Chemical Evolution and a Census of Oxygen in Star-Forming Galaxies

I will present recent results on the chemical evolution of
star-forming Galaxies. I will begin by examining the evolution in the
relation between stellar mass and metallicity over the last ~11
billion years. The mass-metallicity (MZ ) relation taken together with
the relation between stellar mass and SFR can provide strong
constraints on the oxygen content of star-forming galaxies. From these
constraints I will show that there is a large deficit of oxygen in
star-forming galaxies in the sense that the amount of oxygen thought
to be produced in the galaxy is substantially greater than the amount
observed in the stars and gas. By comparing my results with recent
observations of the oxygen content of the hot halo of gas surrounding
galaxies I conclude that the oxygen missing within galaxies resides in
the hot halo. Thus star-forming galaxies must expel a substantial
fraction of their gas as they evolve and increase their stellar mass.