Evolution of Galactic Outflows at z ∼ 0-2 Revealed with SDSS, DEEP2, and Keck Spectra

We conduct a systematic study of galactic outflows in star-forming galaxies at z similar to 0-2 based on the absorption lines of optical spectra taken from SDSS DR7, DEEP2 DR4, and Keck (Erb et al.). We carefully make stacked spectra of homogeneous galaxy samples with similar stellar mass distributions at z similar to 0-2 and perform the multicomponent fitting of model absorption lines and stellar continua to the stacked spectra. We obtain the maximum (v(max)) and central (v(out)) outflow velocities and estimate the mass loading factors (eta), a ratio of the mass outflow rate to the star formation rate (SFR). Investigating the redshift evolution of the outflow velocities measured with the absorption lines whose depths and ionization energies are similar (Na I D and Mg I at z similar to 0-1; Mg II and C II at z similar to 1-2), we identify, for the first time, that the average value of v(max) (v(out)) significantly increases by 0.05-0.3 dex from z similar to 0 to 2 at a given SFR. Moreover, we find that the value of eta increases from z similar to 0 to 2 by eta proportional to (1 + z)(1.2 +/- 0.3) at a given halo circular velocity v(cir), albeit with a potential systematics caused by model parameter choices. The redshift evolution of v(max) (v(out)) and eta is consistent with the galaxy-size evolution and the local velocity-SFR surface density relation and explained by high gas fractions in high-redshift massive galaxies, which is supported by recent radio observations. We obtain a scaling relation of va eta proportional to nu(alpha)(cir) for a = -0.2 +/- 1.1 in our z similar to 0 galaxies that agrees with the momentum-driven outflow model (a = -1) within the uncertainties.