A&A 379, 347-361 (2001)
DOI: 10.1051/0004-6361:20011242
H. Bravo-Alfaro1 - V. Cayatte2 - J. H. van Gorkom3 - C. Balkowski2
1 - Departamento de Astronomía, Universidad de Guanajuato.
Apdo. Postal 144, Guanajuato 36000, México
2 -
Observatoire de Paris DAEC, and UMR 8631,
associé au CNRS et à l'Université Paris 7, 92195 Meudon Cedex, France
3 -
Department of Astronomy, Columbia University, 550 W 120th Street,
New York, NY 10027, USA
Received 11 July 2001 / Accepted 4 September 2001
Abstract
In the first paper of this series we used H I observations of the 19
brightest spirals in Coma to analyze the dynamical state of the cluster. In
this paper we present the detailed H I distribution and kinematics of the
spirals that were detected in H I, and radio continuum data for a sample of
star forming and post starburst galaxies in Coma. We discuss the importance
of ICM-ISM interactions to explain the observed H I morphology. A rough
comparison of observed H I sizes with predicted H I sizes from simulations by
Abadi et al. (1999) gives reasonable agreement. We use the results on radio
continuum emission to estimate the star formation rate in the PSB galaxies we
pointed at. The radio continuum emission in the 11 so called post starburst
galaxies, identified by Caldwell et al. (1993) in the cluster, is
weak. Eight of the 11 were not detected down to a 3 sigma upper limit of
0.6 mJy. This sets an upper limit to the star formation rate in these
galaxies of less than 0.2 yr-1. The three detected post
starburst galaxies have a star formation rate of less than one solar
mass per year. Thus none of the post starburst galaxies in Coma are dust
enshrouded starbursts.
Key words: galaxies: clusters: individual: Coma - radio lines: galaxies
This is our second paper on the imaging of the neutral hydrogen component as a tracer of environmental effects on galaxies in the Coma cluster. Global properties of Coma related with its dynamical state, as derived from our VLA 21 cm line survey, have been discussed in a previous paper (Bravo-Alfaro et al. 2000, hereafter referred to as Paper I). In the present work we give a complete catalogue of H I maps, channel maps, and velocity fields (for cases with sufficient resolution) for 19 detected galaxies. We compare the detailed H I distribution and kinematics with model predictions for the fate of H I in galaxies in a cluster environment. We pay particular attention to a class of so called starburst (SB) and post starburst (PSB) galaxies identified by Caldwell et al. (1993, C93 throughout this paper). For these galaxies we derive star formation rates (SFR) from the radio continuum emission and discuss the implications of current gas content and star formation rate on the evolution of the galaxies.
Several physical mechanisms have been proposed to play a major role affecting the evolution of galaxies in clusters; these processes can be classified according to three types: (a) interactions between the ICM and the interstellar medium (Gunn & Gott 1972; Nulsen 1982; Kenney & Young 1989), (b) interactions with the cluster gravitational field (e.g. Bekki 1999), and (c) galaxy-galaxy interactions (see Icke 1985; Lavery & Henry 1994; Moore et al. 1996, 1999). In many cases more than one of these processes will be at work, but which mechanism dominates under specific physical conditions and how it depends on the galaxy trajectory through the cluster is still a matter of debate. For instance Solanes et al. (2000) conclude from an observational study of eighteen clusters that ICM-ISM interactions are most important to explain the observed H I deficiency. In contrast, Moore et al. (1999) suggest in a numerical study that, in combination with ram-pressure stripping, galaxy harassment may convert disk galaxies into S0s. In spite of this debate, the fact that spirals in the central region of clusters like Virgo show smaller H I than optical disks (Warmels 1988; Cayatte et al. 1990, 1994), is unquestionably associated with the interaction with the ICM. These studies demonstrate the importance of detailed H I imaging of individual galaxies in clusters with a range of ICM conditions to assess what gas removal processes are at work.
Another fundamental process linked with the evolution of galaxies in clusters is the triggering and stopping of bursts of star formation (van den Bergh et al. 1990). As the ISM is the raw material from which stars form, the evolution of H I deficient galaxies and their star formation properties are undoubtedly affected by the interaction with the environment. From statistical analyses on large scales the SFR is known to decrease with increasing density (Dressler et al. 1999), whereas at smaller scales the ICM may produce significant enhancements to the star formation activity in individual galaxies, as shown in clusters like A1367 (Gavazzi et al. 1995) and Coma (Bothun & Dressler 1986). Recent work supports different scenarios: (1) On the theoretical side, Bekki (1999) concludes that the tidal gravitational field of a group-cluster merger may trigger a burst of star formation, accounting for the PSB population in clusters like Coma, while Vollmer et al. (2001b) suggest that local enhancement of star formation could be due to re-accretion of gas clouds after a ram pressure stripping event. (2) Observationally, Dressler et al. (1999) find a trend in the position of SB and PSB galaxies around the cluster, making the hot gas environment the best explanation for this phenomenon (see also Poggianti et al. 1999). From a study of spectroscopically selected post starburst galaxies, which turned out to be mostly located in the field, Zabludoff et al. (1996) conclude that in those more isolated galaxies the post starburst phenomenon is most likely caused by galaxy interactions and mergers.
