Resolving the Nature of the Stellar Halo of the Sombrero, the Nearest Giant Early-Type Spiral Galaxy

Early-type spiral galaxies lie at the apex of the Hubble sequence of galaxies between spirals and ellipticals, and likely play a key role in the morphological and dynamical evolution of galaxies. At a distance of 9 Mpc, the Sombrero (NGC 4594) is the nearest giant early-type spiral galaxy. As such, it is a keystone object for understanding the evolution and early star formation history of such galaxies.

Metallicity distribution functions (MDFs) of galaxy halos place important constraints on the hierarchical formation and assembly histories of galaxies. We therefore propose to obtain deep V- and I-band imaging of halo fields in the Sombrero galaxy using WFC3 and ACS in parallel to determine MDFs of individual red giant stars, extending in radius from the outer bulge to the limits of the halo. These data will enable us to address fundamental questions regarding early-type spiral galaxy assembly such as: At what galactocentric radius does the metal-poor field star component start dominating the metal-rich one? How do the radial distributions of metal-poor and metal-rich globular cluster subpopulations compare with those of the halo field stars at the same metallicities? What is the number of massive globular clusters per unit field star for populations of different metallicity? Is there a stellar metallicity gradient in the halo? This is the first such study for any early-type spiral galaxy, and it will provide a new window on the early star formation history of such galaxies, providing unique constraints on the hierarchical assembly scenario.