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 H I 21 cm spectroscopy of six damped and sub-damped Lyman-α absorbers (DLAs and sub-DLAs, respectively) at z ≲ 0.1, which have yielded estimates of their H I 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 H I masses ≈(0.24–5.2) × 10⁹ M_⊙, lower than the knee of the local H I 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, ≈10⁷–10^8.4 M_⊙, in all cases, consistent with the hosts being dwarf galaxies. We obtain high H I 21 cm or CO emission line widths, ΔV20 ≈ 100–290 km s^-1, and high gas fractions, fH I ≈ 5–100, suggesting that the absorber hosts are gas-rich galaxies with low star formation efficiencies. However, the H I 21 cm velocity spreads (≳100 km s−1) appear systematically larger than the velocity spreads in typical dwarf galaxies.