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Description
We present new Atacama Large Millimeter/Submillimeter Array (ALMA) observations of the [CII] 158um transition and the dust continuum in HZ4, a typical star-forming galaxy when the Universe was only ~1Gyr old (z~5.5). Our high ~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 ~2kpc. In the central ~4kpc of HZ4 the [CII]/FIR is ~3x10^-3^ on global scales as well as on spatially resolved scales of ~2kpc, comparable to the ratio observed in local moderate starburst galaxies such as M82 or M83. 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 ~400km/s, 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 [CII] emission, or [CII] halo, that extends beyond the star-forming disk and has a diameter of ~12kpc. 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~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.
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