Kiloparsec view of a typical star-forming galaxy when the Universe was ∼1 Gyr old

Abstract

We present new Atacama Large Millimeter/Submillimeter Array observations of the [C II] 158 μm transition and the dust continuum in HZ4, a typical star-forming galaxy when the Universe was only ∼1 Gyr old (z ≈ 5.5). Our high ≈0.3″ spatial resolution allows us to study the relationships between [C II] line emission, star formation rate, and far-infrared emission on spatial scales of ∼2 kpc. In the central ∼4 kpc of HZ4, the [C II]/FIR is ∼3 × 10−3 on global scales as well as on spatially resolved scales of ∼2 kpc, comparable to the ratio observed in local moderate starburst galaxies such as M 82 or M 83. For the first time in an individual normal galaxy at this redshift, we find evidence for outflowing gas from the central star-forming region in the direction of the minor axis of the galaxy. The projected velocity of the outflow is ∼400 km s−1, and the neutral gas-mass outflow rate is ∼3 − 6 times higher than the star formation rate in the central region. Finally, we detect a diffuse component of [C II] emission, or [C II] halo, that extends beyond the star-forming disk and has a diameter of ∼12 kpc. The outflow, which has a velocity approximately half of the escape velocity of the system, most likely partly fuels the [C II] extended emission. Together with the kinematic analysis of HZ4 (presented in a forthcoming paper), the analysis supports the hypothesis that HZ4 is a typical star-forming disk at z ∼ 5 with interstellar medium conditions similar to present-day galaxies forming stars at a similar level, driving a galactic outflow that may already play a role in its evolution.