The M33 Cluster System: Fossil Records of a Galaxy's Evolution

Abstract

This study uses star clusters of the Triangulum galaxy (M33) as fossil records to trace its evolutionary history. We synthesize previous observational studies of the cluster system’s population, kinematics, and chemical properties to construct a unified evolutionary picture. The cluster system (GS) shows a clear age-metallicity relation (AMR), with younger clusters more metal-rich and metallicity dispersion increasing with age. The age distribution shows a prominent peak at approximately 10 Myr, indicating a distinct period of cluster formation. An age-velocity relation (AVR) is also observed, as random cluster velocities increase with age, reflecting progressive disk heating. The radial metallicity gradients evolve significantly over time: young clusters (< 150 Myr) exhibit a positive or nearly flat gradient of approximately +0.033 dex/kpc, intermediate-age clusters (1-4 Gyr) show a negative gradient of -0.036 dex/kpc, and older clusters > 4 Gyr within a radius of 4.5 kpc display a steeper negative gradient of -0.065 dex/kpc. These trends support an inside-out growth scenario for the M33 disk. Comparisons with other galaxies show that M33 resembles the Large Magellanic Cloud (LMC) in fraction of old clusters and mean cluster mass but differs from M31 and the Milky Way. Overall, the M33 cluster system preserves valuable fossil evidence of the galaxy’s chemical, kinematic, and structural evolution.