A too-many dwarf satellite galaxies problem in the MATLAS low-to-moderate density fields

TL;DR

This study tests the Lambda-CDM model's ability to reproduce dwarf galaxy abundances in low-to-moderate density fields beyond the Local Volume, finding a significant underestimation of dwarfs in simulations compared to observations.

Contribution

It highlights a 'too-many-satellites' problem in Lambda-CDM simulations for environments outside the Local Group, emphasizing the need for model refinement.

Findings

Simulations underestimate dwarf counts by 6 sigma in MATLAS-like fields.

Photometric properties of simulated dwarfs match observations, but abundance does not.

Background galaxy contamination estimates align with observations, but do not resolve the discrepancy.

Abstract

Dwarf galaxy abundances can serve as discernment tests for models of structure formation. Previous small-scale tensions between observations and dark matter-only cosmological simulations may have been resolved with the inclusion of baryonic processes; however, these successes have been largely concentrated on the Local Group dwarfs the feedback models were initially calibrated on. We investigate whether the $\Lambda$CDM model can reliably reproduce dwarf abundances in the MATLAS low-to-moderate density fields that are centred upon early-type host galaxies beyond the Local Volume. We carried out mock observations of MATLAS-like fields with the high-resolution hydrodynamic simulation IllustrisTNG-50. We used matching selection criteria and compared the properties of dwarfs contained within them with their MATLAS analogues. Although simulated MATLAS-like dwarfs demonstrate photometric properties that are consistent with the observed galaxy population and follow the same scaling relations, TNG50 underestimates the number of dwarf galaxies in isolated MATLAS fields at the 6 sigma level. This significance is maintained within crowded fields containing more than a single bright host. Our 55-62% estimate of the fraction of background galaxies is in agreement with estimates by MATLAS, but is wholly insufficient to alleviate this discrepancy in dwarf abundances. Any incompleteness in the observed fields further exacerbates this tension. We identified a "too-many-satellites" problem in $\Lambda$CDM, emphasising the need for the continued testing and refining of current models of galaxy formation in environments beyond the Local Group.