A Sub-Damped Ly$\alpha$ Absorber with Unusual Abundances: Evidence of
  Gas Recycling in a Low-Redshift Galaxy Group
  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$\pm$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] = $-1.0\pm 0.1$. Photoionization models indicate that the
nitrogen-to-silicon relative abundance is solar, yet magnesium is underabundant
by a factor of $\approx$ 2. We also report spatially resolved emission-line
spectroscopy of the nearby galaxy, and we extract its rotation curve. The
galaxy's metallicity is $\approx 8 \times$ 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
(CGM) 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.
