A Census of Metals in Low-Mass Galaxies: Quantifying the Metal Retention as a Function of Mass

The metallicities of galaxies scale with stellar mass. Theoretical notions have long held that, in addition to lower star formation rates and efficiencies, the low metallicities of dwarf galaxies reflect their increasing inability to retain metals. This is seen in cosmological simulations where feedback-driven galactic winds transport metals from galaxy disks to large distances, with a steep inverse dependence of the amount of metals lost with galaxy mass.

At long last, it is possible to test this framework of stellar feedback and metal loss by actually tracking the metals in real galaxies. We propose to measure the production, distribution, and retention of metals in the stars and in the ISM in a large sample of low-mass, nearby galaxies over the critical mass range where metal loss is predicted to change the most but has yet to be investigated (i.e., 10^6 ~ 10^9 Msun). We will use an innovative technique that couples star formation histories derived from resolved stars with stellar and ISM metallicity constraints. Our program will measure, for the first time in the low-mass galaxy regime, the mean metal retention fraction as a function of galaxy mass, the dispersion on the relationship, and the differences in retention between the stars and the ISM. This ''accounting'' of the metals will provide significant constraints for models that will be far more detailed than global scaling relations with interdependent variables. Such analysis is only possible on nearby galaxies and our archival program capitalizes on the rich datasets available from the HST, supplemented by additional data from the VLA and ground-based observatories.