Large-scale structure in absorption: gas within and around galaxy voids

We investigate the properties of the H i Ly&#0945 absorption systems (Ly&#0945 forest) within and around galaxy voids at z ≲ 0.1. We find a significant excess (>99 per cent confidence level, c.l.) of Ly&#0945 systems at the edges of galaxy voids with respect to a random distribution, on ∼5 h-1 Mpc scales. We find no significant difference in the number of systems inside voids with respect to the random expectation. We report differences between both column density (inline image) and Doppler parameter (inline image) distributions of Ly&#0945 systems found inside and at the edge of galaxy voids at the ≳98 and ≳90 per cent c.l., respectively. Low-density environments (voids) have smaller values for both inline image and inline image than higher density ones (edges of voids). These trends are theoretically expected and also found in Galaxies-Intergalactic Medium Interaction Calculation (GIMIC), a state-of-the-art hydrodynamical simulation. Our findings are consistent with a scenario of at least three types of Ly&#0945 systems: (1) containing embedded galaxies and so directly correlated with galaxies (referred to as 'halo-like'), (2) correlated with galaxies only because they lie in the same overdense large-scale structure (LSS) and (3) associated with underdense LSS with a very low autocorrelation amplitude (≈random) that are not correlated with luminous galaxies. We argue that the latter arise in structures still growing linearly from the primordial density fluctuations inside galaxy voids that have not formed galaxies because of their low densities. We estimate that these underdense LSS absorbers account for 25-30 ±6 per cent of the current Ly&#0945 population (inline image cm-2), while the other two types account for the remaining 70-75 ± 12 per cent. Assuming that only inline image cm-2 systems have embedded galaxies nearby, we have estimated the contribution of the 'halo-like' Ly&#0945 population to be ≈12-15 ± 4 per cent and consequently ≈55-60 ± 13 per cent of the Ly&#0945 systems to be associated with the overdense LSS