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∼22,500), near-infrared (1.5-1.7 μm), 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.
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BibTeX
@misc{gibson2026probing,
title = {{Probing the Assembly of Triangulum: Precision Metallicities and $α$ Abundances of M33's Star Clusters from APOGEE Integrated-Light Spectroscopy}},
author = {Benjamin J. Gibson and Borja Anguiano and Sten Hasselquist and Gail Zasowski and Anil Seth and Rachael L. Beaton and Christian R. Hayes and Geraint F. Lewis and Steven R. Majewski and Giulia Pagnini},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.26565},
eprint = {2610.26565},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.26565},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Probing the Assembly of Triangulum: Precision Metallicities and α Abundances of M33's Star Clusters from APOGEE Integrated-Light Spectroscopy
AU - Benjamin J. Gibson
AU - Borja Anguiano
AU - Sten Hasselquist
AU - Gail Zasowski
AU - Anil Seth
AU - Rachael L. Beaton
AU - Christian R. Hayes
AU - Geraint F. Lewis
AU - Steven R. Majewski
AU - Giulia Pagnini
PY - 2026
DA - 2026/10/19
PB - arXiv
DO - 10.48550/arxiv.2610.26565
UR - https://arxiv.org/abs/2610.26565
AB - 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∼22,500), near-infrared (1.5-1.7 μm), 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.
ER -