Redshift Evolution of Galactic Outflows Revealed with Spectra of Large Surveys

We present observational results of galactic outflows from star-forming galaxies at z~0--2 based on optical spectra with absorption lines of NaID, MgI, MgII, CII, and CIV. The spectra of galaxies at z~0, 1, and 2 are taken from the large-survey data sets of SDSS DR7, DEEP2 DR4, and Erb et al. 2006, respectively. To remove possible systematics of galaxy samples at different redshifts, we carefully construct large and homogeneous samples with similar stellar mass distributions ranging from 10^9 to 10^11.5 solar mass. We stack the galaxy spectra in our galaxy samples, and perform the multi-component fitting of model absorption lines and stellar continua to the stacked spectra. We thus obtain the central (v_out) and maximum (v_max) outflow velocities and estimate the mass loading factors (eta) that are defined by the ratio of the mass outflow rate to the star formation rate (SFR). Because our optical spectra do not cover all of the absorption lines at each redshift, for investigating the redshift evolution, we compare galactic outflow velocities of different redshifts with the absorption lines of atoms and ions whose ionization energies (IEs) are similar: NaID and MgI at z~0--1 (IE ~ 5--8 eV) and MgII and CII at z~ 1--2 (IE ~ 20 eV). We identify, for the first time, that the average value of v_out (v_max) monotonically increases. Moreover, based on the absorption lines of NaID at z~0, MgI at z~1, and CII at z~2, we find that the mass loading factor increases from z~0 to 2 by a factor of (1+z)^{1.3 +/- 0.5} at a given halo circular velocity (v_cir), albeit with a potential systematics caused by model parameter choices. The redshift evolution of v_out (v_max) and eta are probably explained by high gas fractions in high-redshift massive galaxies, which is supported by recent radio observations