What can the outskirts of galaxies tell us about dark matter?

TL;DR

Deep observations of galaxy outskirts, including faint stellar components, combined with simulations, can provide insights into dark matter models, but differences are subtle and challenging to detect observationally.

Contribution

This study compares how Cold Dark Matter and Warm Dark Matter models influence the properties of stellar outskirts in galaxies using cosmological simulations.

Findings

WDM models show up to 10 times lower stellar density at large radii compared to CDM.

Differences between models are small within observationally allowed WDM parameters.

System-to-system variation in CDM is comparable to differences between CDM and WDM models.

Abstract

Deep observations of galaxy outskirts reveal faint extended stellar components (ESCs) of streams, shells, and halos, which are ghostly remnants of the tidal disruption of satellite galaxies. We use cosmological galaxy formation simulations in Cold Dark Matter (CDM) and Warm Dark Matter (WDM) models to explore how the dark matter model influences the spatial, kinematic, and orbital properties of ESCs. These reveal that the spherically averaged stellar mass density at large galacto-centric radius can be depressed by up to a factor of 10 in WDM models relative to the CDM model, reflecting the anticipated suppressed abundance of satellite galaxies in WDM models. However, these differences are much smaller in WDM models that are compatible with observational limits, and are comparable in size to the system-to-system variation we find within the CDM model. This suggests that it will be challenging to place limits on dark matter using only the unresolved ESC.