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Recycling intergalactic and interstellar matter
IAU Symposium Series, Vol. 217, 2004
Pierre-Alain Duc, Jonathan Braine and Elias Brinks, eds.
H i Tidal Tails, Bridges and Clouds
Barbel S. Koribalski
Australia Telescope National Facility, CSIRO
Abstract. There is plenty of intergalactic H i gas without any obvious
stellar content ranging from (1) extended gas envelopes around some nor-
mal and peculiar galaxies, (2) tidal tails/bridges in interacting or merging
galaxy systems, (3) large-scale rings around early type galaxies, and (4)
detached clouds at varying distances from associated galaxies, but there
are few or no isolated H i clouds.
The HIPASS Bright Galaxy Catalog, which covers the whole south-
ern sky, contains only one denite extragalactic H i cloud which is located
close to the galaxy NGC 2442 whereas it is sensitive to isolated H i clouds
with M HI > 10 6  D 2 M . The space density of H i clouds is therefore
about 1/1000th that of galaxies with the same M HI .
Introduction
The best resource to marvel at the peculiar H i distribution in interacting galaxy
pairs/groups and some individual galaxies is the "H i Rogues Gallery" compiled
by Hibbard et al. (2001; see www.nrao.edu/astrores/HIrogues). Here we will
concentrate on extragalactic H i gas that has no obvious stellar counterpart.
This gas is found mostly in the outskirts of galaxies, in the form of extended
envelopes, tidal tails, bridges, rings and detached clouds. Prominent nearby
examples include the Magellanic Stream (Putman et al. 2003) and the extended
H i bridges in the M 81 group (Yun et al. 1994, Makarova et al. 2002). The
questions we want to address are: how much intergalactic H i gas is there, where
is it located and what is its fate ? In particular, how much gas is ejected into the
intergalactic medium by tidal forces ? We will look at the amount and location
of the H i gas surrounding peculiar/interacting galaxies.
Hibbard & Mihos (1995) used numerical simulations to study what happens
to tidal debris over a long period of time. They found that a large fraction of
the ejected H i gas will fall back onto the parent galaxy. The closest material
falls back fastest, while more distant debris return ever more slowly to accrete
at larger distances from the galaxy centre. That leaves plenty of time for the
slowly returning material as well as any escaped gas to form bound entities and
potentially stars. Some of these star forming clumps will build new galaxies,
such as tidal dwarf galaxies (Barnes & Hernquist 1992; Duc et al. 2000, Braine
et al. 2001). But, how long can these young, recycled objects survive in an
interacting galaxy environment ?
1

2 B. S. Koribalski
NGC 3136B
IC 2554
Figure 1. H i distribution (contours) towards the spiral galaxy
IC 2554 overlaid onto a DSS II R-band image. The large, one-sided
H i plume possibly results from tidal interaction with the massive, el-
liptical galaxy NGC 3136B. For details see Koribalski et al. (2003).
Some galaxies have very large H i envelopes extending way beyond their stel-
lar distribution (Broeils & van Woerden 1994, Salpeter & Homan 1996; Broeils
& Rhee 1997). The nature of the extended gas distribution can be very dierent
and ranges from smooth disks to chaotic extensions. Because of its large extent,
the outer neutral hydrogen disk is much more aected by tidal interactions than
the stellar disk and therefore an excellent tracer of these. Collisions of galaxies
with extended H i envelopes lead to large H i extensions, tails and bridges which
can be found out to large distances from the galaxy centre. Recycling and re-
accretion of these distant debris are increasingly important for the evolution of
these systems. We will look at galaxies with (1) extended H i envelopes, (2) tidal
H i tails/plumes, (3) large, possibly accreted H i rings, (4) detached H i clouds,
and examine their extent, mass, and structure. In each category we can only
give a few examples. A thorough compilation of such systems, incl. a set of
standard properties for each, would be most useful for many observational and
numerical studies.
Galaxies with extended H i envelopes
While the average H i to optical extent of galaxies is less than two, a few galaxies
have very extended H i distributions. We give a few examples (see Table 1) and
compare the measured H i extent to the optical Holmberg radius, R Ho . Ideally we
would like to quote standard H i radii at certain H i column or surface densities
similar to the standard optical radii at certain magnitudes.

H i Tidal Tails, Bridges and Clouds 3
NGC 3263
NGC 3262
ESO 263-G044 WPV 060
Figure 2. H i distribution (contours) towards the peculiar galaxy
NGC 3263 overlaid onto a DSS II B-band image. The spectacular H i
plume to the west/south-west of NGC 3263/2 extends over  175 kpc
 200 kpc. The dwarf galaxy WPV 060 coincides with a peak in the H i
plume and is marked with an arrow. For details see English, Koribalski
& Freeman (these proceedings).
Note that while the H i masses of NGC 2915, DDO 154 and ESO215-G?009
are similar, their H i mass to light ratios are very dierent with M HI =LB =
2.6, 8, and 21, respectively. The H i distribution of the Magellanic irregular
NGC 4449, also known as the "Jekyll & Hyde" galaxy is enormous and peculiar.
Hunter et al. (1998) nd  10 9 M in the H i streamers alone (> 35 kpc).
Galaxies with H i tails/plumes
The galaxy systems listed in Table 2 have enormous, one-sided tidal tails and
are typically mergers. These are quite distinct from the more diuse, one-sided
H i plumes/clouds observed towards, e.g., the spiral galaxies IC 2554 (see Fig. 1)
and NGC 3263 (see Fig. 2). The prominent plume emerging to the east of
the peculiar galaxy IC 2554 (D = 16 Mpc) is possibly caused by interaction
with the massive elliptical galaxy NGC 3136B (Koribalski et al. 2003). It has

