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

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 Lyα has a very broad H I (b ≈ 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)× 10⁶ 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 ρ 105 kpc, and |Δ v| 130 km s⁻¹ of the absorber, one of them a dwarf galaxy (M_*≈ 10⁶ M_) overlapping with the quasar PSF, and the other a larger galaxy (R1/2≈ 4 kpc) with M_*≈ 3× 10¹⁰ M_ and Mₕ≈ 7× 10¹¹ M_, 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.