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 with $\log N({{\rm H~}{\rm \small I}})/{\rm cm^{-2}}\approx 16.8$. 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 1/10-th 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 5 are within 500 kpc. The twin sightlines are at normalized impact parameters of ρ ∼ 1.1Rvir, and ρ ∼ 0.8Rvir from a M* ∼ 1010.7 M⊙, L ∼ 0.07L*, and SFR <0.1 M⊙ yr−1 galaxy, potentially probing its CGM. The next closest in normalized separation are a dwarf galaxy with M* ∼ 108.7 M⊙, and SFR ∼0.06 M⊙ yr−1, and an intermediate mass galaxy with M* ∼ 1010.0 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 leads to a warm gas mass limit of M ≳ 4.5 × 109 M⊙.
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