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 _* ∼ 10^10.1 {M}_⊙ . Although it is challenging to observationally determine the outflow velocities, we overcome this by using ALMA [C II] 158 μm 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 [C II]-systemic velocities, and fit outflow-line profiles to the Si II λ1260, C II λ1335, and Si IV λλ1394,1403 absorption lines in the composite spectrum. We measure the maximum (90%) and central outflow velocities to be {v}_\max ={700}_-110⁺¹⁸⁰ {km} {{{s}}}^-1 and {v}_out}={400}_-150⁺¹⁰⁰ {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 _* ∼ 10^10.1 {M}_⊙ , we find that the {v}_\max value of our z = 5-6 galaxies is 3 times higher than those of z ∼ 0 galaxies and comparable to z ∼ 2 galaxies. Estimating the halo circular velocity {v}_cir} from the stellar masses and the abundance matching results, we investigate a {v}_\max -{v}_cir} relation. Interestingly, {v}_\max for galaxies with M _* = 10^10.0-10.8 {M}_⊙ shows a clear positive correlation with {v}_cir} and/or the galaxy star formation rate over z = 0-6 with a small scatter of ≃ +/- 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}_\max at fixed M _* is explained by the increase in {v}_cir} toward high redshift.