Hot accretion onto spiral galaxies: the origin of extended and warped HI discs

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 idealised hydrodynamic simulations, we demonstrate that the central regions of hot galaxy atmospheres continuously condense into cool ($\sim10^4\,{\rm K}$) 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 smooth accretion from hot atmospheres can thus both provide the necessary fuel for star formation and explain the observed ubiquity of extended and warped HI discs around local spirals. In this hot accretion scenario, cool gas observations cannot be used to trace the source of the HI, warps out to the halo, consistent with recent indications of a lack of 21 cm emission from the halos of nearby galaxies (the `HI desert'). Observations of HI warps formed via hot accretion 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.