In this paper we give the atlas of our VLA H I imaging of spirals in Coma, showing that the scenario depicted by the neutral hydrogen constitutes strong evidence in favor of ICM-ISM interaction to explain the H I morphologies and the star formation histories in the cluster. We also present results on the radio continuum emission obtained as a byproduct of the spectral line observations. These results are used to derive the star formation rate in the so called post starburst galaxies to explore the possibility that these are in fact dust enshrouded starbursts as has recently been suggested by Smail et al. (1999) for the intermediate redshift cluster CL0939+4713 (see also Miller & Owen 2001).
We organize the paper as follows. In Sect. 2 we review the
observations. In Sect. 3 we describe H I observational results of
individual detected galaxies, in Sect. 4 we discuss the role of ram
pressure affecting galaxies in Coma, and compare H I observations with
numerical simulations of ram pressure stripping by Abadi et al. (1999) and
Vollmer et al. (2001b). In Sect. 5 we discuss the PSB galaxies; we compare
our H I mass limits and current star formation rates based on the continuum
with those of the more vigorously star forming galaxies and discuss the
implications. The results are summarized in Sect. 6. In Appendix A,
available in electronic form, we give the complete catalogue of H I channel
maps. Throughout the paper we assume a distance of 70 Mpc to the Coma
cluster (with H0=100 kms-1Mpc-1), where an angular size of 1
arcmin corresponds to a linear size of 20 kpc.
Our data consist of 21 cm line data obtained with the VLA. Twelve fields within one Abell radius of the Coma cluster
(equivalent to 1.2
)
were observed with the VLA in its C configuration,
and two of them reobserved in D configuration. The observed fields and the
distribution of the 19 galaxies detected in H I around the cluster are shown
in Paper I (Figs. 1 and 2). Most of the fields are devoted to the
center of Coma and the regions where SB and PSB galaxies had been reported
from optical observations. Our velocity resolution is 21 kms-1, and
43 kms-1 for some of the galaxies observed in both C and D configurations. The
angular resolution ranges between 20 and 35 arcsec. The rms in the final
cubes is between 0.35 and 0.40 mJy beam-1, except in those fields
observed in both C and D configurations, reaching rms values as low as 0.20 mJy beam-1. Most of the channel maps shown in the Appendix (electronic
version) are smoothed in velocity. More details on the H I observations are
given in Paper I.
Eight out of the 19 detected galaxies are projected within 0.5 Mpc from the
cluster center (we consider the position of the cD NGC4874 as the Coma
center). These central galaxies lie inside or near the hot ICM as traced by
the ROSAT X-ray emission (Briel et al. 1992; Vikhlinin et al. 1997), and most
of them are very H I deficient. The H I deficiency is measured following
Giovanelli & Haynes (1985), comparing the observed H I mass with the
expected H I mass of an equivalent isolated spiral. Several galaxies
detected in H I near the center of Coma show H I truncated disks as well as
offsets between the optical and H I distributions (see Sect. 3). In a few
cases unexpected differences between optical and H I velocities are
found. Six of the galaxies detected in H I are reported by Bothun &
Dressler (1986) as having blue disks; five of these are projected inside or
near the cluster X-ray emission. None of the PSB galaxies reported by Caldwell
et al. (1993, 1997) were detected in H I; we obtain for some of them H I mass
upper limits down to
.
Our radio continuum images are obtained as a byproduct of the H I observations. The images are built by averaging a set of those line free channels in a given cube. All of the H I detected spirals (except NGC4907) are also detected in the continuum with flux densities ranging between 2 and 16 mJy (see Paper I).
In this section we briefly describe the H I morphology of detected galaxies,
and the remarkable features concerning galaxy dynamics as revealed by the
H I.
IC3913, KUG1255+275, and Mrk057
These three galaxies were detected in the SW of Coma, between 17 and
25
south of the cD galaxy NGC4839. They display several common
features: they are H I rich galaxies with regular gas distributions, where
H I disks extend well beyond the optical (Figs. 1a, 2 and 3a).
