Intergalactic gas

How much cold or warm debris is out there? Searches for intergalactic HI to date have been discouraging. Systems like the Magellanic Stream have only been seen in one other galaxy system (Schneider et al. 1983; 1989), although tidal tails have been traced out to great distances in a number of objects (Hibbard & Mihos 1995). The only bona fide extragalactic HI cloud is the HI 1225+01 SW complex (Giovanelli & Haynes 1989); the NE cloud is now known to have a weak optical counterpart. Other claims have not been supported by subsequent observations (e.g. Taylor et al. 1995). The situation will no doubt improve with the Parkes Multi-Beam and Arecibo Drift Scan surveys over the next few years in that they are expected to find cloud masses as small as 10 M out to .

There have been various attempts to detect HI structures at large galactocentric radius in optical emission lines. Reynolds et al. (1986) achieved a marginal detection of the Leo Ring in but this has not been confirmed by subsequent studies. The Magellanic Stream was marginally detected (2.7) by the TAURUS-2 group (Bland-Hawthorn, Freeman & Quinn 1994, unpublished) in their observations of the Sculptor group. A problem which remains unresolved is whether some fraction of this emission arises from clouds within the background cluster, a controversy which began with the original HI survey (Haynes & Roberts 1979; Arp 1985). A more concerted campaign has detected ionized gas in clouds MS II-IV along the stream (Weiner & Williams 1996); this result is discussed in the Bland-Hawthorn & Maloney (1997). No detections in optical emission lines have been achieved in the direction of the extragalactic HI clouds, either towards the Haynes-Giovanelli Cloud (q.v. Vogel et al. 1995) or the putative NGC 3256 cloud (McCain et al. 1995). These observations are discussed in .

There have been claims for ionized gas from HI bridges between galaxies, e.g. Donahue, Aldering & Stocke (1995), using on-band/off-band subtraction methods. It is questionable that such methods can be used to make reliable detections at such faint flux levels (emission measures ) as they are extremely susceptible to scattered light. Furthermore, it is important to disperse the light spectrally to avoid any additional contribution from the background (e.g. galaxy, night sky). In particular, the Reynolds Layer [NII] emission, which is transmitted by their filter, is much brighter than the anticipated extragalactic signal. Since the Reynolds Layer is variable and detectable over the entire sky, it could easily provide an excess signal when on and off-band fields are differenced. This could explain why the claimed distribution closely resembles the galaxy continuum distribution.

Is there evidence for H clouds between galaxies? The only such system claimed to date is the `Ant' cloud in Fornax (Bland-Hawthorn et al. 1995). This was originally found from its depolarization signature towards the huge radio lobes in NGC 1316. We determined that the 10 kpc cloud has a mass of roughly 10 M and that it must be almost fully ionized. Follow-up HI surveys with the VLA have revealed nothing (van Gorkom 1995, private communication). There are many similar depolarization `drop outs' across the lobes which might conceivably correspond to a large population of ionized gas clouds. Follow-up observations have been planned to test this hypothesis.

An exciting claim was made by Williams & Schommer (1993) that they had detected ionized gas from a nearby absorption-line system observed towards 3C273, even while HI had not been found in this direction. Indeed, we were encouraged by this result in our efforts to find intergalactic H gas. Unfortunately, subsequent work has been unable to confirm this claim (Bland-Hawthorn et al. 1994; Vogel et al. 1995).

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