Intergalactic Hydrogen Clouds OT Low Redshift: Connections to Voids and Dwarf Galaxies

Abstract

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) survey of intergalactic Lyα clouds located along sightlines to four bright quasars passing through well-mapped galaxy voids (16000 km s^-1^ pathlength) and superclusters (18000 km s^-1^) We report two more definite detections of low-redshift Lyα clouds in voids: one at 3047 km s^-1^ (heliocentric) toward Mrk 421 and a second just beyond the Local Supercluster at 2861 km s^-1^ toward I Zw 1, confirming our earlier discovery of Lyα absorption clouds in voids (Stocke et al., ApJ, 451,24(1995)]. We have now identified ten definite and one probable low- redshift neutral hydrogen absorption clouds toward four targets, a frequency of approximately one absorber every 3400 km s^-1^ above 10^12.7^ cm^-2^ column density. Of these ten absorption systems, three lie within voids; the probable absorber also lies in a void. Thus, the tendency of Lyα absorbers to "avoid the voids" is not as clear as we found previously. If the Lyα clouds are approximated as homogeneous spheres of 100 kpc radius, their masses are ~10^9^ M_sun_ (about 0.01 times that of bright L^*^ galaxies) and they are 40 times more numerous, comparable to the density of dwarf galaxies and of low- mass halos in numerical CDM simulations. The Lyα clouds contribute a fraction {OMEGA}_cl_ ~ 0.003h_75_^-1^ to the closure density of the universe, comparable to that of luminous matter. These clouds probably require a substantial amount of nonbaryonic dark matter for gravitational binding. They may represent extended haloes of low-mass protogalaxies which have not experienced significant star formation or low-mass dwarf galaxies whose star formation ceased long ago, but blew out significant gaseous materiaL.