![]() |
Figure 1a:
H I density distribution of
IC 3913, superposed on a DSS B-band gray scale image. The contours
are 0.3 (2.5 ![]() ![]() ![]() |
![]() |
Figure 2:
H I density distribution of
KUG 1255+275, superposed on a DSS B-band gray scale image. The
contours are 0.3 (2.5 ![]() ![]() ![]() |
![]() |
Figure 3a:
H I density distribution of
Mrk 057, superposed on a DSS B-band gray scale image. The contours
are 0.6 (2.5 ![]() ![]() ![]() |
NGC4848
This Scd galaxy is one of the most interesting objects in our sample.
NGC4848 is a blue disk galaxy showing a very intriguing H I distribution
(see Fig. 4),
![]() |
Figure 4:
H I density distribution of
NGC 4848, superposed on a DSS B-band gray scale image. The
contours are 0.3 (2.5 ![]() ![]() ![]() |
Mergers do not account for the H I morphology or the star formation activity
in NGC4848, because no obvious companion is seen in the DSS optical image
(Fig. 4). However, the more detailed B-band CCD imaging by Gavazzi
et al. (1990) shows a ring-like structure and blue bright zones in the NW, where
the H I and the secondary peaks of CO, H
and 20 cm radio continuum are
found. The hypothesis of a dwarf system crossing the NGC 4848 disk is
explored by Vollmer et al. (2001a) but further observations, both optical and
higher resolution H I imaging, are needed to confirm it. N-body simulations
by the same authors suggest that NGC4848 has already gone through the
cluster core, 4
108 yr ago, and is now moving away from the
cluster. They conclude that re-accretion of some of the stripped gas could
explain the star formation burst.
Mrk58
This Sb, blue disk galaxy is projected onto the X-ray emission, some
20 (
400 kpc) SW of NGC4874, in the zone lying between the main
cluster and the SW group. It is gas deficient by a factor of 3. Its H I map
(Fig. 5) displays a considerably asymmetry,
![]() |
Figure 5:
H I density distribution of
Mrk 058, superposed on a DSS B-band gray scale image. The contours
are 0.2 (2.5 ![]() ![]() ![]() |
CGCG160-058 and CGCG160-076
These late-type spiral galaxies lie in the nearby northern vicinity of Coma, outside the X-ray emission. They are H I rich galaxies displaying regular gas distributions (Figs. 6a and 7a). As expected, considering their position relative to the cluster, no strong environmental effects are seen: their H I disks are larger than the optical, and the H I and optical centroids are coincident. Velocity fields are given in Figs. 6b and 7b.
CGCG160-086
![]() |
Figure 6a:
H I density distribution of
CGCG 160-058, superposed on a DSS B-band gray scale image. The
contours are 0.4 (2.5 ![]() ![]() ![]() |
![]() |
Figure 7a:
H I density distribution of
CGCG 160-076,
superposed on a DSS B-band gray scale image. The contours are 0.2
(2.5 ![]() ![]() ![]() |
![]() |
Figure 8:
H I density distribution of
CGCG 160-086,
superposed on a DSS B-band gray scale image. The contours are 0.4
(2.5 ![]() ![]() ![]() |
IC 4040
This Sdm blue disk galaxy is projected near the very center of Coma, at only
15 (300 kpc) NE from NGC4874. Its H I distribution (see Fig. 9a)
shows the H I contours compressed in the NW,
![]() |
Figure 9a:
H
I density distribution of IC 4040, superposed on a DSS B-band gray
scale image. The contours are 0.2 (5![]() ![]() ![]() |
NGC4907
This Sb galaxy was only marginally detected in H I (Fig. 10).
![]() |
Figure 10:
H I density distribution of
NGC 4907, superposed on a DSS B-band gray scale image. The
contours are 0.2 (2.5 ![]() ![]() ![]() |
KUG1258+287, FOCA195 and CGCG160-098
![]() |
Figure 11a:
H
I density distribution of KUG 1258+287, superposed on a DSS B-band
gray scale image. The contours are 0.3 (2.5 ![]() ![]() ![]() |
![]() |
Figure 12a:
H I density distribution of
FOCA 0195, superposed on a DSS B-band gray scale image. The contours
are 0.2 (2.5 ![]() ![]() ![]() |
![]() |
Figure 13a:
H I density distribution of
CGCG 160-098, superposed on a DSS B-band gray scale image. The
contours are 0.3 (2.5 ![]() ![]() ![]() |
These objects are part of a group dominated by CGCG160-098 (see Biviano
et al. 1996). As all the galaxies in Coma detected in H I outside the X-ray
emission, this group shows a normal H I content and a regular gas
distribution (Figs. 11a, 12a and 13a), with the H I more extended than the
optical disk and coinciding with the optical position.
