CONSTRAINT ON THE INFLOW/OUTFLOW RATES IN STAR-FORMING GALAXIES AT z ∼ 1.4 FROM MOLECULAR GAS OBSERVATIONS

We constrain the rate of gas inflow into and outflow from a main-sequence star-forming galaxy at z similar to 1.4 by fitting a simple analytic model for the chemical evolution in a galaxy to the observational data of the stellar mass, metallicity, and molecular gas mass fraction. The molecular gas mass is derived from CO observations with a metallicity-dependent CO-to-H-2 conversion factor, and the gas metallicity is derived from the H alpha and [N II]lambda 6584 emission line ratio. Using a stacking analysis of CO integrated intensity maps and the emission lines of H alpha and [N II], the relation between stellar mass, metallicity, and gas mass fraction is derived. We constrain the inflow and outflow rates with least-chi-square fitting of a simple analytic chemical evolution model to the observational data. The best-fit inflow and outflow rates are similar to 1.7 and similar to 0.4 in units of star formation rate (SFR), respectively. The inflow rate is roughly comparable to the sum of the SFR and outflow rate, which supports the equilibrium model for galaxy evolution; i.e., all inflow gas is consumed by star formation and outflow.