We present the Atlas3D HI survey of 166 nearby early-type galaxies (ETGs) down to M(HI)~10^7 M_sun. We detect HI in ~40% of all ETGs outside the Virgo cluster and in ~10% of all ETGs inside it. This demonstrates that it is common for non-cluster ETGs to host HI. The HI morphology varies from regular discs/rings (the majority of the detections) to unsettled gas distributions. The former are either small discs (M(HI)<10^8 M_sun) confined within the stellar body and sharing the same kinematics of the stars, or large discs/rings (M(HI) up to 5x10^9 M_sun) extending to tens of kpc from the host galaxy and frequently kinematically decoupled from the stars. Neutral hydrogen provides material for star formation in ETGs. Galaxies with central HI exhibit signatures of star formation in ~70% of the cases, ~5 times more frequently than galaxies without central HI. The central ISM is dominated by molecular gas. In ETGs with a small gas disc the conversion of HI into H_2 is as efficient as in spirals. The ETG HI mass function has M*~2x10^9 M_sun and slope=-0.7. ETGs host much less HI than spirals as a family. However, a significant fraction of them is as HI-rich as spirals. The main difference between ETGs and spirals is that the former lack the high-column-density HI typical of the bright stellar disc of the latter. We find an envelope of decreasing M(HI) with increasing environment density. The gas-richest ETGs live in the poorest environments (where star-formation is more common), galaxies in the centre of Virgo have the lowest HI content, and the cluster outskirts are a transition region. We find an HI morphology-density relation. At low environment density HI is mostly distributed on large discs/rings. More disturbed HI morphologies dominate environment densities typical of rich groups, confirming the importance of processes occurring on a galaxy-group scale for the evolution of ETGs.

The ATLAS3D project - XIII. Mass and morphology of H i in early-type galaxies as a function of environment