Particular features of
these galaxies are their systematic blue
color (see
Table 2 and Fig. 7 in Paper I), and their high radial velocity (between
8400 km s-1 and 8880 km s-1), indicating a fast accretion of the group towards
the main cluster body. Normal rotation patterns are shown by the bluest
objects KUG1258+287 and FOCA195 (see Figs. 11b and 12b). H I emission
equivalent to
was detected at 1.4
W of
KUG1258+287 but no optical counterpart is observed.
NGC4911
This is one of the two brightest spirals in Coma. The H I morphology of
NGC4911 shows a shrunken disk with two central peaks and a normal surface
density in the central region as predicted by ram-pressure stripping
(Fig. 14a).
![]() |
Figure 14a:
H I density distribution of
NGC 4911, superposed on a DSS B-band gray scale image. The contours
are 0.3 (2.5 ![]() ![]() ![]() |
NGC4911, projected 20SE of NGC4874 (
1000 km s-1), is
thought to be the dominant galaxy of a group that recently crossed the
cluster core (Biviano et al. 1996). Vikhlinin et al. (1997) reported an X-ray
filamentary structure in this region, crossing over NGC4911 and culminating
at the position of NGC4921. This cool spot has recently been confirmed with
XMM (Arnaud et al. 2001), who consider the possibility of gas stripped from
an infalling group, but not directly produced in the Coma center. They found
that part of the X-ray excess in this zone is due to NGC4911. No other
galaxies of this group were detected in H I, in support of the stripping
hypothesis. However, the fact that none of the 14 catalogued galaxies
(Biviano et al. 1996) have been classified as spirals weakens this argument
somewhat.
NGC4922
This is the best known case of a merger in Coma, and the only pair detected
in this survey; it is located in the northern most field, 1.4 from the
cluster center. It is very bright in radio (27.5 mJy) and the brightest IR
source in Coma (Mirabel & Sanders 1988). NGC4922 consists of two merging
galaxies, one spiral in the north and one early type galaxy in the south. We
detected H I in emission and absorption. The H I shown in Fig. 15,
NGC4921
This is the brightest spiral in Coma, located 24 (
0.5 Mpc) SE from
NGC4874. It roughly coincides with a second order X-ray peak (Vikhlinin
et al. 1997) also detected with XMM by Arnaud et al. (2001) and Briel et al. (2001). NGC4921 presents a very peculiar picture in 21 cm: a double
peaked H I disk (Fig. 16a) which is considerably smaller than the optical
one,
![]() |
Figure 16a:
H I density distribution of
NGC 4921, superposed on a DSS B-band gray scale image. The contours
are 0.3 (2.5 ![]() ![]() ![]() |
NGC4921 is perhaps the best example in Coma where several mechanisms are
present simultaneously. (a) The shrunken H I disk and the shift between
optical and H I positions look like clear signatures of ram-pressure
stripping, which is also supported by the supersonic velocity of the galaxy
relative to the cluster, 1521 km s-1. (b) NGC4921 shows the largest cross
section in our sample, which could produce, by viscous stripping, a mass loss
rate value up to 20 yr-1. Another feature supporting viscous
stripping is the relatively low surface gas density in the central region of
NGC4921, previously classified as anemic by van den Bergh (1976). (c)
The NW zone, where the brightest HII regions are seen (Amram et al. 1992),
appears depleted of H I, similar to the case of NGC4848. Furthermore,
NGC4921 displays a high total gas content,
+
=
,
with only a small
fraction (0.36) in atomic form, suggesting that H I is actively converted to
molecular gas. Gas re-accretion in the NW may also be present, triggering the
HII regions along the spiral arm. As this galaxy does not show any optical
distortion, processes involving gravitational effects are unlikely to be
important.
IC842 and IC4088
![]() |
Figure 17a:
H I density distribution of
IC 842 superposed on a DSS B-band gray scale image. The contours are
0.3 (2.5 ![]() ![]() ![]() |
These galaxies lie in the far northern region of Coma, along the supercluster
NE filament, and far from the X-ray emission. As expected, they display
normal H I content and no effects of interaction with the ICM are seen
(Figs. 17a and 18a).
They also display normal rotation patterns (Figs. 17b
and 18b). Three dwarf systems were detected in H I around IC4088, one of
them ([GMP 83] 1866) lies at only 2 (
40 kpc) north of the spiral,
but no optical distortions are evident. The dwarfs show velocity dispersions
between 43 km s-1 and 173 km s-1, and their H I masses range between 0.2 and
.
All the galaxies detected in this region,
including the merger NGC4922, are likely part of a group falling towards to
the cluster center (Paper I).
CGCG160-106
This blue disk galaxy is located in the SE outskirts of the X-ray emission.
