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

We present the ATLAS^3D H I survey of a volume-limited, complete sample of 166 nearby early-type galaxies (ETGs) brighter than M_K=-21.5. The survey is mostly based on data taken with the Westerbork Synthesis Radio Telescope, which enables us to detect H I down to 5 × 10⁶-5 × 10⁷ M_⊙ within the survey volume.

We detect ∼40 per cent of all ETGs outside the Virgo galaxy cluster and ∼10 per cent of all ETGs inside it. This demonstrates that it is common for non-cluster ETGs to host H I. The morphology of the detected gas varies in a continuous way from regular, settled H I discs and rings to unsettled gas distributions (including tidal or accretion tails) and systems of clouds scattered around the galaxy. The majority of the detections consist of H I discs or rings (1/4 of all ETGs outside Virgo) so that if H I is detected in an ETG it is most likely distributed on a settled configuration. These systems come in two main types: small discs [? M_⊙], which are confined within the stellar body and share the same kinematics of the stars; and large discs/rings [M(H I) up to 5 × 10⁹ M_⊙], which extend to tens of kpc from the host galaxy and are in half of the cases kinematically decoupled from the stars.

Neutral hydrogen seems to provide material for star formation in ETGs. Galaxies containing H I within ∼1R_e exhibit signatures of on-going star formation in ∼70 per cent of the cases, approximately five times more frequently than galaxies without central H I. The interstellar medium (ISM) in the centre of these galaxies is dominated by molecular gas, and in ETGs with a small gas disc the conversion of H I into H₂ is as efficient as in spirals.

The ETG H I mass function is characterized by M*∼ 2 × 10⁹ M_⊙ and by a slope α∼-0.7. Compared to spirals, ETGs host much less H I as a family. However, a significant fraction of all ETGs are as H I-rich as spiral galaxies. The main difference between ETGs and spirals is that the former lack the high-column-density H I typical of the bright stellar disc of the latter.

The ETG H I properties vary with environment density in a more continuous way than suggested by the known Virgo versus non-Virgo dichotomy. We find an envelope of decreasing M(H I) and M(H I)/L_K with increasing environment density. The gas-richest galaxies live in the poorest environments (as found also with CO observations), where the detection rate of star formation signatures is higher. Galaxies in the centre of Virgo have the lowest H I content, while galaxies at the outskirts of Virgo represent a transition region and can contain significant amounts of H I, indicating that at least a fraction of them has joined the cluster only recently after pre-processing in groups. Finally, we find an H I morphology-density relation such that at low environment density (measured on a local scale) the detected H I is mostly distributed on large, regular discs and rings, while more disturbed H I morphologies dominate environment densities typical of rich groups. This confirms the importance of processes occurring on a galaxy-group scale for the evolution of ETGs.