Kiloparsec view of a typical star-forming galaxy when the Universe was ∼1 Gyr old: I. Properties of outflow, halo, and interstellar medium

We present new Atacama Large Millimeter/Submillimeter Array observations of the [CII] 158 mu m transition and the dust continuum in HZ4, a typical star-forming galaxy when the Universe was only similar to 1 Gyr old (z approximate to 5.5). Our high approximate to 0.3 '' spatial resolution allows us to study the relationships between [CII] line emission, star formation rate, and far-infrared emission on spatial scales of similar to 2 kpc. In the central similar to 4 kpc of HZ4, the [CII]/FIR is similar to 3x10(-3) on global scales as well as on spatially resolved scales of similar to 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 similar to 400 km s(-1), and the neutral gas-mass outflow rate is similar to 3-6 times higher than the star formation rate in the central region. Finally, we detect a diffuse component of [CII] emission, or [CII] halo, that extends beyond the star-forming disk and has a diameter of similar to 12 kpc. The outflow, which has a velocity approximately half of the escape velocity of the system, most likely partly fuels the [CII] 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 similar to 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.

Automated summary

The new ALMA observations of the star-forming galaxy HZ4 at a redshift of approximately 5.5 reveal relationships between [CII] line emission, star formation rate, and far-infrared emission on spatial scales of about 2 kpc. Evidence of outflowing gas and diffuse [CII] emission beyond the star-forming disk indicate potential roles in the galaxy's evolution.