The ICM does not exert important effects on this galaxy because of the low
ICM density at the galaxy position, the low value of
,
and the small
cross section. CGCG160-106 is not very H I poor (it is deficient by a factor of
1.5), and the H I morphology, barely resolved in Fig. 19,
![]() |
Figure 19:
H I density distribution of
NGC 4926-A, superposed on a DSS B-band gray scale image. The
contours are 0.3 (2.5 ![]() ![]() ![]() |
We detect a clear shift in the H I position, lying 15
(some 5 kpc) SW
from the optical disk, and an intriguing difference in velocity between the
H I and optical: 6876 km s-1 and 7188 km s-1, respectively. Amram et al. (1992)
observed this galaxy in H
and reported a velocity of 7100 km s-1 and an
extension to the SW, coincident with the position of a dwarf companion
separated by
20
.
We detect weak H I emission in the same zone, but
higher resolution is needed to resolve the dwarf system.
As mentioned earlier, physical mechanisms affecting galaxies in clusters are produced by interactions with one or more of the next three elements: the hot ICM, neighbor galaxies, and the cluster gravitational field. Two effects observed in H I in this work are explained on the basis of ICM-ISM interactions: the position of H I deficient galaxies relative to the ICM as drawn by the X-ray emission, and the fact that the most central galaxies detected in H I appear stripped in their outer regions, as predicted for ram-pressure stripping. If tidal interactions were predominant there should be evidence of optical signatures of the interaction, but none of the H I deficient galaxies in this work (except the merger NGC4922), neither detected nor undetected, display peculiarities in their optical morphology. It was also shown in Paper I that tidal interactions are unlikely to be the explanation for the disturbed H I disks in Coma or the triggering of starburst events, because there is no correlation between these effects and the number of close neighbors.
In order to confirm the role of ram pressure stripping in producing the observed H I distributions and in enhancing the star formation in Coma, it is fundamental to compare the H I imaging with 3D simulations of the ISM-ICM interaction. A thorough comparison will help to estimate the role played by different infalling orbits, inclination angles, and ICM densities, as well as to determine the stripping time scale and the regions of the disk where ram-pressure is more effective. To this aim we compare our H I observations with predictions made by 3D-simulations carried out by Abadi et al. (1999) and Vollmer et al. (2001b).
Simulations by Abadi et al. (1999) are a good point of departure even if the
clumpiness of the ISM and different elapsed times after crossing the cluster
core are not taken into account. They computed the expected radius of a ram
pressure stripped gas disk (
)
for a Coma cluster-like density and
dispersion velocity. For instance, a typical galaxy in Coma with
1000 kms-1 would have
6kpc. In Table 1 we compare
the observed H I radius (or upper limits if resolution is marginal) with the
prediction of
made by Abadi et al. (1999). Columns 1 and 2
give the galaxy identification, Col. 3 gives the radial velocity relative
to the cluster. In Col. 4 we give the observed H I radius in kpc
estimated at a level of
,
taking the average
between major and minor H I axis (none of the galaxies in Table 1 are very
elongated). Column 5 gives the observed H I radii corrected for the beam;
for galaxies marginally resolved (indicated with *) we give 0.5 times the
beam size as an upper limit of the beam corrections, following Wild
(1970). Column 6 gives the predicted value of
from Abadi
et al. (1999), considering the corresponding velocity of the galaxy relative to
the cluster.
CGCG | Other |
![]() |
Obs | Corr | Pred |
name | kms-1 |
![]() |
![]() |
![]() |
|
(1) | (2) | (3) | (4) | (5) | (6) |
160-055 | NGC4848 | 41 | 6.8 | 5.0* | 17.0 |
160-073 | Mrk058 | 1575 | 7.9 | 5.8 | 4.5 |
160-086 | 481 | 6.0 | 5.6* | 8.5 | |
160-252 | IC4040 | 758 | 10.2 | 8.0 | 7.5 |
160-257 | NGC4907 | 1180 | 6.6 | 5.8* | 5.5 |
160-260 | NGC4911 | 997 | 12.7 | 12.2 | 6.5 |
160-095 | NGC4921 | 1521 | 11.2 | 10.2 | 5.0 |
160-106 | NGC4926-A | 124 | 8.2 | 5.7 | 14.0 |
The predicted H I radius is calculated assuming that the galaxy's
distance to the center equals its projected distance and its velocity
through the cluster equals its radial velocity. Considering those
assumptions there is good agreement between observed and predicted
values of the HI radius. The two galaxies (NGC4848 and 4926-A) that
have significantly smaller radii than predicted must have a non
negligible velocity in the plane of the sky, while the giants NGC
4911 and 4921 are probably at larger distance from the center and only
in projection very close. For the three unresolved galaxies the
H I distribution is limited to a central region of 6 kpc. Our
results for H I deficient yet detected galaxies in Coma, confirm that
most of the restoring force is coming from the central parts of the
disk where the presence of the bulge is more important, as found by
Abadi et al.'s simulation. They found that the gas is not completely
removed by ram pressure, suggesting that other processes may be at
work affecting those spirals which are not detected in H I in this
survey.
