Unveiling M33’s hidden merger history: a potential population of star clusters on retrograde orbits

ABSTRACT We report the discovery of a possible subpopulation of stellar clusters that appear to follow retrograde orbits around the third largest galaxy in the Local Group, M33 (Triangulum). This spiral disc galaxy has apparently had a mostly quiescent existence, although recent discoveries, particularly of a pronounced warp in the gas and stellar disc, suggest that M33’s relatively quiet past was interrupted at least once by a dynamical interaction with another galaxy. We suggest that this subpopulation 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}$$\, {\rm M}_{\odot }$, accounting for about 10 per cent 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 disc.