Galaxies at Redshift ~0.5 Around Three Closely Spaced Quasar Sightlines

TL;DR

This study investigates the relationship between galaxies and the intergalactic medium at z<1 using multiple quasar sightlines, revealing excess galaxy-absorber groups, their association with large-scale structures, and metal absorption linked to galaxy star formation history.

Contribution

Introduces a new grouping algorithm for galaxy-absorber associations across multiple sightlines and demonstrates their connection to large-scale structures and galaxy evolution.

Findings

Excess of galaxy-absorber groups at 99.9% confidence level.

Groups generally trace large-scale filamentary structures in simulations.

Evidence of metal absorption linked to galaxy star formation history.

Abstract

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.