2019ApJ...886...29S

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⁺¹⁸⁰kms^–1 and v_out=400_–150⁺¹⁰⁰kms^–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.