Three dimensional simulations taking into account the clumpiness of the ISM
were carried out for the Virgo cluster by Vollmer et al. (2001b). These
authors found that stripping is very effective for galaxies that are on
radial orbits through the cluster, in agreement with observational evidence
provided by Dressler (1984) and Solanes et al. (2000). Vollmer
et al. (2001b) found that time scales for ram pressure effects in Virgo may be
as short as
yr, and in Coma they would likely be shorter
because of its larger core size and more hostile ICM conditions.
Interestingly, those authors found that galaxies showing important gas
disruptions are not infalling but have already gone through the cluster
core. If this is valid in Coma, it would confirm that all the central spirals
(see Fig. 2 of Paper I) have already gone through the cluster core, while
the
H I regular galaxies in the outskirts of the X-ray emission are in the
process of infalling (see Sect. 3). Some of the consequences in Coma are
discussed in the next section.
ID | Other name | Morph. |
![]() |
![]() |
rms noise | L1.4 | SFR | SFR/![]() |
type | 108 ![]() |
mJy | mJy Beam-1 | 1020 WHz-1 | ![]() |
![]() ![]() |
||
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9)a |
D 77 |
Leda 83676 | S0/a (SB) | <0.6 | - | 0.18 | <3.17 | <0.19 | <0.23 |
D 94 | Leda 83682 | SA0 (PSB) | <0.5 | - | 0.18 | <3.17 | <0.19 | <0.17 |
D 112 | Leda 83684 | SB0 (PSB) | <0.4 | - | 0.18 | <3.17 | <0.19 | <0.03 |
D 21 | MCG 5-31-037 | SBa (PSB) | <0.7 | - | 0.19 | <3.35 | <0.20 | <0.09 |
D 73 | RB 183 | SA0 (PSB) | <0.9 | - | 0.17 | <3.00 | <0.18 | <0.27 |
D 44 | KUG1256+278A | S0 (SB) | <0.6 | - | 0.18 | <3.17 | <0.19 | <0.18 |
D 43 | NGC 4853 | SA0 (SB) | <0.5 | 1.2 | 0.18 | 7.06 | 0.42 | 0.06 |
D 89 | IC 3949 | SA0 (PSB) | <0.3 | 2.1 | 0.18 | 12.35 | 0.73 | 0.18 |
D 127 | RB 042 | S0 (PSB) | <0.6 | 1.0 | 0.18 | 5.88 | 0.35 | 0.46 |
D 216 | RB 160 | Sa (PSB) | <0.7 | - | 0.18 | <3.17 | <0.19 | <0.28 |
D 99 | Mrk 060 | SB0 (PSB) | <0.7 | 1.2 | 0.19 | 7.06 | 0.42 | 0.23 |
D 146 | RB 110 | S0 (PSB) | <0.3 | - | 0.18 | <3.17 | <0.19 | <0.16 |
D 61 | CGCG 160-104 | SA0 (PSB) | <0.7 | - | 0.10 | <1.76 | <0.10 | <0.03 |
D 189 | Leda 83763 | S0 (PSB) | <1.5 | - | 0.17 | <3.00 | <0.18 | <0.31 |
160-055 | NGC 4848 | Scd | 4.3 | 16.6 | 0.18 | 97.37 | 5.75 | 0.38 |
160-073 | Mrk 058 | Sb | 2.0 | 5.5 | 0.17 | 32.10 | 1.89 | 0.56 |
160-086 | Sb | 1.7 | 3.8 | 0.15 | 22.11 | 1.30 | 0.88 | |
160-252 | IC 4040 | Sdm | 3.3 | 15.0 | 0.18 | 88.20 | 5.20 | 1.00 |
160-098 | Sbc | 7.3 | 5.7 | 0.18 | 33.34 | 1.97 | 0.62 | |
160-106 | NGC 4926-A | Sa | 6.0 | 3.1 | 0.15 | 18.17 | 1.07 | 0.49 |
a In units of 10-9.
