Issue |
A&A
Volume 508, Number 1, December II 2009
|
|
---|---|---|
Page(s) | 201 - 207 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200912658 | |
Published online | 21 October 2009 |
A&A 508, 201-207 (2009)
The HI properties of galaxies in the Coma
I cloud revisited
(Research Note)
A. Boselli1 - G. Gavazzi2
1 - Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS, 38 rue F.
Joliot-Curie, 13388 Marseille, France
2 - Universita degli Studi di Milano-Bicocca, Piazza delle Scienze 3,
20126 Milano, Italy
Received 9 June 2009 / Accepted 19 September 2009
Abstract
Context. Pre-processing within small groups has been
proposed to explain several of the properties of galaxies lying in rich
clusters.
Aims. The aim of the present work is to see whether
pre-processing is acting in the nearby universe, where the structures
that are merging to form rich clusters are large and massive.
Methods. We study the HI gas properties of a large
sample of late-type galaxies belonging to the Coma I cloud, an
association of objects close to the Virgo cluster.
Results. Contrary to what was previously claimed,
late-type galaxies in the Coma I cloud are not deficient in
HI gas (
).
Conclusions. If the Coma I cloud is
representative of infalling groups in nearby clusters, this result
suggests that, in the local universe, the evolution of late-type
galaxies belonging to loose structures with high velocity dispersions (300 km s-1)
associated with rich clusters such as Virgo is not significantly
perturbed by pre-processing.
Key words: galaxies: general - galaxies: ISM - galaxies: distances and redshifts - galaxies: clusters: general
1 Introduction
The morphology segregation effect (Dressler 1980; Whitemore
& Gilmore 1991)
is the strongest evidence that the environment plays a major role in
shaping galaxy evolution.
Recent surveys such as SDSS (Gomez et al. 2003)
and 2dF (Lewis et al. 2002), which have
allowed us to continuously trace galaxy properties from the highest
density regions in the core of rich clusters down to the field, have
shown that the star formation activity decreases at the periphery of
clusters, probably because the interactions responsible for removal of
gas, the principal feeder of star formation (e.g. Boselli
et al. 2001),
were already in place in the infalling groups prior to the formation of
rich clusters. These results are consistent with our own studies of the
gas and star formation properties of galaxies in nearby clusters
(Gavazzi et al. 2002,
2005, 2006a,b, 2008).
Although in some cases the presence of hot gas might trigger galaxy
interactions with
the intergalactic medium, the low velocity dispersion of small groups (200 km s-1)
suggests that
gravitational interactions are probably at the origin of the
pre-processing of galaxies even before they enter rich clusters
(Dressler 2004).
Pre-processing is probably efficient at high redshift, when clusters are under formation (Gnedin 2003). Pre-processing is less evident in the nearby universe (Boselli & Gavazzi 2006), where clusters are rather accreting large structures characterized by high velocity dispersions (Donnelly et al. 2001; Ferrari et al. 2003; Cortese et al. 2004) or single galaxies, making gravitational interactions rather rare. For instance, the velocity dispersion of the M and W clouds in the Virgo cluster are relatively high, of the order of 450-650 km s-1 (Gavazzi et al. 1999), thus almost comparable to that of an already formed cluster. The only exception found in the local universe is the blue infalling group in A1367 (Sakai et al. 2002; Gavazzi et al. 2003a; Cortese et al. 2006), a compact group of galaxies with a velocity dispersion of only 150 km s-1 falling into the cluster A1367. Here pre-processing is efficiently perturbing the galaxy morphology and star formation activity, creating long tails of ionized gas.
The study of the Virgo cluster, the nearest rich cluster of
galaxies, and its surroundings, however, revealed the presence of
satellite clouds with HI-deficient objects revealing an ongoing
interaction, thus making these clouds of particular interest in
studying pre-processing in the nearby universe. Being loosely anchored
to the galaxy potential, the HI component can be
easily removed during any kind of interaction, and is thus an ideal
tracer of ongoing perturbations (Boselli & Gavazzi 2006).
Among these, the Coma I cloud, a loose aggregation of galaxies
in the projected direction of the Coma/A1367 supercluster located at 5 Mpc
from M 87 (see Sects. 4 and 5), is the most
promising target since
previous studies have shown that this loose cloud is composed of
HI-deficient galaxies (Garcia-Barreto et al. 1994).
The availability of new HI data more than doubled the sample
of Garcia-Barreto et al. (1994),
suggesting the importance of reanalyzing the HI gas properties
of the Coma I cloud galaxies in the framework of
pre-processing
in the nearby universe.
