Investigating Ionized Gas Emission in Massive Quiescent Galaxies at Cosmic Noon with JWST

The study of massive, quiescent galaxies at high redshift is crucial for understanding the physical processes driving galaxy evolution. The aim of this work is to analyze ionized gas emission lines of quiescent galaxies at z ∼ 2, in order to investigate the underlying sources of ionization and thus constrain the quenching mechanism. This work is based on the Blue Jay survey which observed 147 galaxies at 1.7 < z < 3.5 in the COSMOS field with JWST. From these, a sample of 22 massive quiescent galaxies was selected. Emission lines in their spectra were fitted using single Gaussian model profiles. At least one emission line from ionized gas was detected with SNR>3 in 82% of the sample, indicating the presence of continued ionizing sources in this passive population. I obtained the line flux ratios and used them in line diagnostic diagrams. All of the diagrams found that most of the sample’s galaxies fall under the AGN-powered excitation range: of the whole sample, 54.5% were able to be classified using diagnostic diagrams, and 67% of these were confirmed to host an active nucleus. The current SFR of each galaxy was measured from the Hα line luminosity. The majority of the sample was confirmed to be quiescent. Finally, in 4 galaxies I unveiled the presence of powerful ionized gas outflows. These ionized gas outflows have measured velocities of the order of 1000s of km/s and are thus only traceable back to AGN-feedback at work. The general picture emerging from this study is that massive quiescent galaxies at Cosmic Noon may harbour a significant fraction of never-before-detected AGNs, strongly indicating AGN feedback as the primary channel of star-formation quenching at work. The detection of ionized gas outflows in a subset of active galaxies in the sample is direct evidence of AGN feedback at work through gas ejection at very high velocities, reinforcing the link between quenching of star formation in massive quiescent galaxies and AGN activity.