The ATLAS^(3D) project – XXVI. HI discs in real and simulated fast and slow rotators

Abstract

One quarter of all nearby early-type galaxies (ETGs) outside Virgo host a disc/ring of H i with size from a few to tens of kpc and mass up to ∼10^9 M_⊙. Here we investigate whether this H i is related to the presence of a stellar disc within the host making use of the classification of ETGs in fast and slow rotators (FR/SR). We find a large diversity of H i masses and morphologies within both families. Surprisingly, SRs are detected as often, host as much H i and have a similar rate of H i discs/rings as FRs. Accretion of H i is therefore not always linked to the growth of an inner stellar disc. The weak relation between H i and stellar disc is confirmed by their frequent kinematical misalignment in FRs, including cases of polar and counterrotating gas. In SRs the H i is usually polar. This complex picture highlights a diversity of ETG formation histories which may be lost in the relative simplicity of their inner structure and emerges when studying their outer regions. We find that Λ CDM hydrodynamical simulations have difficulties reproducing the H i properties of ETGs. The gas discs formed in simulations are either too massive or too small depending on the star formation feedback implementation. Kinematical misalignments match the observations only qualitatively. The main point of conflict is that nearly all simulated FRs and a large fraction of all simulated SRs host corotating H i. This establishes the H i properties of ETGs as a novel challenge to simulations.

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