Abstract
We study galactic outflows of star-forming galaxies at $z\sim$0-2 based on
optical spectra with absorption lines of NaID, MgI, MgII, CII, and CIV. The
spectra of galaxies at $z\sim$0, 1, and 2 are taken from the large-survey data
sets of SDSS DR7, DEEP2 DR4, and Erb et al. (2006), respectively. We carefully
construct large and homogeneous galaxy samples with similar stellar mass
distributions. We stack the galaxy spectra in our samples and perform the
multi-component fitting of model absorption lines to the stacked spectra. We
obtain the central ($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 outflow velocities at
different redshifts with the absorption lines whose depths and ionization
energies are similar. We identify, for the first time, that the average value
of $v_out$ ($v_max$) monotonically increases by 0.1-0.4 dex from
$z\sim$0 to 2 at the $\gtrsim5\sigma$ significance levels at a given SFR.
Moreover, based on the absorption lines of NaID at $z\sim0$, MgI at $z\sim1$,
and CII at $z\sim2$, we find that $\eta$ increases from $z\sim0$ to 2 by
$\eta\propto(1+z)^1.8\pm0.5$ 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. We obtain a scaling relation of
$\etav_cir^a$ for $a=-0.5\pm1.1$ in our $z\sim0$ galaxies. This
scaling relation agrees well with the momentum-driven outflow model ($a=-1$)
within the uncertainty.
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