Hints against the cold and collisionless nature of dark matter from the galaxy velocity function

TL;DR

This paper explores alternative dark matter models like warm, mixed, and self-interacting dark matter to explain the observed scarcity of dwarf galaxies, suggesting these models can resolve discrepancies with standard cosmology.

Contribution

It demonstrates that warm, mixed, and velocity-dependent self-interacting dark matter models can reconcile observed dwarf galaxy counts with theoretical predictions.

Findings

Warm and mixed dark matter models eliminate the galaxy abundance discrepancy.

Self-interacting dark matter can produce extended cores in dwarf galaxies.

Velocity-dependent cross sections are necessary for consistent self-interacting models.

Abstract

The observed number of dwarf galaxies as a function of rotation velocity is significantly smaller than predicted by the standard model of cosmology. This discrepancy cannot be simply solved by assuming strong baryonic feedback processes, since they would violate the observed relation between maximum circular velocity ($v_{\rm max}$) and baryon mass of galaxies. A speculative but tantalising possibility is that the mismatch between observation and theory points towards the existence of non-cold or non-collisionless dark matter (DM). In this paper, we investigate the effects of warm, mixed (i.e warm plus cold), and self-interacting DM scenarios on the abundance of dwarf galaxies and the relation between observed HI line-width and maximum circular velocity. Both effects have the potential to alleviate the apparent mismatch between the observed and theoretical abundance of galaxies as a function of $v_{\rm max}$. For the case of warm and mixed DM, we show that the discrepancy disappears, even for luke-warm models that evade stringent bounds from the Lyman-$\alpha$ forest. Self-interacting DM scenarios can also provide a solution as long as they lead to extended ($\gtrsim 1.5$ kpc) dark matter cores in the density profiles of dwarf galaxies. Only models with velocity-dependent cross sections can yield such cores without violating other observational constraints at larger scales.