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

We study the prevalence of thin galaxies as a function of stellar mass in the range 10 7 < M / < 10 11 using data from the GAMA, DESI, ALFALFA, and Nearby Galaxy catalogs. We use the distribution of projected axis ratios, q, 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 40% of galaxies in the mass range 10 9<M/ <10 10 are intrinsically flatter than 1: 5 (i.e., c/a<0.2), a fraction that rises to about 80% for c/a<0.3. Although the incidence of thin galaxies decreases towards lower and higher M, they are still quite common in dwarfs: about 30% and about 65% of about 10 8 galaxies are inferred to be intrinsically flatter than c/a=0.2 and 0.3, 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 M < 10 9 simulated galaxies flatter than c/a=0.2, 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.