We present velocities of galactic outflows in seven star-forming galaxies at z = 5-6 with stellar masses of M ∗ ∼ 1010.1M⊙. 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 b Vmax=700+180 -110km s-1and = -v 400+ out 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 ∗ ∼ 1010.1M ⊙. we find that the vmax 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 vcir from the stellar masses and the abundance matching results, we investigate a vmax-vcir relation. Interestingly, vmax for galaxies with M*=1010.0-10.8M⊙ shows a clear positive correlation with vcir 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 vmax at fixed M is explained by the increase in vcir toward high redshift.
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