SIMBAD references

We report a characterization of an X-ray-detected quiescent galaxy at $z=2.09$, named COS-XQG1, using JWST/Near-Infrared Camera (NIRCam) and Near-Infrared Spectrograph (NIRSpec) data. This galaxy is detected in Chandra imaging, suggesting the presence of an active galactic nucleus (AGN) with a high black hole accretion rate of $\dot{M}_{\rm BH}=0.22\pm 0.03\, {\rm M_\odot \, yr^{-1}}$. Using multiwavelength photometry from X-ray to submillimetre, including the latest JWST imaging, we confirm that COS-XQG1 is massive ($M_\star = (1.6\pm 0.2)\times 10^{11}\, {\rm M}_\odot$) and quiescent (${\rm sSFR}< 10^{-10}\, {\rm yr^{-1}}$) as reported previously, even considering the contribution from AGN emission. Noticeably, COS-XQG1 displays a broad ${\rm H} \, \beta$ and ${\rm H} \, \alpha$ emission component with a full width at half-maximum of $4365^{+81}_{-81}\, {\rm km\, s^{-1}}$ in its NIRSpec spectrum, which is typical of Type 1 AGNs. The line width and luminosity of the broad ${\rm H} \, \alpha$ emission give a black hole mass of $\log {(M_{\rm BH}/{\rm M}_\odot)} = 8.43\pm 0.02\, (\pm 0.5)$. With a stellar velocity dispersion measurement ($\sigma _\star =238\pm 32\, {\rm km\, s^{-1}}$), 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 $z\ge 2$ 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 disc 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 quiescent galaxy with lower AGN activity by redshift 0. This study shows the usefulness of X-ray-detected quiescent galaxies in investigating the coevolution between supermassive black holes and galaxies in the early Universe.