Hidden Disruption in Milky Way-like Satellite Galaxy Populations

Abstract

Determining whether a satellite galaxy is tidally disrupting from photometric observations alone remains a fundamental challenge in near-field cosmology. Cosmological simulations of Milky Way-mass galaxies predict that satellite populations should be highly disrupted, yet the observed Milky Way satellite population appears largely intact. Understanding this tension requires either uncovering disruption that has gone undetected in Milky Way satellites, or identifying limitations in the simulations themselves. We use the Auriga suite of cosmological simulations to evaluate several commonly used indicators of disruption. We find that, in the simulated satellite population, the shape, size, and proper motion alignment of a satellite's bound component are unreliable indicators of its present-day disruption state despite their widespread use in the literature. In contrast, the extended stellar distribution, specifically the ratio of stellar mass between $3r_{1/2}$ and $10r_{1/2}$ to that within $r_{1/2}$, is strongly correlated with disruption. We introduce the extended mass ratio $μ_{\rm EMR}$ as a metric for comparing extended stellar distributions across satellites of varying masses and exploring their connection to disruption in Milky Way observations. Applying this metric to a sample of Milky Way satellites with measured extended stellar distributions, we find that many are consistent with significant ongoing disruption, even among systems traditionally classified as intact. These results suggest that tidal disruption may be substantially more widespread in the observed Milky Way satellite population than currently recognized, and establish extended stellar distributions as a promising new tool for uncovering this hidden disruption.