On the connection between the intergalactic medium and galaxies: The hi-galaxy cross-correlation at z < 1*

We present a new optical spectroscopic survey of 1777 'star-forming' ('SF') and 366 'non-starforming' ('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 GeminiMulti-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 < N_HI < 10¹⁷ cm^-2) and 496 'weak' (10¹³ < N_HI ≤ 10¹⁴ cm^-2) intervening HI (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 HI 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 HI-galaxy (ξ_gg) and galaxy-galaxy (ξ;_gg) correlations at transverse scales r_⊥ ≤ 10 Mpc, and the HI-H I autocorrelation (ξ;_gg) at transverse scales r_⊥ ≤ 2Mpc. 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 ξ_ag, ξ_gg and ξ_a, we constrain the HI-galaxy statistical connection, as a function of both HI column density and galaxy star formation activity. Our results are consistent with the following conclusions: (i) the bulk of HI 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' HI 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' HI systems at scales <2Mpc; and (iv) ≥50 per cent of 'weak' HI 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 HI systems within galaxy voids might still be evolving in the linear regime even at scales <2Mpc.