The Triangulum Extended (TREX) Survey: The Stellar Disk Dynamics of M33 as a Function of Stellar Age

TL;DR

This study uses the TREX Survey to analyze the stellar disk dynamics of M33 across different ages, revealing unexpectedly high and structured velocity dispersions that challenge existing models of galaxy evolution.

Contribution

It provides the first detailed age-dependent stellar kinematic analysis of M33's disk, highlighting discrepancies with simulated galaxy analogs and offering new insights into dynamical heating processes.

Findings

M33's stellar disk has low overall velocity dispersion (~16 km/s).

No significant trend in velocity dispersion with stellar age was observed.

Young stars are as dynamically hot as older stars, contrary to expectations.

Abstract

Triangulum, M33, is a low mass, relatively undisturbed spiral galaxy that offers a new regime in which to test models of dynamical heating. In spite of its proximity, the dynamical heating history of M33 has not yet been well constrained. In this work, we present the TREX Survey, the largest stellar spectroscopic survey across the disk of M33. We present the stellar disk kinematics as a function of age to study the past and ongoing dynamical heating of M33. We measure line of sight velocities for ~4,500 disk stars. Using a subset, we divide the stars into broad age bins using Hubble Space Telescope and Canada-France-Hawaii-Telescope photometric catalogs: massive main sequence stars and helium burning stars (~80 Myr), intermediate mass asymptotic branch stars (~1 Gyr), and low mass red giant branch stars (~4 Gyr). We compare the stellar disk dynamics to that of the gas using existing HI, CO, and Halpha kinematics. We find that the disk of M33 has relatively low velocity dispersion (~16 km/s), and unlike in the Milky Way and Andromeda galaxies, there is no strong trend in velocity dispersion as a function of stellar age. The youngest disk stars are as dynamically hot as the oldest disk stars and are dynamically hotter than predicted by most M33 like low mass simulated analogs in Illustris. The velocity dispersion of the young stars is highly structured, with the large velocity dispersion fairly localized. The cause of this high velocity dispersion is not evident from the observations and simulated analogs presented here.