DISK GALAXIES NEAR THE LINE OF SIGHT TO 3C 273 AND CONJECTURES ON LYMAN-ALPHA ABSORPTION

摘要

Using data from the preceding paper (Hoffman, Lewis, & Salpeter 1995) and recent literature, we augment compilations by Salzer (1992) and by Morris et al. (1993) to select (by projected distance) 22 disk galaxies near the line of sight to the quasar 3C 273. These galaxies are correlated with the seven Lyα absorption lines (from HST observations) below 10,000 km s~(-1) on velocity scales typical of galaxy groups (a few hundred km s~(-1)). For three of the seven lines an individual galaxy is close enough in position and velocity that it may provide the absorption. This includes the galaxy MGC 0-32-16, which has a projected separation of 204 kpc and a velocity difference of 93 km s~(-1) from the lowest velocity absorption line. For a fourth of the seven lines, there is a small group of galaxies fairly close by. For the remaining three of the seven absorption lines, however, no ordinary galaxies are nearby. Thus, while some of the absorption lines are likely to arise in extensions of ordinary galaxy disks, the majority must be caused by otherwise invisible objects. Whatever the physical nature of these objects, many of them are likely to reside in "clouds or filaments" in voidlike regions which also contain some ordinary galaxies or galaxy groups. As an alternative to "minihalos" for the invisible objects, we postulate an "almost invisible" class of galaxies ("Vanishing Cheshire Cats") which are born with central column densities already about 10 times smaller than for ordinary disk galaxies and which then suffer further mass loss from the inner disk. Recent observations that show a larger fraction of Lyman-limit systems with H I column density below 10~(17.7) cm~(-2) for z < 1 than for z > 2 provide further motivation for this model.