Another Problem With The ΛCDM Standard Model Of Cosmology

Every time you turn around, it seems, a new problem with the ΛCDM Standard Model of Cosmology emerges. The latest is that it predicts far too few thin disk galaxies, which are the most common type of galaxies in the real world.

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, ΛCDM). 

Using the latest state-of-the-art cosmological ΛCDM 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∗>10^10M⊙ 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 ΛCDM simulations disagrees with the Galaxy And Mass Assembly (GAMA) survey and Sloan Digital Sky Survey at ≥12.52σ (TNG50-1) and ≥14.82σ (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 ΛCDM 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 85 times better mass resolution realization than TNG50-1 would reduce the tension with GAMA to the 5.58σ 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 ΛCDM, 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.

Moritz Haslbauer, Indranil Banik, Pavel Kroupa, Nils Wittenburg, Behnam Javanmardi, "The High Fraction of Thin Disk Galaxies Continues to Challenge ΛCDM Cosmology" arXiv:2202.01221 (February 2, 2022) (published at 925 ApJ, 183 (2022) DOI: 10.3847/1538-4357/ac46ac.