpart of this compact steep spectrum radio source is undetected down to a very deep level (R=25.6).
There is a hint of a central radio component. The NS extended optical object is
offset from this component by
to the North, which is within
the astrometric error, and is therefore the likely optical counterpart.
The suggested optical ID partially overlaps with the image of a 1.6 mag
brighter elliptical galaxy located about
to the South. A 2 h spectrum
obtained with EFOSC 2 at the ESO/MPI 2.2 m telescope shows a rich emission-line
spectrum at z=0.92. The adjacent elliptical galaxy is at z=0.6.
The best candidate appears to be the slightly extended object located about
West of the radio position. A very faint fuzz is coincident with the radio peak.
The spectral index of this radio source has a large uncertainty of 0.2(see Sect. 2.1). No object is seen within the radio contours above
the detection limit of the image (
).
A tight clustering of bright stars is seen within
West of this
radio source. This considerably degrades the quality of detection of the faint
(
)
optical counterpart observed between the two radio lobes.
This compact steep-spectrum radio source coincides with a barely detected fuzz
with
situated in a crowded field.
The results of our detailed optical observations of this source have been reported by Melnick et al. (1993). The distorted double radio source is identified with a z=0.552 cD galaxy which is member of a cluster (Kulkarni et al. 2005, in prep.). Between this galaxy and another similarly bright galaxy to the NE, which is a member of the same cluster, an unusually bright and straight arc-like feature was found by Melnick et al. (1993). We interpreted it as the image of a pair of merging galaxies at z=1.116, highly magnified due to gravitational lensing by the foreground cluster (see also Kneib et al. 1994).
On the assumption that the two radio components of this source are
physically associated, the most likely optical identification is the bright
(R=20.8
0.1) object roughly located midway between them. On the
other hand, it is quite possible that the two radio components are
independent radio sources (particularly since the western component is itself
a
double). In that case, the optical features detected towards
each of them would be their likely optical IDs. The magnitudes of these
objects are R=22.7
0.2 for the western lobe and R=20.8
0.1 for
the source close the eastern lobe.
More sensitive radio imaging is needed to confirm if this source is a wide-angle tail. The likely optical identification is coincident with the radio peak.
We have presented a catalogue of 52 powerful radio sources with very
steep spectra at decimetre wavelengths (median
).
For
15% of the sources, the observing conditions were poor or
the optical fields are confused, so even if the optical counterparts may not be
very faint, they could not be identified with the available material. Out
of the 41 (i.e.,
80%) identified sources, in spite of
considerable efforts, we were also able to obtain redshifts for only about
a third of them. This is most likely due to the fact that the CCDs of
those days had rather high read-out noise and poor blue sensitivity. We are
confident that using modern detectors on the same telescopes we should be
able to do a lot better.
Our sample is found to contain six USSRS that are not only optically very
faint ()
but also have small radio sizes (<
). These
sources, namely, 0001+058, 0634-196, 1509-158, 1631-222, 2011-169 and
2105-119, are prime candidates to be at very large redshifts, and indeed,
we plan to make a new round of optical identifications to complete
the catalogue.
Finally, we note that, even though optical spectroscopy has been completed for
only about a quarter of the sample (14 sources), already this small subset is
found to contain two powerful ultra-steep spectrum radio sources
having LINER-type ultra-soft emission-line spectra (0410-198 and
1411-192 at, respectively, z=0.79 and 0.48) plus at least one good
candidate (1523-017 at z=0.93). Another three candidates,
namely, 0423-199 (z=1.12), 1146+052 (z=0.42) and
2057-179 (z=0.92), show strong [O II] emission but since
their existing spectra do not extend to the region of the [O III] line,
spectral softness based on the strengths of these two emission lines remains
to be confirmed. Our observations indicate that the former two sources are
associated with distant () galaxy clusters having a dense
intra-cluster medium, as is the case for the nearby massive cooling
flow cluster Hydra A (Melnick et al. 1997;
also, McNamara 1995). Thus, together with an ultra-steep radio
spectrum and relaxed radio morphology, the presence of a LINER spectrum in
the optical can be used as a powerful indicator of high-z clusters which
are not merely concentrations of galaxies, but have acquired an intra-cluster
medium dense enough to sustain cooling flow activity.
Acknowledgements
G.K. thanks ESO for the hospitality in Chile during a visit when this project was started. JM thanks NCRA, Pune, for their hospitality when this project was finished. We also thank Samir Dhurde and Alok Gupta for their invaluable help with data handling. This work made use of the MIDAS and AIPS data reduction and analysis packages as well as the NASA/IPAC extragalactic database (NED). VLA is operated by Associated Universities, Inc., under contract with the National Science Foundation.
Table 1:
Overall properties of the sample galaxies.
Table 2: Optical imaging observations.
Table 3: Optical spectroscopy observations.
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Figure 1: Radio-optical overlays. The insets show the optical counterparts without radio contour. The radio contours are based on VLA snapshots taken at 4.8 GHz. |
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