The physical mechanisms which trigger and quench the star formation activity, and the question if the star burst exhausts the H I reservoir is still a matter of debate. We address this problem pointing at 14 galaxies in Coma for which abnormal spectra are reported by Caldwell et al. (1993, 1997), in order to get H I and radio continuum information. The radio continuum at 1.4 GHz could be due to synchrotron emission from relativistic electrons accelerated by supernovae hence probing star formation, or it could come from AGN. At intermediate redshift Smail et al. (1999) detect in the cluster CL0939+4713 (z=0.41) several post starburst galaxies in radio continuum and these authors argue that the PSBs are most likely dust enshrouded starbursts. In the local universe Chang et al. (2001) observed in H I five of the spectroscopically selected sample of E+A galaxies (Zabludoff et al. 1996). They detected one in H I and none in radio continuum. The implied low star formation rates and limits on the SFR for these galaxies rule out that any of these is in fact a dust enshrouded starburst.
Eleven of the 14 abnormal spectrum galaxies we pointed at in Coma are defined
as PSBs because of their spectra with strong absorption lines and no emission
at all; the three remaining are star forming systems (SBs), as indicated by
their emission line spectrum superposed to absorption lines (C93). Their
position relative to the X-ray emission is shown in Paper I (Fig. 6). We
detected none of these objects in H I, and report very low H I mass upper
limits between 3 and
(see Table 2). We do detect
weak radio continuum emission in three PSB (IC3949, Mrk060 and RB042)
and one SB galaxy (NGC4853, see Table 2). Our radio continuum levels in
Coma imply even lower limits to the star formation rates than found by Miller
& Owen (2001) and Chang et al. (2001) for the field E+A's. Thus in Coma,
contrary to in CL 0939+4713, the post starburst galaxies have at best only
very modest levels of star formation. Chang et al. (2001) detected one of
five E+A's in H I, while our H I upper limits for the cluster PSBs are almost
two orders of magnitude below this detection. This may indicate a real
difference between cluster and field post starburst galaxies. Whatever
triggers the starburst, whether collisions or ICM interaction, in a cluster
the gas is cleaned out after the starburst, something that would not happen
in the field. We show that the post starburst galaxies in Coma have very
little, if any, H I.
Table 2 gives in the upper part the H I mass upper limits and radio continuum
parameters for the sample of abnormal spectrum galaxies that we observed. For
comparison, we show in the bottom the same results for the blue
disks. RB042 is projected very close to the cD galaxy NGC4874 and was only
marginally detected in radio. Three of four galaxies are new detections, the
fourth, NGC4853, is catalogued in the FIRST Survey (Becker
et al. 1995). Table 2 gives in Cols. 1 and 2 the galaxy identification, Col. 3 gives the morphological and spectral classification, following C93. In
Col. 4 we list the H I upper limits, in Col. 5 the radio continuum
flux, Col. 6 lists the rms noise in the continuum images. In Col. 7 we
give the radio power, with upper limits set at 3 sigma. The associated SFR,
listed in Col. 8, is calculated following Yun et al. (2001):
[
WHz-1]
yr-1. In Col. 9 we give the relation between
the SFR and the blue luminosity (
)
obtained from the blue
total magnitude, using the Mean Data from the LEDA database for homogeneity.
Table 2 shows that the SFR in the PSBs is well below 1 yr-1. It comes
as somewhat of a surprise that also the SB galaxies have SFR less than
1
yr-1. This contrasts sharply with the blue disk galaxies reported by
Bothun & Dressler (1986), which were all detected in continuum (see Table 2) and for which the associated SFR are from 1.0 to 5.5
yr-1. We note
that these rates are in good agreement with the rates reported from the
H
equivalent widths by Bothun & Dressler (1986). Thus the blue
"starforming" disks have a significantly higher SFR than the "starbursting"
galaxies identified by C93. This is also valid if we consider the factor
SFR/
given in Col. 9 of Table 2, which is probably a better criteria
than merely SFR if the sampled galaxies span a large range in size and
luminosity. There is a consistent trend between Cols. 8 and 9, with blue
disks showing systematically higher values of SFR/
than galaxies from
C93. The explanation is simple: C93 only selected early type galaxies for
their sample of abnormal spectrum galaxies. The emission lines typically have
narrow H
lines (
3
), while the blue disks have
3
H
7
.
Thus the SB in the C93 sample
means a higher than usual SFR for an early type galaxy; the absolute
levels are very low. It is interesting that the C93 sample is spatially
distinct from the blue disks. The latter are located at the edge of the X-ray emission, while most of the C93 galaxies are in the cluster core or in
the zone between the core and the SW group. The difference in environment may
well account for the different star formation properties of these galaxies.
