The relationship between baryons and dark matter in extended galaxy halos

The relationship between gas-rich galaxies and Ly alpha absorbers is addressed in this paper in the context of the baryonic content of galaxy halos. Deep Arecibo H I observations are presented of two gas-rich spiral galaxies within 125 h(70)(-1) kpc projected distance of a Ly alpha absorber at a similar velocity. The galaxies investigated are close to edge-on, and the absorbers lie almost along their major axes, allowing for a comparison of the Ly alpha absorber velocities with galactic rotation. This comparison is used to examine whether the absorbers are diffuse gas rotating with the galaxies' halos, outflow material from the galaxies, or intergalactic gas in the low-redshift cosmic web. The results indicate that if the gas resides in the galaxies' halos, it is not rotating with the system and is possibly counterrotating. In addition, simple geometry indicates that the gas was not ejected from the galaxies, and there are no gas-rich satellites detected down to 3.6-7.5 x 10(6) M-circle dot or remnants of satellites to 5.6 x 10(18) cm(-2). The gas could potentially be infalling from large radii, but the velocities and distances are rather high compared to the high-velocity clouds around the Milky Way. The most likely explanation is that the galaxies and absorbers are not directly associated, despite the vicinity of the spiral galaxies to the absorbers (58-77 h(70)(-1) kpc from the H I edge). The spiral galaxies reside in a filament of intergalactic gas, and the gas detected by the absorber has not yet come into equilibrium with the galaxy.