2 The sample
The Coma I cloud has been defined by Gregory & Thompson (1977) as the
cloud of nearby galaxies (20 Mpc)
in the foreground
of the Coma/A1367 supercluster. The analysis presented in this work is
thus based on a sample composed of all galaxies extracted from NED in
the
sky region 11
30
< RA
(2000) < 13
30
;
20
< Dec < 34
with a recessional velocity
2000 km s-1.
Excluding misclassified HII regions associated with bright
galaxies, the resulting sample is composed of 161 galaxies.
Since no limits on the magnitude or diameter of the selected galaxies
are applied, the selected sample is not complete in any sense.
3 The data
The set of data necessary for the following analysis, restricted to those galaxies with available HI data (72 objects), are listed in Table 1. This includes morphological type, optical and near IR magnitudes, optical diameters and HI flux and line width measurements. Coordinates and morphological type have been taken from NED, in its updated version including the SDSS data release 6. For galaxies without a morphological classification, we assigned a morphology type according to, in order of preference, the presence of emission lines in the SDSS spectra, their optical color on the SDSS composed image or their optical appearance on the POSS plates. Thanks to their proximity, the morphological classification of the brightest galaxies, those with available HI data and thus the most concerned by the present analysis, is very accurate, to less than one bin in the Hubble sequence. It is poor for the very compact sources that dominate at low luminosity. The discrimination between early and late-type galaxies, based on spectroscopic measurements or optical colors, however, should be reliable.
Optical diameters have been taken from GOLDMine (Gavazzi et al. 2003b) whenever available, or from NED otherwise. We preferred to use the GOLDMine values whenever possible to be as consistent as possible in the definition and in the determination of the HI-deficiency parameter, which here is based on the calibrations of Solanes et al. (1996). Near infrared JHK total magnitudes have been taken from 2MASS (Jarrett et al. 2003) whenever available, or from Gavazzi & Boselli (1996). The comparison of 2MASS and GOLDMine total magnitudes that we made using an extended sample indicates that they differ by less than 0.1 mag.
HI data have been taken from several sources in the
literature: to be consistent with the distance determination using the
Tully-Fisher calibration given by Masters et al. (2008),
HI fluxes and velocity widths have been taken whenever
available from Springob et al. (2005). These
data also have the advantage of being accurately homogenized.
HI line widths from other sources have been corrected for
smoothing, redshift stretch and turbulent motion
(6.5 km s-1) as described in
Springob et al. (2005).
HI fluxes of sources others than Springob et al. (2005)
have been corrected consistently for pointing offsets and beam
attenuation. The accuracy in the fluxes should be of the order of
10-15% (Springob et al. 2005),
and of the order of 10 km s-1
in the line width measurements.
Galaxies in Table 1
are arranged as follow:
- - Column 1: galaxy name. Column 2: morphological type, from NED whenever available, or from our own classification;
- - Column 3 and 4: major and minor optical diameters, from GOLDMine whenever available, from NED elsewhere. These are B band isophotal diameters at the 25 mag arcsec-2;
- - Column 5: optical (generally B band (mB)) magnitude, from NED;
- - Column 6: heliocentric velocity, in km s-1, from NED;
- - Column 7: Tully-Fisher distance, in Mpc, determined as explained in Sect. 4.2. Distances for those galaxies with available primary indicators are taken from Ferrarese et al. (2000). For galaxies without a direct distance estimate, we assume 14.52 Mpc;
- - Column 8: Coma I cloud (CI) members and background (Bg) objects, whose identification has been determined as explained in Sect. 4.1;
- - Column 9: HI flux, in Jy km s-1 corrected for pointing offset and beam attenuation consistent with Springob et al. (2005);
- - Column 10: HI line width at 50%, defined as in Springob et al. (2005; WF50), corrected for smoothing, redshift stretch and turbulent motion as prescribed by Springob et al. (2005);
- - Column 11: logarithm of the HI mass, in solar units;
- - Column 12: reference to the HI data;
- - Column 13: the HI deficiency parameter, determined as described in the text;
- - Column 14: a code for the HI line profile, from Springob et al. (2005) whenever available, or with the following criteria: 1 for high signal to noise two horn profiles; 2 for high signal to noise one horn profiles; 3 for fair profiles; 4 for low signal to noise, bad quality profiles and 5 for unavailable profiles.
Table 1: Galaxies with HI data.
