A warm dark matter cosmogony may yield more low-mass galaxy detections in 21-cm surveys than a cold dark matter one

TL;DR

This study predicts that warm dark matter models could result in more low-mass galaxy detections in 21-cm surveys than cold dark matter models, due to differences in galaxy formation and gas retention.

Contribution

It demonstrates that forward-modeling galaxy formation simulations into observational data can reveal differences between WDM and CDM in 21-cm survey predictions.

Findings

WDM models predict more low-$w_{50}$ galaxies than CDM.

Low-mass galaxies in WDM form later and retain more gas.

Future simulations with improved cold gas modeling can better constrain dark matter models.

Abstract

The 21-cm spectral line widths, $w_{50}$, of galaxies are an approximate tracer of their dynamical masses, such that the dark matter halo mass function is imprinted in the number density of galaxies as a function of $w_{50}$. Correcting observed number counts for survey incompleteness at the level of accuracy needed to place competitive constraints on warm dark matter (WDM) cosmological models is very challenging, but forward-modelling the results of cosmological hydrodynamical galaxy formation simulations into observational data space is more straightforward. We take this approach to make predictions for an ALFALFA-like survey from simulations using the EAGLE galaxy formation model in both cold (CDM) and WDM cosmogonies. We find that for WDM cosmogonies more galaxies are detected at the low-$w_{50}$ end of the 21-cm velocity width function than in the CDM cosmogony, contrary to what might na\"ively be expected from the suppression of power on small scales in such models. This is because low-mass galaxies form later and retain more gas in WDM cosmogonies (with EAGLE). While some shortcomings in the treatment of cold gas in the EAGLE model preclude placing definitive constraints on WDM scenarios, our analysis illustrates that near-future simulations with more accurate modelling of cold gas will likely make strong constraints possible, especially in conjunction with new 21-cm surveys such as WALLABY.