A Partial Lyman Limit Absorber in the Halo of a Galaxy Pair: A Possible Signature of Gas Inflow

Abstract

We present an analysis of a partial Lyman limit system at z = 0.87641 detected in the HST/COS spectrum of the background quasar LBQS 0107-0235. The absorber exhibits a simple kinematic structure, with the metal-lines and the H I Lyman-series absorption well described by a single component. Photoionization modeling yields a gas metallicity of one-tenth solar and a hydrogen number density of n_H \approx 8.5 \times 10^{-4} cm^{-3} (log_{10}(n_H/cm^{-3}) \approx -3.1). At the absorber redshift, the VLT/MUSE data show two galaxies (G1 and G2) at normalized impact parameters of ρ/R_{vir} \approx 0.9 and velocity separations of |Δv| = 18 and 99 km s^{-1}, respectively, from the absorber. Both galaxies have rotating disks with stellar masses of M_* \approx 6 \times 10^9 and \approx 2.2 \times 10^{10} M_{\odot}. Their 100-Myr-averaged star formation rates are \approx 2.5 and \approx 2.2 M_{\odot} yr^{-1}, though their instantaneous rates place G2 on the star-forming main sequence and G1 above it, which is actively star-forming at this redshift. The absorber is positioned very close to the projected major axis of both galaxies. The absorber's orientation, kinematics, and sub-solar metallicity (log_{10}(Z/Z_{\odot}) = -1.05) are consistent with the absorption tracing a sub-solar metallicity inflowing stream, though a galaxy-galaxy interaction origin cannot be excluded. We discuss these scenarios in the context of cosmological simulations of cold-mode accretion and CGM gas flows around galaxies with halos of mass M_h \lesssim 10^{12} M_{\odot}.