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

We present velocities of galactic outflows in seven star-forming galaxies at $z=$5-6 with stellar masses of $M_* \sim10^{10.1}\ \rm{M_\odot}$. 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 $M_* \sim10^{10.1}\ \rm{M_\odot}$, we find that the v_\rm{max} value of our $z=$5-6 galaxies is 3 times higher than those of $z\sim0$ galaxies and comparable to $z\sim2$ 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 $M_*=10^{10.0-10.8}\ \rm{M_\odot}$ 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 $M_*$ is explained by the increase in v_\rm{cir} toward high redshift.