Active Galactic Nucleus Feedback in Quiescent Galaxies at Cosmic Noon Traced by Ionized Gas Emission

We analyze ionized gas emission lines in deep rest-frame optical spectra of 14 massive ( log M* > 10.2 M⊙) quiescent galaxies at redshifts 1.7 < z < 3.5 observed with JWST/NIRSpec by the Blue Jay survey. Robust detection of emission lines in 71% of the sample indicates the presence of ongoing ionizing sources in this passive population. The Hα line luminosities confirm that the population is quiescent, with star formation rates that are at least 10 times lower than the main sequence of star formation at z ∼ 2. The quiescent sample is clearly separate from the star-forming population in line diagnostic diagrams, and occupies a region usually populated by active galactic nuclei (AGN). Analysis of the observed line ratios, equivalent widths, and velocity dispersions leads us to conclude that in most cases the gas is ionized by AGN activity, despite the lack of X-ray detections. We measure generally low values of the bolometric luminosity Lbol ∼ 1044 erg s−1 and Eddington ratio λ ∼ 10−2-10−3 for the central engines, typical of low-luminosity AGN. A subset of the sample also hosts ionized and neutral outflows, with ionized outflow velocities of the order of ∼1000 km s−1. Our results show, for the first time using a representative sample, that low-luminosity AGN are extremely common among quiescent galaxies at high redshift. These low-luminosity AGN may play a key role in quenching star formation and in maintaining massive galaxies quiescent from Cosmic Noon to z ∼ 0.