2021MNRAS.503.3243A

Using HST/COS spectra of the twin quasar lines of sight Q 0107-025A & Q 0107-025B, we report on the physical properties, chemical abundances, and transverse sizes of a multiphase medium in a galaxy field at z = 0.399. The angular separation between the quasars corresponds to a physical separation of 520 kpc at the absorber redshift. The absorber towards Q 0107-025B is a partial Lyman limit system (pLLS) with logN(H I)/cm^-2^~16.8. The H I column density in the absorber along the other sightline is ≃ 2 orders of magnitude lower. The O VI along both sightlines have comparable column densities and broad b-values (b > 30 km s^–1) whereas the low ionization lines are considerably narrower. The low ionization gas is inconsistent with the O VI when modelled assuming photoionization from the same phase. In both lines of sight, O VI and the broad H I coinciding, are best explained through collisional ionization in a cooling plasma with solar metallicity. Ionization models infer 1/10th solar metallicity for the pLLS and solar metallicity for the lower column density absorber along the other sightline. Within ± 250 km s^–1 and 2 Mpc of projected distance from the sightlines 12 galaxies are identified, of which five are within 500 kpc. The twin sightlines are at normalized impact parameters of ρ ∼ 1.1R_vir, and ρ ∼ 0.8R_vir from a M_* ∼ 10^10.7 M_☉, L ∼ 0.07L*, and star formation rate (SFR) < 0.1 M_☉ yr^–1 galaxy, potentially probing its CGM (circumgalactic medium). The next closest in normalized separation are a dwarf galaxy with M_* ∼ 10^8.7 M_☉, and SFR ∼ 0.06 M_☉ yr^–1, and an intermediate mass galaxy with M_* ∼ 10^10.0 M_☉, and SFR ∼ 3 M_☉ yr^–1. Along both sightlines, O VI could be either tracing narrow transition temperature zones at the interface of low ionization gas and the hot halo of nearest galaxy, or a more spread-out warm component that could be gas bound to the circumgalactic halo or the intragroup medium. The latter scenarios lead to a warm gas mass limit of M >= 4.5 x 10⁹ M_☉.