# Evolution of Galactic Outflows at $z\sim0$-$2$ Revealed with SDSS, DEEP2, and Keck spectra

## TL;DR

This study analyzes galactic outflows from $z o0$ to 2$ using stacked optical spectra, revealing that outflow velocities and mass loading factors increase with redshift, consistent with galaxy evolution models and high gas fractions.

## Contribution

First systematic measurement of outflow velocity and mass loading factor evolution from $z o0$ to 2$ using homogeneous stacked spectra and multi-component absorption line fitting.

## Key findings

- Outflow velocities increase by 0.05-0.3 dex from $z o0$ to 2$ at fixed SFR.
- Mass loading factors $ o$ increase with redshift as $(1+z)^{1.2 extpm0.3}$.
- Results align with galaxy size evolution and high gas fractions at high redshift.

## Abstract

We conduct a systematic study of galactic outflows in star-forming galaxies at $z\sim0$-$2$ 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 $z\sim0$-$2$, and perform the multi-component fitting of model absorption lines and stellar continua to the stacked spectra. We obtain the maximum ($v_\rm{max}$) and central ($v_\rm{out}$) 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 $z\sim0$-$1$; Mg II and C II at $z\sim1$-$2$), we identify, for the first time, that the average value of $v_\rm{max}$ ($v_\rm{out}$) significantly increases by 0.05-0.3 dex from $z\sim0$ to $2$ at a given SFR. Moreover, we find that the value of $\eta$ increases from $z\sim0$ to $2$ by $\eta \propto (1 + z)^{1.2\pm0.3}$ at a given halo circular velocity $v_\rm{cir}$ , albeit with a potential systematics caused by model parameter choices. The redshift evolution of $v_\rm{max}$ ($v_\rm{out}$) 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 $\eta \propto v_\rm{cir}^a$ for $a = -0.2 \pm 1.1$ in our $z\sim0$ galaxies that agrees with the momentum-driven outflow model ($a = -1$) within the uncertainty.