INTERGALACTIC HYDROGEN CLOUDS AT LOW-REDSHIFT - CONNECTIONS TO VOIDS AND DWARF GALAXIES

We provide new post-COSTAR data on one sightline (Mrk 421) and updated data from another (I Zw 1) from our Hubble Space Telescope (HST) surv ey of intergalactic Ly alpha clouds located along sightlines to four b right quasars passing through well-mapped galaxy voids (16 000 km s(-1 ) pathlength) and superclusters (18 000 km s(-1)). We report two more definite detections of low-redshift Ly alpha clouds in voids: one at 3 047 km s(-1) (heliocentric) toward Mrk 421 and a second just beyond th e Local Supercluster at 2861 km s(-1) toward I Zw 1, confirming our ea rlier discovery of Ly alpha absorption clouds in voids [Stocke et at, ApJ, 451, 24 (1995)]. We have now identified ten definite and one prob able low-redshift neutral hydrogen absorption clouds toward four targe ts, a frequency of approximately one absorber every 3400 km s(-1) abov e 10(12.7) cm(-2) column density. Of these ten absorption systems, thr ee lie within voids; the probable absorber also lies in a void. Thus, the tendency of Ly alpha absorbers to ''avoid the voids'' is not as cl ear as we found previously. If the Ly alpha clouds are approximated as homogeneous spheres of 100 kpc radius, their masses are similar to 10 (9) M. (about 0.01 times that of bright L galaxies) and they are 40 t imes more numerous, comparable to the density of dwarf galaxies and of low-mass halos in numerical CDM simulations. The Ly alpha clouds cont ribute a fraction Omega(cl) approximate to 0.003h(75)(-1) to the closu re density of the universe, comparable to that of luminous matter. The se clouds probably require a substantial amount of nonbaryonic dark ma tter for gravitational binding. They may represent extended haloes of low-mass protogalaxies which have not experienced significant star for mation or low-mass dwarf galaxies whose star formation ceased long ago , but blew out significant gaseous material. (C) 1996 American Astrono mical Society.