The weak connection between the stellar haloes and merger histories of Milky Way-mass galaxies

TL;DR

Stellar haloes are not reliable indicators of galaxy merger histories, as disc galaxies with low stellar halo mass can still experience significant mergers that do not contribute to the halo.

Contribution

This study reveals that the fraction of stellar mass in haloes does not reliably indicate merger activity, challenging previous assumptions about galaxy merger diagnostics.

Findings

Low stellar halo mass does not imply quiescent merger history.

Mergers on co-planar, circular orbits can avoid building stellar haloes.

Disc galaxies can have active merger histories despite low stellar halo mass.

Abstract

Stellar haloes form through the disruption of satellite galaxies over time, making them a promising observable for constraining galaxy merger histories. We use a dynamical decomposition technique to isolate the stellar haloes of Milky Way-mass galaxies in the $100\,{\rm Mpc}$ EAGLE simulation and study their relationship to the merger histories of their hosts. We define the stellar halo as the stellar mass that is bound to the central subhalo but not associated with the disc or bulge components of a galaxy, and we quantify their merger histories using the most significant merger since $z=1$. Surprisingly, we find that the fraction of a galaxy's total stellar mass in the stellar halo, $f_{\rm SH}$, is not a reliable indicator of its merger activity. Contrary to common assumptions, disc galaxies with low $f_{\rm SH}$ do not necessarily have quiescent merger histories. In fact, roughly one quarter experienced a merger at $z \leq 1$ with a satellite whose stellar mass was at least 10 per cent of the host galaxy's stellar mass. These galaxies undergo mergers with satellites on circular orbits that are roughly co-planar with the pre-existing disc and thereby avoid contributing mass to the stellar halo. Instead, such mergers build thick, extended discs and supply fresh gas that often triggers a significant episode of star formation in the disc. Our results suggest that disc galaxies with low-mass stellar haloes, such as the Milky Way, can have varied and active merger histories, and that stellar haloes may not be a reliable tool for inferring galaxy merger histories.