Satellites Around Milky Way Analogs: Tension in the Number and Fraction of Quiescent Satellites Seen in Observations Versus Simulations

TL;DR

This study compares observed and simulated satellite galaxy properties around Milky Way-like hosts, revealing a significant discrepancy in the fraction of quenched satellites below a certain stellar mass, which challenges current models.

Contribution

It provides a detailed comparison of star-formation activity in satellites from observations and simulations, highlighting a persistent discrepancy that suggests missing physics in models.

Findings

Observed satellites have fewer quenched low-mass satellites than simulations.

Good agreement in star-forming satellite counts and SFRs between observations and simulations.

Discrepancy persists across different tracers and simulation resolutions.

Abstract

We compare the star-forming properties of satellites around Milky Way (MW) analogs from the Stage~II release of the Satellites Around Galactic Analogs Survey (SAGA-II) to those from the APOSTLE and Auriga cosmological zoom-in simulation suites. We use archival GALEX UV imaging as a star-formation indicator for the SAGA-II sample and derive star-formation rates (SFRs) to compare with those from APOSTLE and Auriga. We compare our detection rates from the NUV and FUV bands to the SAGA-II H$\alpha$ detections and find that they are broadly consistent with over $85\%$ of observed satellites detected in all three tracers. We apply the same spatial selection criteria used around SAGA-II hosts to select satellites around the MW-like hosts in APOSTLE and Auriga. We find very good overall agreement in the derived SFRs for the star-forming satellites as well as the number of star-forming satellites per host in observed and simulated samples. However, the number and fraction of quenched satellites in the SAGA-II sample are significantly lower than those in APOSTLE and Auriga below a stellar mass of $M_*\sim10^{8}\,M_{\odot}$, even when the SAGA-II incompleteness and interloper corrections are included. This discrepancy is robust with respect to the resolution of the simulations and persists when alternative star-formation tracers are employed. We posit that this disagreement is not readily explained by vagaries in the observed or simulated samples considered here, suggesting a genuine discrepancy that may inform the physics of satellite populations around MW analogs.