The formation and evolution of late -type galaxies

Late-type spiral and irregular galaxies are the most abundant type of galaxy in the local universe and provide an analog to the environments in which star formation occurred at high redshift. Studying these systems locally gives us the opportunity to use individual stars and star clusters as tools to examine their history and morphology in detail. For my thesis, I have conducted three separate projects focusing on how late-type galaxies are built up.

The first project presents a study of the globular cluster systems of 11 irregular galaxies in Virgo and Fornax. From analysis of cluster sizes, colors, and magnitudes, I show that the globular cluster systems in these galaxies: (1) are metal poor, (2) have core radii significantly larger than Galactic globular clusters, and (3) have a larger number of clusters per unit luminosity than more massive late-type galaxies, suggesting differences in their early histories.

The second project presents an analysis of the vertical disk structure of resolved stars in a sample of nearby, edge-on spiral galaxies observed with HST. In all six galaxies studied, I find that the scale height of a stellar population increases with stellar age. The oldest, thickest population of red giant branch stars have metallicities similar to the Milky Way thick disk, and have no gradient in the metallicity with increasing height above the disk. These observations significantly constrain the formation mechanism of these disks, indicating a need both for mergers and slow disk-heating.

In the final project, I examine the very prominent star clusters located in the nuclei of late-type galaxies. From the edge-on perspective, I find that three of nine nuclear cluster candidates in our sample are resolved into both a spheroid and disk component. The disk components are bluer than the spheroid components, with the relative colors suggesting an age for the disk of <1 Gyr. The multiple populations are confirmed spectroscopically for one cluster. These observations strongly favor an in situ formation mechanism in which stars form episodically from a gas disk accreted onto the nucleus and then gradually lose angular momentum to end up in a spheroid.