On the connection between the intergalactic medium and galaxies: the H I-galaxy cross-correlation at z ≲ 1

We present a new optical spectroscopic survey of 1777 `star-forming' (`SF') and 366 `non-star-forming' (`non-SF') galaxies at redshifts z ˜ 0-1 (2143 in total), 22 AGN and 423 stars, observed by instruments such as the Deep Imaging Multi-Object Spectrograph, the Visible Multi-Object Spectrograph and the Gemini Multi-Object Spectrograph, in three fields containing five quasi-stellar objects (QSOs) with Hubble Space Telescope (HST) ultraviolet spectroscopy. We also present a new spectroscopic survey of 173 `strong' (1014 ≤ NHI≲ 1017 cm-2) and 496 `weak' (1013 ≲ NHI <1014 cm-2) intervening H I (Lyα) absorption-line systems at z ≲ 1 (669 in total), observed in the spectra of eight QSOs at z ˜ 1 by the Cosmic Origins Spectrograph and the Faint Object Spectrograph on the HST. Combining these new data with previously published galaxy catalogues such as the Very Large Telescope Visible Multi-Object Spectrograph Deep Survey and the Gemini Deep Deep Survey, we have gathered a sample of 654 H I absorption systems and 17 509 galaxies at transverse scales ≲50 Mpc, suitable for a two-point correlation function analysis. We present observational results on the H I-galaxy (ξag) and galaxy-galaxy (ξgg) correlations at transverse scales r⊥ ≲ 10 Mpc, and the H I-H I autocorrelation (ξaa) at transverse scales r⊥ ≲ 2 Mpc. The two-point correlation functions are measured both along and transverse to the line of sight, ξ(r⊥, r∥). We also infer the shape of their corresponding `real-space' correlation functions, ξ(r), from the projected along the line-of-sight correlations, assuming power laws of the form ξ(r) = (r/r0)-γ. Comparing the results from ξag, ξgg and ξaa, we constrain the H I-galaxy statistical connection, as a function of both H I column density and galaxy star formation activity. Our results are consistent with the following conclusions: (i) the bulk of H I systems on ˜ Mpc scales have little velocity dispersion (≲120 km s-1) with respect to the bulk of galaxies (i.e. no strong galaxy outflow/inflow signal is detected); (ii) the vast majority (˜100 per cent) of `strong' H I systems and `SF' galaxies are distributed in the same locations, together with 75 ± 15 per cent of `non-SF' galaxies, all of which typically reside in dark matter haloes of similar masses; (iii) 25 ± 15 per cent of `non-SF' galaxies reside in galaxy clusters and are not correlated with `strong' H I systems at scales ≲2 Mpc; and (iv) >50 per cent of `weak' H I systems reside within galaxy voids (hence not correlated with galaxies), and are confined in dark matter haloes of masses smaller than those hosting `strong' systems and/or galaxies. We speculate that H I systems within galaxy voids might still be evolving in the linear regime even at scales ≲2 Mpc.