A Sub-damped Ly alpha Absorber with Unusual Abundances: Evidence of Gas Recycling in a Low-redshift Galaxy Group

Abstract

Using Hubble Space Telescope/Space Telescope Imaging Spectrograph G140M spectroscopy, we investigate an absorption-line system at z = 0.07489 in the spectrum of the quasi-stellar object PG 1543+489 (z(QSO) = 0.401). The sightline passes within rho = 66 kpc of an edge-on 2L* disk galaxy at a similar redshift, but the galaxy belongs to a group with four other galaxies within rho = 160 kpc. We detect H I [log N(H I/cm(-2)) = 19.12 +/- 0.04] as well as N I, Mg II, Si II, and Si III, from which we measure a gas-phase abundance of [N/H] = -1.0 +/- 0.1. Photoionization models indicate that the nitrogen-to-silicon relative abundance is solar, yet magnesium is underabundant by a factor of approximate to 2. We also report spatially resolved emission-line spectroscopy of the nearby galaxy, and we extract its rotation curve. The galaxy's metallicity is approximate to 8x higher than [N/H] in the absorber, and interestingly, the absorber velocities suggest that the gas at rho = 66 kpc is corotating with the galaxy's stellar disk, possibly with an inflow component. These characteristics could indicate that this sub-damped Ly alpha absorber system arises in a "cold-accretion" flow. However, the absorber abundance patterns are peculiar. We hypothesize that the gas was ejected from its galaxy of origin (or perhaps is a result of tidal debris from interactions between the group galaxies) with a solar nitrogen abundance, but that subsequently mixed with (and was diluted by) gas in the circumgalactic medium or group. If the gas is bound to the nearby galaxy, this system may be an example of the gas "recycling" predicted by theoretical galaxy simulations. Our hypothesis is testable with future observations.