Thursday

The number density of HI sources as a function of the line width - the HI velocity width function (HIWF) - serves as an indirect constraint on the dark matter halo mass function, a fundamental prediction of the LCDM concordance cosmology. I present a new determination of the HIWF based on the final data release of the ALFALFA survey. I find that the shape of the HIWF in the two spatially contiguous survey regions, approximately toward and diametrically opposed to the Virgo cluster on the sky, are indistinguishable. This is in contrast to the number density of sources as a function of their HI masses - the HI mass function - which has been shown by Jones et al. (2018) to differ significantly between the two regions, notably with a steeper low-mass slope in the direction of Virgo. While the HI content of galaxies, reflected in the HIMF, is delicately sensitive to their environment, their HI kinematics appear to remain a reliable tracer, through the HIWF, of their dark matter content, at least as long as there remains HI to be detected. This implies that future measurements of the HIWF, e.g. from WALLABY, may be exploited as precision tests of the current cosmological paradigm.