ALMA suggests outflows in z ∼ 5.5 galaxies

We present the first attempt to detect outflows from galaxies approaching the Epoch of Reionization (EoR) using a sample of nine star-forming (SFR = 31 ± 20 M⊙ yr- 1) z ˜ 5.5 galaxies for which the [C II]158 μm line has been previously obtained with Atacama Large Millimeter Array (ALMA). We first fit each line with a Gaussian function and compute the residuals by subtracting the best-fitting model from the data. We combine the residuals of all sample galaxies and find that the total signal is characterized by a flux excess of ˜0.5 mJy extended over ˜1000 km s-1. Although we cannot exclude that part of this signal is due to emission from faint satellite galaxies, we show that the most probable explanation for the detected flux excess is the presence of broad wings in the [C II] lines, signatures of starburst-driven outflows. We infer an average outflow rate of dot{M}=54± 23 M_{⊙} yr^{-1}, providing a loading factor η =dot{M}/SFR=1.7± 1.3 in agreement with observed local starbursts. Our interpretation is consistent with outcomes from zoomed hydro-simulations of Dahlia, a z ˜ 6 galaxy (SFR˜ 100 M_{⊙} yr^{-1}), whose feedback-regulated star formation results into an outflow rate dot{M}˜ 30 M_{⊙} yr^{-1}. The quality of the ALMA data is not sufficient for a detailed analysis of the [C II] line profile in individual galaxies. Nevertheless, our results suggest that starburst-driven outflows are in place in the EoR and provide useful indications for future ALMA campaigns. Deeper observations of the [C II] line in this sample are required to better characterize feedback at high-z and to understand the role of outflows in shaping early galaxy formation.