Fast Outflows Identified in Early Star-Forming Galaxies at $z=5-6$

Abstract

We present velocities of galactic outflows in seven star-forming galaxies at $z=$5-6 with stellar masses of $M_{\ast }\sim 10^{10.1}{\mathrm{M}}_{\odot }$. Although it is challenging to observationally determine the outflow velocities, we overcome this by using ALMA [CII]158 $\mu$m emission lines for systemic velocities and deep Keck spectra with metal absorption lines for velocity profiles available to date. We construct a composite Keck spectrum of the galaxies at $z=$5-6 with the [CII]-systemic velocities, and fit outflow-line profiles to the SiII1260, CII1335, and SiIV1394,1403 absorption lines in the composite spectrum. We measure the maximum (90%) and central outflow velocities to be $v_\rm{max}=700^{+180}_{-110}\ \rm{km\ s^{-1}}$ and $v_\rm{out}= 400^{+100}_{-150}\ \rm{km\ s^{-1}}$ on average, respectively, showing no significant differences between the outflow velocities derived with the low to high-ionization absorption lines. For $M_{\ast }\sim 10^{10.1}{\mathrm{M}}_{\odot }$, we find that the $v_\rm{max}$ value of our $z=$5-6 galaxies is 3 times higher than those of $z\sim 0$ galaxies and comparable to $z\sim 2$ galaxies. Estimating the halo circular velocity $v_\rm{cir}$ from the stellar masses and the abundance matching results, we investigate a $v_\rm{max}$-$v_\rm{cir}$ relation. Interestingly, $v_\rm{max}$ for galaxies with $M_{\ast }=10^{10.0-10.8}{\mathrm{M}}_{\odot }$ shows a clear positive correlation with $v_\rm{cir}$ and/or the galaxy star formation rate over $z=$0-6 with a small scatter of $\simeq \pm 0.1$ dex, which is in good agreement with theoretical predictions. This positive correlation suggests that the outflow velocity is physically related to the halo circular velocity, and that the redshift evolution of $v_\rm{max}$ at fixed $M_{\ast }$ is explained by the increase in $v_\rm{cir}$ toward high redshift.

13 pages, 7 figures, 4 tables; Accepted for publication in ApJ