The High Fraction of Thin Disk Galaxies Continues to Challenge {\Lambda}CDM Cosmology

TL;DR

This study compares the distribution of galaxy shapes in b1CDM simulations with observations, revealing a significant deficit of thin disk galaxies in simulations and discussing potential causes and implications for cosmology.

Contribution

It provides a detailed analysis of galaxy morphology distributions in b1CDM simulations and highlights discrepancies with observations, suggesting possible reasons and solutions.

Findings

Simulations show fewer thin disk galaxies than observed.

Significant statistical disagreement between simulations and surveys.

Thinner galaxies are associated with fewer major mergers.

Abstract

Any viable cosmological framework has to match the observed proportion of early- and late-type galaxies. In this contribution, we focus on the distribution of galaxy morphological types in the standard model of cosmology (Lambda cold dark matter, $\Lambda$CDM). Using the latest state-of-the-art cosmological $\Lambda$CDM simulations known as Illustris, IllustrisTNG, and EAGLE, we calculate the intrinsic and sky-projected aspect ratio distribution of the stars in subhalos with stellar mass $M_* > 10^{10}\,M_\odot$ at redshift $z=0$. There is a significant deficit of intrinsically thin disk galaxies, which however comprise most of the locally observed galaxy population. Consequently, the sky-projected aspect ratio distribution produced by these $\Lambda$CDM simulations disagrees with the Galaxy And Mass Assembly (GAMA) survey and Sloan Digital Sky Survey at $\geq 12.52\sigma$ (TNG50-1) and $\geq 14.82\sigma$ (EAGLE50) confidence. The deficit of intrinsically thin galaxies could be due to a much less hierarchical merger-driven build-up of observed galaxies than is given by the $\Lambda$CDM framework. It might also arise from the implemented sub-grid models, or from the limited resolution of the above-mentioned hydrodynamical simulations. We estimate that an $8^5$ times better mass resolution realization than TNG50-1 would reduce the tension with GAMA to the $5.58\sigma$ level. Finally, we show that galaxies with fewer major mergers have a somewhat thinner aspect ratio distribution. Given also the high expected frequency of minor mergers in $\Lambda$CDM, the problem may be due to minor mergers. In this case, the angular momentum problem could be alleviated in Milgromian dynamics (MOND) because of a reduced merger frequency arising from the absence of dynamical friction between extended dark matter halos.