Lyman Limit System with O VI in the Circumgalactic Environment of a Pair of Galaxies

Abstract

We report on the analysis of a multiphase Lyman limit system (LLS) at $z=0.39047$ identified towards the background quasar FBQS J0209-0438. The O VI doublet lines associated with this absorber have a different profile from the low ionization metals and the H I. The Ly$\alpha$ has a very broad H I ($b\approx 150$ km s${}^{-1}$) component well-aligned with one of the O VI components. The Doppler $b$-parameters for the broad H I and O VI indicate gas with $T=(0.8-2.0)\times 10^6$ K, and a total hydrogen column density that is an order of magnitude larger than the cooler phase of gas responsible for the LLS. Observations by VLT/MUSE show two moderately star-forming galaxies within $\rho \lesssim 105$ kpc, and $|\Delta v|\lesssim 130$ km s${}^{-1}$ of the absorber, one of them a dwarf galaxy ($M_{\ast }\approx 10^6$ M${}_{\odot }$) overlapping with the quasar PSF, and the other a larger galaxy ($R_{1/2}\approx 4$ kpc) with $M_{\ast }\approx 3\times 10^{10}$ M${}_{\odot }$ and $M_h\approx 7\times 10^{11}$ M${}_{\odot }$, and the dwarf galaxy within its virial radius. Though the absorption is aligned with the extended major axis of the larger galaxy, the line-of-sight velocity of the absorbing gas is inconsistent with corotating accretion. The metallicity inferred for the LLS is lower than the gas phase [O/H] of the two galaxies. The mixture of cool and warm/hot gas phases for the absorbing gas and its proximity and orientation to the galaxy pair points to the LLS being a high-velocity gas in the combined halo environment of both galaxies.

18 pages, 10 figures, 5 tables, accepted to be published in ApJ