The gas and stellar mass of low-redshift damped Lyman-α absorbers

Abstract

We report Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet and Arecibo Telescope HI 21 cm spectroscopy of six damped and sub-damped Lyman-a absorbers (DLAs and sub-DLAs, respectively) at z ≲ 0.1, which have yielded estimates of their HI column density, metallicity and atomic gas mass. This significantly increases the number of DLAs with gas mass estimates, allowing the first comparison between the gas masses of DLAs and local galaxies. Including three absorbers from the literature, we obtain HI masses ≈(0.24-5.2) × 109Mo˙, lower than the knee of the local HI mass function. This implies that massive galaxies do not dominate the absorption cross-section for low-z DLAs. We use Sloan Digital Sky Survey photometry and spectroscopy to identify the likely hosts of four absorbers, obtaining low stellar masses, ≈107-108.4Modot;, in all cases, consistent with the hosts being dwarf galaxies. We obtain high HI 21 cm or CO emission line widths, ΔV20 ≈ 100-290 km s-1, and high gas fractions, fHI ≈ 5-100, suggesting that the absorber hosts are gas-rich galaxies with low star formation efficiencies. However, the HI 21 cm velocity spreads (≳100 km s-1) appear systematically larger than the velocity spreads in typical dwarf galaxies.