HST images (Caldwell et al. 1999) show that some of the PSB galaxies in Coma
have retained their disk, even with a smooth spiral structure. D61 (Zw
160-104) shows two dust lanes forming an edge-on disk, and D216 (RB 160)
shows clear spiral structure. Interestingly our limits to the SFR and
H I content for these two galaxies are very low. Obviously the time scale
for gas removal from these galaxies was shorter than for a morphological
transformation. Caldwell et al. (1999) suggest that star formation activity in
the PSBs of Coma could be triggered by galaxy harassment and by gravitational
perturbations between the main cluster and the SW group. However, Moore
et al. (1996) estimate a time-scale of 3 Gyr to produce considerable
changes in the optical morphology by this process. This is too long for
galaxies like D61 and D216 to explain the whole process of triggering a
starburst, the loss of a big fraction of their gas reservoir, and the
subsequent quenching of the activity. An alternative process to explain this
scenario is the re-accretion of gas mass after the stripping, as the
infalling gas colliding with the clouds remaining in the disk can trigger
star formation within the disk. In a cluster like Virgo this process starts
at
years after the closest passage of the cluster core,
and ends at
years (Vollmer
et al. 2001b). The same process in Coma should develop with at least the same
time-scales (or shorter, if we consider the more hostile ICM conditions),
suggesting that the interaction with the ICM and gas re-accretion may
account for this scenario. This is reinforced by the fact that galaxies could
reach their present condition during a single pass through the cluster core,
on a time scale
1 Gyr, which is much shorter than the time needed by
galaxy harassment to modify the optical morphology. The cases of the PSBs
D61 and D216 support this scenario: the former presents a very young
starburst age of 0.5 Gyr, and the latter, recently reclassified as a spiral
(Caldwell et al. 1999), should have harbored a significant amount of gas in the
recent past. Both need a faster process than galaxy harassment to account for
the sudden removal of the H I reservoir and quenching of star forming
activity.
The gas which has been accelerated to values below the escape velocity will
accrete back to the galaxy at time scales of 2-
years
(Vollmer et al. 2001b). As a first approach in Coma we apply a conservative
value of
years for the re-accretion time scale, i.e. the same
value found by Vollmer et al. in Virgo. If we consider a typical galaxy moving
across the Coma cluster at
1000 kms-1 (equivalent to the velocity
dispersion) it will travel some 0.5 Mpc before the gas accretes back on to
the disk and triggers a burst of star formation. It is very interesting to
associate this with the fact that a considerable fraction (38%) of the star
forming UV flux in Coma is produced in an annular region between 20
and 30
,
or
0.5 Mpc from the cluster center (Donas et al. 1995), and that
it is in this zone where most of the blue galaxies in Coma are located
(Paper I). Taking into account their position relative to the cluster,
NGC4848 and most of the blue disk galaxies in Coma have had enough time
after passing across the cluster core to re-accrete gas clouds and boost a
major star formation event; this mechanism may in part account for the blue
annulus observed in Coma which is also observed in high redshift clusters
(Oemler 1992).
In this paper we present the H I morphology and kinematics of the brightest spirals in Coma. We compare the H I morphology with numerical simulations on ram pressure stripping by the ICM. We derive star formation rates for a sample of post starburst and actively star forming galaxies, from deep continuum imaging obtained as a byproduct of the H I observations. We conclude that the H I morphology of the spirals in Coma, the location of the H I deficient galaxies and the size of the H I disks are consistent with predictions of the effect of ram pressure stripping by the ICM.
Targeted observations of 11 of the 14 known PSB galaxies in Coma give
H I upper limits between 3 and
in H I. The star
formation rates derived from (upper limits to) the radio continuum are less
than 1
yr-1. Even the early type galaxies with abnormal emission lines
(SB galaxies from Caldwell et al.) have SFR well below 1
yr-1. Thus in
Coma there is no evidence for the presence of the dust enshrouded starburst
galaxies, which may have been found in clusters at intermediate redshift. We
found additional observational evidence confirming a real difference between cluster and
field post starbursts; galaxies in clusters would exhaust the gas after the
starburst, something that is not always observed in the field.
Acknowledgements
We are very grateful to E. Brinks for helping to improve this paper significantly. HBA thanks the DAEC of the Observatoire de Paris, the Astronomy Department of Columbia University, and the AOC of the NRAO, for support and hospitality during his visits. We used the Digital Sky Survey, produced at the Space Telescope Science Institute. We have made use of the Lyon-Meudon Extragalactic Database (LEDA) supplied by the LEDA team at the CRAL-Observatoire de Lyon (France). We used NED, the NASA/IPAC extragalactic database, operated for NASA by the Jet Propulsion Laboratory at Caltech. This work has in part been supported by NSF grant AST-97-17177 to Columbia University. We appreciate the suggestions done by an anonymous referee and the efficiency with which this paper passed through the whole evaluation procedure.
Individual Channel Maps (Presented in increasing Right Ascension Order) This appendix is only available in electronic form at http://www.edpsciences.org