Comparing Galaxies and Lyα Absorbers at Low Redshift

A scenario is explored in which Lyman alpha absorbers at low redshift arisefrom lines of sight through extended galaxy disks, including those of dwarf andlow surface brightness galaxies. A population of galaxies is simulated basedupon observed distributions of galaxy properties, and the gas disks are modeledusing pressure and gravity confinement. Some parameter values are ruled out bycomparing simulation results with the observed galaxy luminosity function, andconstraints may be made on the absorbing cross sections of galaxies. Simulationresults indicate that it is difficult to match absorbers with particulargalaxies observationally since absorption typically occurs at high impactparameters (>200 kpc) from luminous galaxies. Low impact parameter absorptionis dominated by low luminosity dwarfs. A large fraction of absorption lines isfound to originate from low surface brightness galaxies, so that the absorbinggalaxy is likely to be misidentified. Low redshift Lyman alpha absorber countscan easily be explained by moderately extended galaxy disks when low surfacebrightness galaxies are included, and it is easily possible to find a scenariowhich is consistent with observed the galaxy luminosity function, with lowredshift Lyman limit absorber counts, and with standard nucleosynthesispredictions of the baryon density, Omega_Baryon.Comment: 17 pages, 8 figures, accepted to the Astrophysical Journa