An HI census of Loose Groups of Galaxies

Abstract

We present results from our Parkes Multibeam HI survey of 3 loose groups of galaxies that are analogous to the Local Group. This is a survey of groups containing only spiral galaxies with mean separations of a few hundred kpc, and total areas of approximately 1 sq. Mpc; groups similar to our own Local Group. We present a census of the HI-rich objects in these groups down to an M(HI), 1-sigma sensitivity ~7x10^5 M(sun), as well as the detailed properties of these detections from follow-up Compact Array observations. We found 7 new HI-rich members in the 3 groups, all of which have stellar counterparts and are, therefore, typical dwarf galaxies. The ratio of low-mass to high-mass gas-rich galaxies in these groups is less than in the Local Group meaning that the ``missing satellite'' problem is not unique. No high-velocity cloud analogs were found in any of the groups. If HVCs in these groups are the same as in the Local Group, this implies that HVCs must be located within ~300-400 kpc of the Milky Way.

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arXiv:astro-ph/0310094v1 3 Oct 2003
**TITLE**
ASP Conference Series, Vol. **VOLUME***, **YEAR OF PUBLICATION**
**NAMES OF EDITORS**
An H I Census of Loose Groups of Galaxies
D.J. Pisano
NSF Distinguished International Postdoctoral Research Fellow, ATNF,
P.O. Box 76, Epping NSW 1710, Australia
David G. Barnes
School of Physics, Univ. of Melbourne, Victoria 3010, Australia
Brad K. Gibson
Swinburne University of Technology, Centre for Astrophysics and
Supercomputing, Mail # 31, , P.O. Box 218, Hawthorn, Victoria 3122,
Australia
Lister Staveley-Smith
ATNF, P.O. Box 76, Epping NSW 1710, Australia
Ken C. Freeman
RSAA, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611,
Australia
Abstract.
3 loose groups of galaxies that are analogous to the Local Group. This is a
survey of groups containing only spiral galaxies with mean separations of a
few hundred kpc, and total areas of approximately 1 Mpc2; groups similar
to our own Local Group. We present a census of the H I-rich objects in
these groups down to a 1σ MHI sensitivity ∼7×105M⊙, as well as the
detailed properties of these detections from follow-up Compact Array
observations. We found 7 new H I-rich members in the 3 groups, all of
which have stellar counterparts and are, therefore, typical dwarf galaxies.
The ratio of low-mass to high-mass gas-rich galaxies in these groups is
less than in the Local Group meaning that the “missing satellite” problem
is not unique. No high-velocity cloud analogs were found in any of the
groups. If HVCs in these groups are the same as in the Local Group, this
implies that HVCs must be located within ∼300-400 kpc of the Milky
Way.
We present results from our Parkes Multibeam H I survey of
1. Introduction
Loose groups of galaxies are collections of a few (2-3) large galaxies and tens of
smaller galaxies. They are the most diffuse components of structure in the uni-
verse, yet they are relatively understudied despite their importance. About 60%
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of galaxies reside in groups (Tully 1987) including the Milky Way which is part
of the Local Group of galaxies. Loose groups are possibly still forming (Zablud-
off & Mulchaey 1998), and may even be the site of ongoing galaxy formation
as traced by the high-velocity clouds (HVCs; e.g. Blitz et al. 1999). Measuring
the distribution of galaxy masses in groups also provides a useful constraint on
models of galaxy formation. To learn more about groups of galaxies similar to
our own, to constrain the nature of HVCs, and to learn more about galaxy and
structure formation, we have commenced a survey of loose groups of galaxies.
2.Observations
Our survey examined analogs to the Local Group: spiral-rich loose groups with-
out any large elliptical or lenticular galaxies. We selected five such groups from
the LGG catalog of Garcia (1993). A sixth group was selected from the HIPASS
group catalog (Stevens, 2003, private communication) and was not previously
identified optically. Assuming H0= 65 km/s/Mpc, the groups lie between 10.6 -
13.4 Mpc. We observed a projected area of 1 - 1.7 Mpc2centered on each group
using the Parkes Multibeam receiver. The observations involved scanning the
instrument in a “basket-weave” pattern in right ascension and declination mul-
tiple times until an RMS sensitivity of ∼7×105M⊙per 3.3 km/s was reached.
The total velocity coverage of the observations was either 1700 or 3400 km/s
(depending on the group) with a velocity resolution of 1.65 km/s or 3.3 km/s.
The linear resolution of the Parkes observations was ∼50 kpc.
The data were reduced and gridded into cubes using the ATNF livedata
and gridzilla packages in aips++. The final cubes were searched by eye by
three people for detections. Fake sources were inserted into these cubes to assess
the reliability and completeness of the search. If a source was found by at least 2
people, it was considered to be a detection. Based on this analysis, our detection
algorithm was essentially 100% complete down to the 10σ level.
All our detections, not just the new ones, were re-observed with the ATCA.
The ATCA observations not only served to confirm the detections, but the higher
spatial resolution, ∼4 kpc, allowed us to uniquely identify H I detections with
optical counterparts. Furthermore, as the ATCA observations had about the
same sensitivity as the Parkes cubes, we were able to search for H I clouds
which were previously unresolved in the Parkes data.
The combination of the large area observed, high velocity resolution, and
extremely sensitive observations of multiple loose groups makes this survey
uniquely tuned to get a census of the H I content of loose groups of galaxies
and to strongly constrain the origin of HVCs. We will address these topics using
the first 3 groups studied in our survey: LGG 93, LGG 180, and LGG 478.
3.The H I Content of Loose Groups of Galaxies
Cold dark matter simulations of the formation of the Local Group of galaxies
(e.g. Klypin et al. 1999, Moore et al. 1999) reveal an extremely large number
of small dark matter halos (∼300) compared to the number of known luminous
satellite galaxies (∼20). This is the so-called “missing satellite” problem. While
invoking a different type of dark matter, such as warm dark matter, or invoking

