Unveiling M33's Hidden Merger History: A Potential Population of Star Clusters on Retrograde Orbits

TL;DR

This paper uncovers a potential population of star clusters on retrograde orbits in M33, indicating past galaxy interactions and accretion events that have shaped its current structure.

Contribution

It provides evidence of a previously unrecognized sub-population of star clusters on retrograde orbits, suggesting M33's merger history and estimating the mass of accreted material.

Findings

Discovery of star clusters on retrograde orbits in M33.

Estimated lower limit of accreted halo virial mass: ~7 x 10^{10} solar masses.

Proposed link between accretion events and the warp in M33's disk.

Abstract

We report the discovery of a possible sub-population of stellar clusters that appear to follow retrograde orbits around the third largest galaxy in the Local Group, M33 (Triangulum). This spiral disk galaxy has apparently had a mostly quiescent existence, although recent discoveries, particularly of a pronounced warp in the gas and stellar disk, suggest that M33's relatively quiet past was interrupted at least once by a dynamical interaction with another galaxy. We suggest that this sub population provides evidence of accretion of one or more dwarf galaxies in M33's history. We estimate a lower limit for the accreted halo virial mass of $M_{\rm vir} \sim (7 \pm 3) \times 10^{10}$\msun, accounting for about 10\% of the virial mass in the halo of M33 that has an accretion origin. We propose one of these accretion events as the source of the observed warp in M33's disk.