Galaxies at Redshift ~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 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%. The same search is performed with mock spectra generated using
a hydrodynamic simulation, and we find 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 2-d distribution of the HI column density in a ~30 h^-1 Mpc region.
We discover a probable sub-damped Lyman-alpha system at z=0.557 showing strong,
low-ionisation metal absorption lines. Previous analyses of absorption across
the three sightlines attributed these metal lines to HI. 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 the gas causing this metal absorption may have been
enriched and ejected by the galaxy during a burst of star formation 2 Gyr ago.
