The stellar, gas, and dynamical masses of star-forming galaxies at z ∼ 2

We present analysis of the near-infrared spectra of 114 rest-frame UV-selected star-forming galaxies at z similar to 2. By combining the H alpha spectra with photometric measurements from observed 0.3-8 mu m, we assess the relationships among kinematics, dynamical masses, inferred gas fractions, and stellar masses and ages. The H alpha line widths give a mean dynamical mass M-dyn = (6.9 +/- 0: 6) X 10(10) M-circle dot within a typical radius of similar to 6 kpc, after excluding AGNs. The average dynamical mass is similar to 2 times larger than the average stellar mass, and the two agree to within a factor of several for most objects. However, similar to 15% of the sample has M-dyn >> M-*. These objects are best fit by young stellar populations and tend to have high H alpha equivalent widths, W-H alpha greater than or similar to 200 angstrom, suggesting that they are young starbursts with large gas masses. Rest-frame optical luminosity and velocity dispersion are correlated with 4 sigma significance. Using the local empirical correlation between star formation rate per unit area and gas surface density, we estimate the mass of the gas associated with star formation and find a mean gas fraction of similar to 50% and a strong decrease in gas fraction with increasing stellar mass. The masses of gas and stars combined are considerably better correlated with the dynamical masses than are the stellar masses alone, and agree to within a factor of 3 for 85% of the sample. The combination of kinematic measurements, estimates of gas masses, and stellar population properties suggest that the factor of similar to 500 range in stellar mass across the sample cannot be fully explained by intrinsic differences in the total masses of the galaxies, which vary by a factor of similar to 40; the remaining variation is due to the evolution of the stellar population and the conversion of gas into stars.