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The confirmation of massive quiescent galaxies emerging within the first billion years of the universe poses intriguing questions about the mechanisms of galaxy formation. There must be highly efficient process at work to shut down star formation in galaxies at cosmic dawn. I present the detection of neutral outflowing gas in a massive (M_ = 1. 2 10^11 M_) recently quenched AGN-host galaxy at z=4 as a evidence that AGN-driven outflows could be one such mechanism. Based on JWST spectrum, the star formation rate of this has been declining with a rapid e-folding timescale of 50~Myrs. The current specific star formation rate is 510^-11 yr^-1, roughly 40 times lower than that of the star-forming main sequence at comparable redshifts. Emission line ratios of NeIII/OII and OIII/H emission indicate the presence of an AGN. A series of FeII and MgII absorption lines appear blueshifted by 250 km~s^-1 relative to the stellar continuum, suggesting an outflow of neutral gas. The estimate mass outflow rate is approximately 7 times greater than star formation rate derived from the stellar continuum, implying that the suppression of star formation is likely due to gas being depleted by the AGN-driven outflow. This galaxy represents the most distant example of its kind known to date. This study offers a compelling explanation for the existence of massive quiescent galaxies in the first billion years of the universe.
Po-Feng Wu (Sat,) studied this question.