The abundance of thin dwarf galaxies: a challenge for cosmological hydrodynamical simulations

José A. Benavides; Laura V. Sales; Julio F. Navarro; Simon D. M. White; Carlos S. Frenk; Kyle A. Oman; Shaun Cole

Paper

The abundance of thin dwarf galaxies: a challenge for cosmological hydrodynamical simulations

Abstract

We study the prevalence of thin galaxies as a function of stellar mass in the range

using data from the GAMA, DESI, ALFALFA and Nearby Galaxy catalogs. We use the distribution of projected axis ratios,

, to infer the abundance of intrinsically flat galaxies needed to reproduce the observed abundance of highly elongated systems in projection. We find that as many as

of galaxies in the mass range

are intrinsically flatter than

(i.e.,

), a fraction that rises to

for

. Although the incidence of thin galaxies decreases towards lower and higher

, they are still quite common in dwarfs:

and

galaxies are inferred to be intrinsically flatter than

and

, respectively. A comparison of these results with several state-of-the-art cosmological hydrodynamical simulations (TNG50, FIREbox, Romulus25) reveals a distinctive lack of thin simulated dwarfs. In particular, there are no

simulated galaxies flatter than

, in clear contrast with observational samples. This discrepancy likely reflects limitations in resolution and in the treatment of baryonic physics, suggesting that our understanding of the mechanisms regulating the formation of disk galaxies less massive than the Milky Way is still quite incomplete. Our results present a clear challenge to current numerical models of dwarf galaxy formation, which future models should attempt to meet.