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Figure 1.
with measured MHI (circles) and upper limits (arrows). The stars
indicate the sources in LGG 93. The line indicates the 10σ detection
limit
MHIvs. Vrotfor Local Group galaxies (from Mateo 1998)
feedback processes to suppress the collapse of baryons in dark halos can solve
this problem, it is worth asking if this problem exists at the same level in groups
similar to the Local Group.
Our survey detected all the optically identified galaxies in the 3 groups. In
addition, we detected 4 new group members in LGG 93, 2 new group members
in LGG 180, and 1 new group member in LGG 478 (along with 7 background
galaxies). Only 4 of these new detections were not evident in the HIPASS data
(Barnes et al. 2001), however. All of these new detections have been confirmed
with ATCA, and have MHI= 107−9M⊙. All of the detections have associated
stellar components; they are all dwarf galaxies.
Our survey found that the “missing satellite” problem is actually worse in
the groups we surveyed than in the Local Group. Figure 1 shows that if we were
observing the Local Group, we would detect a total of 14 galaxies, 9 of which
have FWHM ≤100 km s−1and could be considered dwarf galaxies. Data for
the Local Group galaxies comes from Mateo (1998). The dwarf-to-giant ratio is
1.45 for the three groups we studied, compared with 1.8 for the Local Group.
This discrepancy will be verified when we finish analyzing the remaining three
groups in our sample, but, at the present, it appears that the Local Group is not
unique in its deficit of luminous satellite galaxies compared to cold dark matter
models of galaxy formation.

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4.The Nature of High-Velocity Clouds
HVCs are clouds of H I seen all around the Milky Way, but which are not in
regular galactic rotation (see Wakker, these proceedings for a review). As such,
their distances and masses are unknown, making their origins uncertain. Some
HVCs are certainly associated with the Magellanic Stream which is the result of
the tidal interaction between the LMC, SMC, and Milky Way, and it is possible
that other complexes have similar origins (e.g. Lockman 2003). Other HVCs
may be ejected material from a Galactic fountain, or may be primordial gas
falling into the Local Group and onto the Milky Way as part of ongoing galaxy
formation.
While the idea of HVCs being associated with galaxy formation is an old
one, interest in the idea was revived by Blitz et al. (1999) and Braun & Burton
(1999) who suggested that HVCs and compact HVCs (CHVCs) may contain
dark matter, have masses of ∼107M⊙, and reside at distances of ∼ 1 Mpc from
the Milky Way. These HVCs may even be associated with the large population
of dark matter halos seen in the simulations discussed above. This idea has
been expanded on by de Heij, Braun, & Burton (2002b), who proposed that
CHVCs were concentrated around the Milky Way and M31 with a Gaussian
distance distribution of width 150 - 200 kpc, and MHI≤107M⊙. If CHVCs are
associated with galaxy formation, then some of their analogs should be visible
in the groups we have observed.
Starting with this premise, we have constructed a simple model for HVCs
to predict how many should be seen in the groups we have studied. We begin
by taking the integrated fluxes and velocity widths of all Milky Way CHVCs
cataloged by Putman et al. (2002) and de Heij, Braun, & Burton (2002a). We
then randomly determine a distance assuming a Gaussian distance distribution
of a given half-width, half-maximum. Given this distance, we get an H I mass
for each cloud. Now we ask if this CHVC were in one of our groups, would we
detect it at a 10σ level. We do this for every CHVC to determine the number
of expected detections per group for a given DHWHM, and then we repeat the
process 100,000 times to get a distribution of expected detections. The results
of these trials for two groups, LGG 93 and LGG 478, and two DHWHM values
are shown in Figure 2.
As we already stated in Section 3, we have found no HVC analogs (H I
clouds without stars) in any of our groups. Therefore, in looking at Figure 2,
we want to know for a given DHWHMwhat the probability of zero detections is.
Since this is a function of the mass detection limit of the group, we get different
limits on DHWHM for each group. For LGG 93, we can see in Figure 2 that
DHWHMis less than 400 kpc at a 99.76% confidence level, but DHWHM< 250
kpc is only 73% likely. For LGG 478, the figure demonstrates that DHWHM<
400 kpc is 99.997% likely, and DHWHM< 250 kpc is 93.8% certain. And for LGG
180, not shown in the figure, the limits are 99.98% and 89% certain, respectively.
There a couple of notes of caution regarding this analysis. First, it is im-
portant to note that the vast majority of CHVC analogs are not detected; we
can only detect the most massive analogs. This means that we are using a small
number of objects to infer the properties of a larger population. If the popula-
tion of CHVCs in these groups is different than in the Local Group, we may not
be able to detect any analogs. We also assume that the same number of CHVCs

