The distribution of dark galaxies and spin bias

Abstract

ABSTRACT In the light of the discovery of numerous (almost) dark galaxies from the ALFALFA and LITTLE THINGS surveys, we revisit the predictions of the existence of dark galaxies, based on the Toomre stability of rapidly spinning gas discs. We have updated the predictions for Λ-cold dark matter with parameters given by the Planck18 collaboration, computing the expected number densities of dark objects, and their spin parameter and mass distributions. Comparing with the data is more challenging, but where the spins are more reliably determined, the spins are close to the threshold for discs to be stable according to the Toomre criterion, where the expected number density is highest, and reinforces the concept that there is a bias in the formation of luminous galaxies based on the spin of their parent halo.