Using H <scp>i</scp> observations of low-mass galaxies to test ultralight axion dark matter

ABSTRACT We evaluate recent and upcoming low-redshift neutral hydrogen (H i) surveys as a cosmological probe of small scale structure with a goal of determining the survey criteria necessary to test ultralight axion (ULA) dark matter models. Standard cold dark matter (CDM) models predict a large population of low-mass galactic haloes, whereas ULA models demonstrate significant suppression in this small-scale regime, with halo mass cutoffs of $10^{12}\, \mathrm{M}_{\odot }$ to $10^{7}\, \mathrm{M}_{\odot }$ corresponding to ULA masses of $10^{-24}\,$ to $10^{-20}\,$ eV, respectively, if ULAs compose all of the dark matter. We generate random, homogeneously populated mock universes with cosmological parameters adjusted to match CDM and ULA models. We simulate observations of these mock universes with hypothetical analogues of the mass-limited ALFALFA and WALLABY H i surveys and reconstruct the corresponding H i mass function (HIMF). We find that the ALFALFA HIMF can test for the presence of ULA DM with $m_{a}\lesssim 10^{-21.5}~{\rm eV}$, while WALLABY could reach the larger window $m_{a}\lesssim 10^{-20.9}~{\rm eV}$. These constraints are complementary to other probes of ULA dark matter, demonstrating the utility of local Universe H i surveys in testing dark matter models.