Probing the Assembly of Triangulum: Precision Metallicities and $α$ Abundances of M33's Star Clusters from APOGEE Integrated-Light Spectroscopy

Abstract

The star cluster system of M33 provides an excellent opportunity to investigate whether galaxy formation physics is dependent on a galaxy's mass. Older globular clusters (GCs) trace hierarchical assembly and chemical enrichment, while younger clusters trace the recent star formation. By studying the M33 star cluster system as a whole, we can probe the formation physics of galaxies at a lower mass than the Milky Way, and look for signals of accretion. To that end, we present analysis of 37 M33 star clusters using high-resolution ($R\sim22,500$), near-infrared (1.5-1.7 $μ$m), integrated light spectra from the APOGEE survey. We use full-spectrum fitting to obtain precision measurements of the clusters' kinematics, metallicities, and $α$ abundances. We identify three populations of clusters from our measurements: young stellar clusters (~100 Myr) with broad Brackett absorption features; intermediate-age clusters (1-2 Gyr) that are metal-rich ([M/H]=-0.5 to +0.3) with roughly solar $α$ abundance; and old (>5 Gyr), kinematically-hot and metal-poor GCs. These old GCs exhibit a broad range of kinematics (RV~-250 to -8 km s$^{-1}$), metallicities ([M/H]=-1.8 to -0.6) and $α$ abundances ([$α$/M]=+0.05 to +0.3), and we identify and discuss three GCs that show strong chemodynamical evidence of accretion into M33's halo. M33's GCs at [M/H]~-1.2 have a spread in [$α$/M] of nearly 0.2 dex. This pattern is strikingly similar to the abundance distribution of Milky Way GCs, and indicates either that M33 and the Milky Way formed clusters similarly at early times, or that M33 hosts a significant population of accreted clusters in its halo.