4 B. S. Koribalski
Table 1. Examples of galaxies with an extended H i distribution
Name type D H i extent  R Ho M HI Refs.
[Mpc] [kpc] [10 9 M ]
NGC 2915 BCD 2.7 16 5 0.25 (1)
DDO 154 IBm 3.2 8.4 5 0.25 (2)
ESO215-G?009 dIrr 4.2 15 5 0.5 (3)
NGC 4449 IBm 3.4 66 6 2 (4)
Circinus SAb 4.2 >73 >4 8 (5)
M 83 SABc 4.5 78 4 8 (6)
(1) Meurer et al. 1996, (2) Carignan & Purton 1998, (3) Warren, Jerjen, Koribalski,
these proceedings, (4) Hunter et al. 1998, (5) Jones et al. 1999, (6) Park et al. 2001.
NGC 6221
NGC 6215
Figure 3. H i distribution (contours) towards the interacting spiral
galaxies NGC 6221 and NGC 6215 overlaid onto a DSS II B-band im-
age. For details see Koribalski & Dickey (2004).

H i Tidal Tails, Bridges and Clouds 5
Table 2. Examples of galaxy pairs with long, one-sided H i tails
Name D H i tail M HI Refs.
[Mpc] [kpc] [10 9 M ]
Arp 85 (M 51, NGC 5194/5) 9.6 90 0.5 (1)
Arp 143 (NGC 2444/5) 9.6 90 0.5 (2)
Arp 270 (NGC 3395/6) 21.7 70 0.5 (3)
Arp 215 (NGC 2782) 34 54 1.4 (4)
Arp 299 (NGC 3690 + IC 694) 48 180 3.3 (5)
(1) Rots et al. 1990, (2) Appleton et al. 1987, (3) Clemens et al. 1999, (4) Smith 994,
Jogee et al. 1998, (5) Hibbard & Yun 1999.
an extent of 30 kpc and contains about a third of the total H i mass of the
system (2  10 9 M ). The NGC 3256 galaxy group (D = 37.6 Mpc) features a
spectacular H i cloud of size  175 kpc  100 kpc with an H i mass of 10 9 M
(English, Koribalski & Freeman; these proceedings). It lies just west of the H i
rich galaxy NGC 3263 which in itself is most peculiar (Koribalski et al. 2004,
in prep.). The dwarf galaxy WPV 060 which appears to be associated with an
H i peak towards the south-western end of the cloud, could be a young galaxy
formed out of the far-reaching tidal debris.
H i rings
In Table 3 we list a few examples of E/S0 galaxies surrounded by large H i rings.
Duc & Mirabel (1998) suggest that the H i ring in the NGC 5291 system, which
appears to be a nursery of recycled dwarf galaxies, was accreted from a gas-rich
galaxy in the cluster.
Table 3. Examples of galaxies surrounded by large H i rings
Name type D H i ring M HI Refs.
[Mpc] [kpc] [10 9 M ]
NGC 5291 E 58 170 50 (1)
Leo Ring (M 96 group) E/S0 10 200 1 (2)
NGC 1533 S0 21 70 7 (3)
NGC 2292/3 S0 27 30 2 (4)
(1) Malphrus et al. 1997, (2) Schneider et al. 1989, (3) Ryan-Weber et al., these
proceedings; Ryan-Weber et al. 2004, (4) Barnes 1999.
Detached H i clouds
H i clouds are seen in many tidally interacting galaxy groups such as NGC 697
(van Moorsel 1988), the Grus Quartet (Koribalski et al., in prep.), and the
NGC 6221/15 group (D = 18 Mpc; Koribalski & Dickey 2004). These may
constitute the gaseous material ejected furthest from the host galaxy and now
disconnected from gas that has already returned. The two-stranded H i bridge
between the galaxies NGC 6221 and NGC 6215 (Fig. 3) spans over a projected
distance of nearly 100 kpc. It has an H i mass of at least 1:4  10 8 M . Note

6 B. S. Koribalski
that Fig. 3 displays only H i within the bridge velocity range. The NGC 6221/15
group also contains three H i-rich dwarf galaxies, only one of which (Dwarf 3)
is shown here, marked with an arrow. The inset at the bottom right shows a
close-up of Dwarf 3. Oosterloo et al. (these proceedings) found several extended
H i clouds more than 100 kpc from the elliptical galaxy NGC 1490 (D = 70
Mpc) with a total M HI of almost 10 1 0 M . H i 1225+01, better known as "the
Virgo Cloud" (D = 20 Mpc) has an H i extent over 100 kpc and an H i mass
of 5  10 9 M (Giovanelli & Haynes 1989). Only the NE component of the H i
distribution has an optical counterpart (Chengalur et al. 1995).
The only intergalactic H i gas cloud detected in the HIPASS Bright Galaxy
Catalog (BGC; Koribalski et al. 2004) lies at a projected separation of 250
kpc from the galaxy NGC 2442 (D = 15.5 Mpc) which is part of a loose group
of galaxies. The cloud, HIPASS J0731{69, for which no optical counterpart
has been detected, has an H i mass of 10 9 M (Ryder et al. 2001; Ryder &
Koribalski, these proceedings).
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