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

We investigate the properties of the H?i Lya absorption systems (Lya forest) within and around galaxy voids at z ? 0.1. We find a significant excess (>99 per cent confidence level, c.l.) of Lya systems at the edges of galaxy voids with respect to a random distribution, on similar to 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 (NHi) and Doppler parameter (bHi) distributions of Lya 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 NHi and bHi 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 Lya 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 (similar to 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 2530 +/- 6 per cent of the current Lya population (NHi?1012.5?cm-2), while the other two types account for the remaining 7075 +/- 12 per cent. Assuming that only NHi=1014?cm-2 systems have embedded galaxies nearby, we have estimated the contribution of the halo-like Lya population to be similar to 1215 +/- 4?per cent and consequently similar to 5560 +/- 13?per cent of the Lya systems to be associated with the overdense LSS.