Star Cluster Formation Efficiency in the Andromeda Galaxy

Thesis (Ph.D.)--University of Washington, 2015This work revolutionizes the study of star clusters in the Local Group galaxy Andromeda (Messier 31) using high spatial resolution, multi-wavelength imaging from the Hubble Space Telescope obtained as part of the Panchromatic Hubble Andromeda Treasury (PHAT) survey. I construct a cluster catalog using visual identification methods, including the use of image classifications collected from citizen scientist volunteers as part of the Andromeda Project. I perform analysis that combines large numbers of volunteer classifications with expert-derived identifications to yield a well-characterized census of star clusters. The resulting cluster catalog surpasses existing Galactic and extragalactic samples in terms of completeness and uniformity, serves as the basis for a wide range of current and future investigations of star formation and stellar evolution in M31, and is an important legacy data product of the PHAT survey. I use the PHAT star cluster catalog and associated cluster age and mass characterizations to study two aspects of star cluster formation. First, I investigate cluster formation efficiency and its dependence on star formation intensity. This study combines detailed measurements of cluster ages and masses with star formation histories of underlying total stellar populations, where both sets of constraints are derived from fitting color-magnitude diagrams of individually resolved stars. I find that ~4% of young stars (10-100 Myr old) in M31 are born in long-lived star clusters, and demonstrate that this fraction varies systematically as a function of star formation rate surface density (ΣSFR) and gas depletion time (τdep). The results derived here agree with trends established by previous observations of nearby galaxies in which cluster formation efficiency correlates with star formation rate intensity. The spatially-resolved measurements of cluster formation efficiency in M31 are also consistent with theoretical predictions, providing observational support for a model of cluster formation in which star clusters are born in regions of high gas density and star formation efficiency within a hierarchically-structured interstellar medium. In addition to star cluster formation efficiency, I also study the mass distribution of the young cluster populations (10-300 Myr old) in M31. I find that the mass function shape is well described by a Schechter function, with a power law index of α = −1.99 ± 0.12 and a characteristic mass of Mc = 8.5+2.8−1.8 M⊙. This exponential high-mass truncation of the cluster mass function occurs at a significantly lower mass in M31 than found for other nearby star forming galaxies. I show for the first time that the exponential truncation of the cluster mass function varies systematically with star formation rate intensity, such that the characteristic Schechter mass increases with star formation rate surface density as Mc ∝ ΣSFR^~1.3. Additionally, I explore the possibility that the Mc–ΣSFR relation derived here might also apply to old globular cluster systems, and thus be useful in constraining properties of star formation environments in the early universe