An X-ray-Detected Quiescent Galaxy at \(z=2.09\): Implications for the Connection between AGNs and Galaxy Quenching at High Redshift

We report a characterization of an X-ray-detected quiescent galaxy at \(z=2.09\), named COS-XQG1, using JWST/NIRCam and NIRSpec data. This galaxy is detected in Chandra imaging, suggesting the presence of an AGN with a high black hole accretion rate of \(\dot{M}_{\rm BH}=0.22\pm0.03\, {\rm M_\odot yr^{-1}}\). Using multi-wavelength photometry from X-ray to sub-millimeter, including the latest JWST imaging, we confirm that COS-XQG1 is massive (\(M_\star = (1.6\pm0.2)\times10^{11}\, M_\odot\)) and quiescent (\({\rm sSFR}=(0.9\pm 1.8)\times10^{-11}\, {\rm yr^{-1}}\)) as reported previously, even considering the contribution from AGN emission. Noticeably, COS-XQG1 displays a broad line H\(\alpha\) emission component with a full width at half maximum of \(4491^{+118}_{-110}\, {\rm km\, s^{-1}}\) in its NIRSpec spectrum. The line width and luminosity of the broad H\(\alpha\) emission give a black hole mass of \(\log{(M_{\rm BH}/M_\odot)} = 8.45\pm0.02\, (\pm 0.5)\). With a stellar velocity dispersion measurement (\(\sigma_\star=235\pm35\, {\rm km\, s^{-1}}\)), we find that this galaxy is consistent with the local relations in the \(M_{\rm BH} - \sigma_\star\) and \(M_{\rm BH}- M_\star\) planes, which might suggest that massive quiescent galaxies at \(z\geq2\) have already been mature in terms of both stellar and black hole masses and will not evolve significantly. In addition, image 2D-decomposition analysis finds that this galaxy comprises disk and point source components. The latter is likely the composition of an AGN and a stellar bulge. Based on a comparison with numerical simulations, we expect that COS-XQG1 will evolve into a typical bulge-dominated quiescent galaxy with lower AGN activity by redshift 0. This study shows the usefulness of X-ray-detected quiescent galaxies in investigating the co-evolution between SMBHs and galaxies in the early Universe.