Hot accretion onto spiral galaxies: the origin of extended and warped H  <scp>i</scp> discs

ABSTRACT Gas accretion, hot ($\sim 10^6\, {\rm K}$) atmospheres, and a tilt between the rotation axes of the disc and the atmosphere are all common predictions of standard galaxy evolution theory for massive star-forming galaxies at low redshift. Using idealized hydrodynamic simulations, we demonstrate that the central regions of hot galaxy atmospheres continuously condense into cool ($\sim 10^4\, {\rm K}$) discs, while being replenished by an inflow from larger scales. The size and orientation of the condensed disc are determined by the angular momentum of the atmosphere, so the condensed disc is expected to often be tilted and more extended than the stellar disc. Continuous accretion from hot atmospheres can thus explain the ubiquity of extended and warped H i discs around local spirals, and also potentially provide the necessary fuel for star formation. This hot accretion scenario predicts the absence of significant H i from galaxy haloes, consistent with recent $21\, {\rm cm}$ constraints on nearby spirals (the so-called ‘H i desert’). Moreover, our analysis indicates that observations of H i warps can be used to constrain the angular momentum, accretion rate, and gas metallicity of hot galaxy atmospheres, important parameters for disc galaxy evolution that are hard to determine by other means.