Outflows and extended [CII] halos in high redshift galaxies

Abstract

Recent stacked ALMA observations have revealed that normal, star-forming galaxies at $z\approx 6$ are surrounded by extended ($\approx 10\mathrm{kpc}$) [CII] emitting halos which are not predicted by the most advanced, zoom-in simulations. We present a model in which these halos are the result of supernova-driven cooling outflows. Our model contains two free parameters, the outflow mass loading factor, $\eta$, and the parent galaxy dark matter halo circular velocity, $v_c$. The outflow model successfully matches the observed [CII] surface brightness profile if $\eta =3.20\pm 0.10$ and $v_c=170\pm 10\mathrm{km}{\mathrm{s}}^{-1}$, corresponding to a dynamical mass of $\approx 10^{11}{\mathrm{M}}_{\odot }$. The predicted outflow rate and velocity range are $128\pm 5{\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1}$ and $300-500\mathrm{km}{\mathrm{s}}^{-1}$, respectively. We conclude that: (a) extended halos can be produced by cooling outflows; (b) the large $\eta$ value is marginally consistent with starburst-driven outflows, but it might indicate additional energy input from AGN; (c) the presence of [CII] halos requires an ionizing photon escape fraction from galaxies $f_{\mathrm{esc}}\ll 1$. The model can be readily applied also to individual high-$z$ galaxies, as those observed, e.g., by the ALMA ALPINE survey now becoming available.

13 pages, 7 figures; accepted for publication in MNRAS