Astronomy Colloquium

I will present observational constraints on the formation and evolution of two separate components of late-type spiral galaxies, their nuclear star clusters and their disks. Both projects stem from HST observations of nearby, edge-on galaxies. First, I will show nuclear clusters that have both compact (<10 pc) disk and spheroidal components. These compact disk components are aligned with the major axis of the galaxy disk and have younger stellar populations than the spheroidal components. Combined with spectroscopic evidence of multiple stellar populations, these observations strongly suggest that nuclear star clusters form in situ, with episodic star formation occurring in compact disks. Second, I will examine the vertical structure of six low mass, late-type spiral galaxies. By using resolved stars to track populations of different ages, I will demonstrate that older populations have larger disk scale heights. The metallicity of the oldest population is Z ~ -1, similar to the Milky Way thick disk, and does not vary with height above the disk. These observations require both disk heating and merging to play a role in the formation of these disks. Finally, I will present the preliminary detection of a low-metallicity stellar halo in one of these galaxies, NGC 4244.