Fast Outflows Identified in Early Star-forming Galaxies at z = 5–6

Abstract

Abstract We present velocities of galactic outflows in seven star-forming galaxies at z = 5–6 with stellar masses of M * ∼ 1010.1 . 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 and on average, respectively, showing no significant differences between the outflow velocities derived with the low- to high-ionization absorption lines. For M * ∼ 1010.1 , we find that the 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 from the stellar masses and the abundance matching results, we investigate a – relation. Interestingly, for galaxies with M * = 1010.0–10.8 shows a clear positive correlation with and/or the galaxy star formation rate over z = 0–6 with a small scatter of 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 at fixed M * is explained by the increase in toward high redshift.