Characterizing circumgalactic gas around massive ellipticals at z ≈ 0.4 - III. The galactic environment of a chemically pristine Lyman limit absorber

This paper presents a study of the galactic environment of a chemically pristine (<0.6 per cent solar metallicity) Lyman Limit system (LLS) discovered along the sightline towards QSO SDSSJ 135726.27+043541.4 (z_QSO = 1.233) at projected distance d = 126 physical kpc (pkpc) from a luminous red galaxy (LRG) at z = 0.33. Combining deep Hubble Space Telescope images, MUSE integral field spectroscopic data, and wide-field redshift survey data has enabled an unprecedented, ultradeep view of the environment around this LRG-LLS pair. A total of 12 galaxies, including the LRG, are found at d ≲ 400 pkpc and line-of-sight velocity Δ v< 600 km s^-1 of the LLS, with intrinsic luminosity ranging from 0.001 L_* to 2 L_* and a corresponding stellar mass range of {M_star}≈ 10^{7-11} M_{⊙}}. All 12 galaxies contribute to a total mass of {M_star}=1.6× 10^{11} M_{⊙} with ≈80 per cent contained in the LRG. The line-of-sight velocity dispersion of these galaxies is found to be σ_group = 230 km s^-1 with the centre of mass at d_group = 118 pkpc and line-of-sight velocity offset of Δ v_group=181 km s^-1 from the LLS. Three of these are located at d ≲ 100 pkpc from the LLS, and they are all faint with intrinsic luminosity < 0.02 L_* and gas-phase metallicity of ≈10 per cent solar in their interstellar medium. The disparity in the chemical enrichment level between the LLS and the group members suggests that the LLS originates in infalling intergalactic medium and that parts of the intergalactic gas near old and massive galaxies can still remain chemically pristine through the not too distant past.