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Figure 2.
(left) and LGG 478 (right). The solid line represents the probability
distribution for DHWHM = 400 kpc and the dashed line represents
DHWHM= 250 kpc.
The probability of a given number of detections for LGG 93
is present in each group. If the number of CHVCs is a function of group mass,
for example, we would expect less CHVCs in LGG 478 than in LGG 93 or LGG
180, so the constraints on DHWHM may not be as strong as suggested above.
Nevertheless by analogy to other groups, it appears that CHVCs should be clus-
tered with DHWHM ≤ 300-400 kpc of the Milky Way. At these distances, the
total H I mass in CHVCs is only ∼108M⊙, making them an important source
of fuel for star formation, but not dynamically important to the Local Group.
5.Conclusions
We have surveyed ∼1 Mpc2around the centers of 3 loose groups of galaxies
using the Parkes Multibeam and the ATCA. The goal of these observations is
to get a census of the H I-rich galaxies in these groups and to search for analogs
to HVCs. This will permit us to constrain models for the origin of HVCs and
to begin to test models of galaxy formation. These groups were chosen to be
analogs to the Local Group, so that they only contain a few large spiral galaxies
which are separated by a few hundred kiloparsecs. Our observations have very
high velocity resolution, <3.3 km/s, in order to facilitate the detection of low-
mass objects, so that our 1σ detection limit is ∼7×105M⊙per 3.3 km/s. Using
fake sources to determine our completeness and ATCA observations to confirm
our detections, we are roughly 100% complete and reliable at 10× the theoretical
noise limit.
We found 7 new H I-rich objects in the groups, plus 7 background galaxies,
only 4 of which were not seen in HIPASS, and all of which have stellar coun-
terparts. The ratio of low-mass to high-mass galaxies in these groups is less
than expected from simulations and also less than the ratio in the Local Group,
therefore the “missing satellite” problem is not unique to our local neighborhood.
We found no analogs to HVCs in any of the three groups. Assuming the
HVCs in these groups are the same as in the Local Group, this implies that
compact HVCs must be clustered within ∼300-400 kpc of the Milky Way, oth-

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erwise we would have seen their analogs in our survey. This is strong evidence
against the Blitz et al. (1999) and Braun & Burton (1999) models for the origin
of HVCs, but is still consistent with the de Heij et al. (2002b) model. At these
distances, however, the total H I mass in CHVCs is only ∼108M⊙, making them
unimportant to the dynamics of the Local Group, but still a useful reservoir of
fuel for future star formation.
We have Parkes data in hand for an additional 3 groups, and will be fol-
lowing up our detections in these groups with the ATCA and the VLA in the
near future. The ensemble of the data on these 6 groups will place stronger
constraints on the nature of HVCs and provide a better test of different models
of galaxy formation.
Acknowledgments.
and the ATCA for their assistance with observing. We wish to thank Martin
Zwaan for his assistance with inserting fake sources into our data cubes. D.J.P.
acknowledges generous support from NSF MPS Distinguished Research Fellow-
ship grant AST0104439.
The authors wish to thank the excellent staff at Parkes
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