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 using data from the GAMA, DESI, ALFALFA and Nearby Galaxy catalogs. We use the distribution of projected axis ratios, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>q</mml:mi> </mml:math> , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>40</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> of galaxies in the mass range are intrinsically flatter than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>1</mml:mn> </mml:math> : <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>5</mml:mn> </mml:math> (i.e., <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>c</mml:mi> <mml:mi>/</mml:mi> <mml:mi>a</mml:mi> <mml:mo>&lt;</mml:mo> <mml:mn>0.2</mml:mn> </mml:mrow> </mml:math> ), a fraction that rises to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>80</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>c</mml:mi> <mml:mi>/</mml:mi> <mml:mi>a</mml:mi> <mml:mo>&lt;</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> </mml:math> . Although the incidence of thin galaxies decreases towards lower and higher <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⋆</mml:mo> </mml:msub> </mml:math> , they are still quite common in dwarfs: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>30</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>65</mml:mn> <mml:mi>%</mml:mi> </mml:mrow> </mml:math> of galaxies are inferred to be intrinsically flatter than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>c</mml:mi> <mml:mi>/</mml:mi> <mml:mi>a</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.2</mml:mn> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mn>0.3</mml:mn> </mml:math> , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>c</mml:mi> <mml:mi>/</mml:mi> <mml:mi>a</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.2</mml:mn> </mml:mrow> </mml:math> , 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.