|Annu. Rev. Astron. Astrophys. 1998. 36:
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Is the Ly forest absorption caused by galaxies ? To match the large rate of incidence of typical Ly absorbers, "normal" (i.e., known types of) galaxies must possess very large absorption cross-sections (Burbidge et al 1977). Alternatively, since the rate of incidence is constraining only the product of number density and geometric cross-section of the absorbers, there could be a population of unknown, more numerous objects, subtending a smaller cross-section. Two key observations have fuelled the interest in the nature of the absorber-galaxy connection. One was the detection of apparently ordinary galaxies at the same (intermediate) redshifts as high column density, MgII metal absorbers (Bergeron 1986). Subsequent work (Bergeron & Boissé 1991; Steidel 1995) established an incontrovertible connection between gaseous galactic halos and high column density, metal absorption systems (which we will not discuss here, as it is most relevant for optically thick Lyman limit systems). The other was the widely unanticipated detection of a remnant population of Ly absorbers in the local universe with HST (Morris 1991; Bahcall et al 1991, 1993), where the properties of an galaxy can be studied in much greater detail than at high redshift.
6.1. The Low Redshift Lyman Alpha Forest
Soon after the discovery of the low z absorbers, galaxy surveys in the fields of the QSOs observed by HST, especially near 3C273, were undertaken to investigate a possible link between absorbers and galaxies. Salzer (1992), Morris et al (1993) and Salpeter & Hoffman (1995) found redshift coincidences with galaxies for some of the lines, hundreds of kpc up to Mpcs away from the absorbers, but there was no unambiguous correspondence between absorbers and individual galaxies. Searches for any low surface brightness objects closer to the LOS to 3C273 which could have escaped detection, have been performed in HI radio emission (van Gorkom et al 1993), H emission (Morris et al 1993, Vogel et al 1995), and in deep broad band optical images (Rauch et al (1996), but were all equally unsuccessful. Morris et al (1993) concluded that there is some correlation between absorbers and galaxies, but the galaxy-absorber cross correlation function is weaker than the galaxy-galaxy correlation. By trying to model the correlation results as a mixture of randomly distributed objects and galactic halos, Mo & Morris (1994) deduced that galaxy halos constitute only about 25% of the local absorber population. At first glance this is at variance with the result of Lanzetta et al (1995), who, based on their large galaxy redshift survey, concluded that most absorption systems are associated with galactic envelopes of typical radius 160 h-1 kpc (see also Chen et al 1998). Moreover, a significant anticorrelation between the equivalent width and the impact parameter of the LOS from the center of the galaxies was measured. Le Brun et al (1996)'s survey showed a weaker anticorrelation, and Bowen et al (1996) saw none at all, although they both confirm Lanzetta et al's results in that there is a region with a high covering factor for absorption around each galaxy, within impact parameters < 200 - 300h-1 kpc, dropping rapidly at larger separations. It appears that the conclusions of Morris et al (1991) and Lanzetta et al (1995) can be reconciled if the Lanzetta et al's main result, i.e., the existing of well defined galactic envelopes, is valid only for the relatively strong absorption lines used in their comparison. The weaker lines, from which Morris et al (1991) derived a weak galaxy-absorber correlation, could still be caused by a truly intergalactic medium. Absorption systems do exist in voids known from galaxy surveys (Stocke et al 1995; Shull et al 1996), but there appears to be a general trend for the absorbers to trace the same large scale structure as galaxies (Stocke et al. 1995; Hoffman et al 1995). In any case, the very large structures causing the stronger common absorption systems in QSO pairs (Bechtold 1994; Dinshaw et al 1994, 1995) cannot be explained by single objects like giant disks or halos: in those cases where several galaxies have been found at the same redshift as the absorber the typical transverse separation between the galaxies is smaller than the transverse absorber size (Rauch et al 1996). This is reminiscent of Oort's (1981) "uncondensed gas in superclusters". There is amazing redshift agreement (velocity differences 0 < v < 20 kms-1) between the HI velocity centroids of galaxies and absorption systems with impact parameters as large as 300 h-1 kpc (van Gorkom et al 1996).
MODELLING LOW REDSHIFT LYMAN ALPHA ABSORBERS Attempts at modelling have mainly been concerned with the large cross-section required if the low redshift Ly clouds are parts of the known galaxy population. Maloney (1992) has suggested that the absorption arises in the distant, ionized outer parts of the known population of disk and irregular galaxies. Clouds that are pressure confined by a hotter gas in a extended galactic halo were discussed by Mo (1994). Salpeter (1993) has invoked a new type of extended low redshift disk galaxies which, in a short burst of star formation blow out their denser centers and disappear from view, while their gas cross-section remains (the so-called "Cheshire Cat model"). The large sizes and low column densities postulated require that these objects must have formed late (Salpeter & Hoffman 1995). Again, observationally the large coherence lengths of the absorbers on the sky seem to correspond to groups of galaxies. To get the large covering factor right these objects must be so close together as to run into each other like circular saws at only slightly higher redshift, which puts us basically back at a continuous intergalactic medium. A number of other sources of low z Ly absorption has been considered, among them tidal tails (Morris & Van den Berg 1994), and galactic winds (Wang 1995). Galaxy clusters have been seen to cause at least some absorbers (Lanzetta et al 1996), and the strong clustering found among low z absorption lines (Boksenberg 1995; Bahcall et al 1996; Ulmer 1996) hints at the increasing importance of such structures with decreasing redshift.
Clearly, one has to be open-minded regarding all these possibilities, many of which plausibly may contribute a significant fraction to low z Ly absorption. How much exactly may be hard to quantify (Sarajedini et al 1996). Again, believers in hierarchical structure formation can expect some consolation from simulations. Petitjean et al (1995) predicted a bi-modal distribution of absorbers, with large galaxy halos with typical radii on the order of 0.5 h-1 Mpc on one hand, and more frequent, lower column density intergalactic absorption occuring in filaments up to several Mpc away from the nearest galaxy, on the other. Miraculously, this is consistent with all the observational evidence we have.