4 The derived parameters
4.1 Cloud membership
The distribution on the sky and in the velocity space of the selected
galaxies
is shown in Fig. 1.
The wedge diagram shown in the lower panel clearly
indicates that the Coma I cloud is confined within recessional
velocities 1500 km s-1.
To avoid any possible contamination from unclassified objects possibly
associated to our galaxy, we
arbitrarily remove any object with recessional velocity
<100 km s-1 (none of
these objects have HI data).
![]() |
Figure 1:
The sky distribution ( upper panel) and the wedge
diagram ( lower panel) of galaxies in the studied
region. Red circles are for early-type ( |
Open with DEXTER |
The average recessional velocity of the galaxies identified as
Coma I cloud members (132 objects
in the 100
1500 km s-1)
is vel
km s-1.
The velocity dispersion is relatively small for a structure
extended over
250 square
degrees, in particular if compared to that observed in the
W and M clouds (see Table 2) located in the
background of the Virgo cluster, at
32 Mpc (Gavazzi et al. 1999).
As defined, the Coma I cloud is an aggregation of objects with
a spiral fraction of 68%, thus
slightly higher than the W (54%) and M (62%) clouds in the Virgo
cluster. An overdense region is visible around NGC 4278 (
galaxies/Mpc3,
where
is the local density of galaxies brighter than MB
= -16, in galaxies/Mpc3, within a
three-dimensional grid 0.5 Mpc wide; Tully 1988b):
this density is slightly lower than that observed in the periphery of
the Virgo cluster (
galaxies/Mpc3;
Tully 1988a).
4.2 Distance determination
An accurate determination of the gas properties of the target galaxies needs a precise distance determination. Five galaxies in the studied region have distance measurements from primary indicators such as cepheids (NGC 4414, NGC 4725), planetary nebulae and globular cluster luminosity functions (NGC 4278, NGC 4494, NGC 4565) and surface brightness fluctuations (NGC 4278, NGC 4494, NGC 4565, NGC 4725) (Ferrarese et al. 2000).
For inclined galaxies (inclination 30 deg) with
available HI line widths and JHK total
magnitudes,
the distance can be inferred using the Tully-Fisher relation determined
adopting the Masters et al. (2008)
calibration.
For these 27 galaxies, we estimate their distance as the
average of the JHK Tully-Fisher distance. We notice
that for the three galaxies having both distance estimates, the
Tully-Fisher distance is systematically lower than that obtained from
the primary indicators by
3.5 Mpc.
The average distance of the Coma I cloud members defined in
the previous section
is
Mpc, while it is
Mpc
considering only
the five galaxies with primary distance indicators. In the following
analysis we assume a distance of 14.52 Mpc for those galaxies
belonging to the Coma I cloud without any direct distance
measurement. For the few background galaxies (
km s-1)
the distance is determined assuming a Hubble constant of H0
= 73 km s-1 Mpc-1once
their recessional velocity is corrected for a Virgocluster infall of
224 km s-1.
Table 2: Coma I and Virgo clouds properties.
![]() |
Figure 2:
Same as Fig. 1:
filled symbols are for galaxies with a distance within 5 Mpc
of the average distance of the Coma I cloud as determined from primary
indicators (
|
Open with DEXTER |
4.3 The gas mass and the HI-deficiency parameter
The HI gas mass has been determined using the relation:
![]() |
(1) |
where the distance is determined as described in the previous section.
The HI-deficiency parameter (
)
is defined
as the logarithmic difference between the average HI mass of a
reference sample of isolated galaxies of similar type and linear
dimension and the HI mass actually observed in individual objects:
.
According to Haynes & Giovanelli (1984),
log
,
where cand d
are weak functions of the Hubble type, diam is the linear
diameter of the galaxy (see Gavazzi et al. 2005) and h
= H0/100.
In the present analysis we use the calibration of Solanes
et al. (1996) for late-type
galaxies, extended to Scd-Im-BCD objects as prescribed in Gavazzi
et al. (in preparation) (see Table 3). This calibration
is based on a sample of 98 galaxies of type
Scd
in the local supercluster (excluding the Virgo cluster),
observed by ALFALFA in the sky region 11
< RA(2000) < 16
,
4
< Dec < 16
and in
the velocity range
< 2000 km s-1
and is, at present,
the best available calibration for this morphological class. It is
preferred to the highly uncertain
calibration of Haynes & Giovanelli (1984) which is
based on a small sample of 38 Scd-Im-BCD
galaxies mostly of large diameter (Gavazzi et al. 2008; Solanes
et al. 1996).
