Galaxies at a redshift of ∼0.5 around three closely spaced quasar sightlines

We examine the relationship between galaxies and the intergalactic medium at z < 1 using a group of three closely spaced background quasi-stellar objects (QSOs) with z_em ≈ 1 observed with the Hubble Space Telescope. Using a new grouping algorithm, we identify groups of galaxies and absorbers across the three QSO sightlines that may be physically linked. There is an excess number of such groups compared to the number we expect from a random distribution of absorbers at a confidence level of 99.9 per cent. The same search is performed with mock spectra generated using a hydrodynamical simulation, and we find that the vast majority of such groups arise in dense regions of the simulation. We find that at z < 0.5, groups in the simulation generally trace the large-scale filamentary structure as seen in the projected 2D distribution of the H i column density in an ∼30 h^-1 Mpc region. We discover a probable sub-damped Lyman α system at z = 0.557 showing strong, low-ionization metal absorption lines. Previous analyses of absorption across the three sightlines attributed these metal lines to H i. We show that even when the new line identifications are taken into account, evidence remains for planar structures with scales of ∼1 Mpc absorbing across the three sightlines. We identify a galaxy at z = 0.2272 with associated metal absorption in two sightlines, each 200 kpc away. By constraining the star formation history of the galaxy, we show that the gas causing this metal absorption may have been enriched and ejected by the galaxy during a burst of star formation 2 Gyr ago.