Table 3: The calibration of the HI deficiency parameter.
![]() |
Figure 3:
Same as Fig. 1:
filled symbols are for HI-deficient galaxies (
|
Open with DEXTER |
The average HI deficiency of galaxies in the Coma I cloud is ,
thus slightly higher
than the average value for unperturbed field objects (
;
Haynes & Giovanelli 1984).
This result is robust against the adopted calibration of the
HI-deficiency parameter since it does not change significantly using
the c = 7.00 and d = 0.94
coefficients for Scd-Im-BCD galaxies of Haynes & Giovanelli (1984):
,
thus consistent with our estimate. Out of the 55 late-type
Coma I cloud members with available HI data, only 13
(24%) can be considered as deficient in HI gas with
HI deficiencies greater than 0.3. These most
deficient objects (filled squares in Fig. 3) do not seem to be
located in particular zones of the sky or of the velocity space, nor
are objects at the average distance of the Coma I cloud but
with high velocity with respect to the cloud (Fig. 4).
![]() |
Figure 4:
The distance-velocity diagram of galaxies in the studied region. Filled
symbols are for HI-deficient galaxies (
|
Open with DEXTER |
5 Discussion and conclusion
![]() |
Figure 5: Comparison of the HI fluxes (in Jy km s-1) used in this work to those determined from Table 1 of Garcia-Barreto et al. (1994) using a distance of 10 Mpc and the relation given in Eq. (1). Red crosses indicates data from Springob et al. (2005), red filled squares from Lewis (1987), blue filled triangles from HyperLeda, green filled squares from Schneider et al. (1990), orange filled dots from Fisher & Tully (1981), blue empty dots from Huchtmeier (1982) and green empty triangles from Huchtmeier & Richter (1989). The dotted line shows the one to one relation. |
Open with DEXTER |
By studying the HI properties of 32 galaxies in the
Coma I cloud with data taken at Effelsberg, Garcia-Barreto
et al. (1994)
concluded that these objects are generally devoid of gas. In their
sample of 23 late-type galaxies the average HI-deficiency is
,
significantly higher than the value found in this work (
)
on a sample more than
doubled in size (55 objects). The difference with
Garcia-Barreto et al. (1994) might
result from several reasons. The HI fluxes used in this work,
mostly (64%) taken from the compilation of Springob
et al. (2005),
are systematically higher (22%) than those of Garcia-Barreto
et al. (1994)
(see Fig. 5).
The resulting HI-deficiency parameter is thus lower by a factor
of 0.09 on average than the previous estimate.
This difference can be due to the fact that Springob et al. (2005) correct
the data for beam attenuation and pointing offsets, while it is unclear
whether Garcia-Barreto et al. (1994) used
similar corrections.
An additional difference of 0.03 in
is due to the fact that, to transform
fluxes into gas
masses, Garcia-Barreto et al. (1994)
in Eq. (1) used a constant value of
instead of
as in this work.
The relationship between optical linear diameters and the HI mass being
non linear, the HI-deficiency parameter is not a distance independent
value: for a given galaxy the HI-deficiency increases if its distance
decreases. Garcia-Barreto et al. (1994) used
a distance of
10 Mpc in the determination of the HI mass of their
sample, while we used the Tully-Fisher distance whenever available, or
14.52 Mpc elsewhere. This difference in distance leads to an
overestimate of the HI-deficiency parameter of
0.04
for a typical Sc galaxy in the Garcia-Barreto et al.
calculations with respect to ours.
Conversely, the use of the calibration of Solanes et al.
(1996) for Sa-Sc galaxies, which is based on H0
=100 km s-1 Mpc-1,
induces a decrease of the HI-deficiency parameter by a factor
(1 - d)Log h2
(from 0.11 for Sa to 0.04 for Sc). Since the present
sample is dominated by galaxies of type
Scd (78%), whose distance has been determined using H0
=73 km s-1 Mpc-1,
the average
is only marginally affected by the choice of H0
=100 km s-1 Mpc-1
for Sa-Sc galaxies of Solanes et al. (1996).
The rest of the difference (0.18 in
)
might be due to statistical reasons, our sample (55 objects)
being more than twice the size of that of Garcia-Barreto
et al. (1994)
(23 objects), or to the adopted calibration. Garcia-Barreto
et al. (1994)
determined the HI-deficiency parameter using the B
band luminosity relation of Giovanelli et al. (1981), while
our estimate is
based on a diameter relation. The calibration of the HI-deficiency on
optical diameters
is less dispersed than that based on optical luminosities (Haynes
& Giovanelli 1984).
We can thus conclude that late-type galaxies in the Coma I
cloud are not as deficient in HI gas as previously claimed.
The Coma I cloud is thus composed of galaxies with a similar spiral
fraction but richer in gas content than the Virgo M and
W clouds. Being at a distance along the line of sight similar
to that of Virgo (14.52 Mpc for Coma I and
16.5 Mpc for Virgo), and at a distance of 5 Mpc
on the plane of the sky to the core of the cluster, it could be
considered as a cloud of Virgo (for comparison the M and W clouds are
located at
16 Mpc
from the core of Virgo, Gavazzi et al. 1999). Is
pre-processing acting on the late-type galaxy population in the Coma
I cloud?
From a statistical point of view, the present analysis excludes it.
There exists, however, a fraction of objects with a significant
HI-deficiency (
).
What is its origin?
Because of the relatively poor statistics and the low density contrast
within the cloud, it is impossible to disentangle gravitational
interactions from interactions with the intergalactic medium within the
Coma I cloud itself or during the crossing of the whole cloud through
the core of the Virgo cluster.
The spread of the HI-deficient galaxies (
)
within the cloud and in the velocity-distance space (Fig. 4)
do not seem to favor the former scenario, since gravitational
interactions or ram-pressure stripping within the cloud would be more
efficient in the highest density regions or for galaxies with the
highest velocities with respect to the mean value of the
Coma I cloud.
Indeed using the prescription of Boselli & Gavazzi (2006) we can
estimate that the frequency of galaxy encounters within
the Coma I cloud is very low, the relaxation time being
40 Gyr.
Despite the process in place, however, if the Coma I cloud is
representative of infalling groups in nearby clusters,
we conclude that in the nearby universe the gas properties of late-type
galaxies belonging to large substractures of rich clusters do not
appear significantly perturbed by their environment.
The ongoing ALFALFA survey (Giovanelli et al. 2005) will
soon provide us with an unprecedent sky coverage in HI
of 7000 sq. degrees of the sky, thus covering a large range in
galaxy density from the core of rich clusters to the local voids.
In particular, given its sensitivity (2.4 mJy at
5 km s-1, Giovanelli
et al. 2005)
combined with a multi-beam detector, ALFALFA will be perfectly suited
for observing at the same time extended sources and point-like objects
as those populating the Coma I cloud.
This survey will thus be a unique opportunity for studying, using a
homogenous dataset and with an unprecedent statistical significance,
the gas properties of galaxies in different density regimes of the
local universe,
including loose groups and substructures probably infalling into rich
clusters.
We want to thank C. Marinoni, L. Cortese, C. Pacifici and S. Boissier for interesting discussions, and the anonymous referee for useful comments. This research has made use of 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. We acknowledge the usage of the HyperLeda database (http://leda.univ-lyon1.fr) and the GOLDMine database (http://goldmine.mib.infn.it/).
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Footnotes
- ... section
- For galaxies with both primary distance indicators and Tully-Fisher distances, the former are adopted.
All Tables
Table 1: Galaxies with HI data.
Table 2: Coma I and Virgo clouds properties.
Table 3: The calibration of the HI deficiency parameter.
All Figures
![]() |
Figure 1:
The sky distribution ( upper panel) and the wedge
diagram ( lower panel) of galaxies in the studied
region. Red circles are for early-type ( |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Same as Fig. 1:
filled symbols are for galaxies with a distance within 5 Mpc
of the average distance of the Coma I cloud as determined from primary
indicators (
|
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Same as Fig. 1:
filled symbols are for HI-deficient galaxies (
|
Open with DEXTER | |
In the text |
![]() |
Figure 4:
The distance-velocity diagram of galaxies in the studied region. Filled
symbols are for HI-deficient galaxies (
|
Open with DEXTER | |
In the text |
![]() |
Figure 5: Comparison of the HI fluxes (in Jy km s-1) used in this work to those determined from Table 1 of Garcia-Barreto et al. (1994) using a distance of 10 Mpc and the relation given in Eq. (1). Red crosses indicates data from Springob et al. (2005), red filled squares from Lewis (1987), blue filled triangles from HyperLeda, green filled squares from Schneider et al. (1990), orange filled dots from Fisher & Tully (1981), blue empty dots from Huchtmeier (1982) and green empty triangles from Huchtmeier & Richter (1989). The dotted line shows the one to one relation. |
Open with DEXTER | |
In the text |
Copyright ESO 2009
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