Understanding the rapid formation of supermassive black holes in the early Universe requires insights into stellar mass growth in host galaxies. Here we present NIRSpec rest-frame optical spectra and NIRCam imaging from JWST of two galaxies at z > 6, both hosting moderate-luminosity quasars. These galaxies exhibit Balmer absorption lines, like low-redshift post-starburst galaxies. Our analyses of the medium-resolution spectra and multiband photometry show that the bulk of the stellar mass (log(M-*/M-circle dot) >= 10.6) formed in starburst episodes at redshift 9 and 7. One of the galaxies shows a clear Balmer break and lacks spatially resolved H alpha emission. It falls well below the star-formation main sequence at z = 6, indicating quiescence. The other is transitioning to quiescence; together, these massive galaxies are among the most distant post-starburst systems known. The blueshifted wings of the quasar [O iii] emission lines indicate quasar-driven outflow, which possibly influences star formation. Direct stellar velocity dispersion measurements reveal that one galaxy follows the local black hole mass versus sigma(*) relation whereas the other is overmassive. The existence of massive post-starburst galaxies hosting billion-solar-mass black holes in short-lived quasar phases indicates that supermassive black holes and host galaxies played a principal role in each other's rapid early formation.

We report the spectroscopic confirmation of a merging pair of massive quiescent galaxies at z = 3.44. Using JWST observations, we confirm that the two galaxies lie at a projected separation of 4.5 kpc with a velocity offset of similar to 680 km s(-1) (delta(z) similar to 0.01). The pair resides in the core of a known rich overdensity of galaxies, dubbed the "Cosmic Vine". For both pair members, modeling of the spectral energy distributions and faint rest-frame optical emission lines indicate high stellar masses (log(M-star/M-circle dot) similar to 10.9) and suppressed star formation (log(sSFR/yr(-1)) < -10), more than an order of magnitude below the level of the star formation main sequence at this redshift. We then explore the Illustris-TNG simulation and the GAEA and SHARK semi-analytical models to examine whether they produce a pair of massive quiescent galaxies akin to that of the Cosmic Vine. While all models produce close pairs of massive quiescent galaxies at 2 < z < 4 with comparable separations and velocity offsets, their predicted number densities are 10-80 times lower than our observational constraint. This discrepancy cannot be fully explained by coarse time sampling in these models or the general challenge of forming early massive quiescent galaxies in simulations. Given that > 90% of simulated pairs in the models that we analyzed merge by z = 0, our findings suggest that our observed pair will likely coalesce into a single massive galaxy. The merger, occurring in the dense core of a large-scale structure, might represent a critical event in the formation of a brightest cluster galaxy and the morphological transformation of high-redshift disky quiescent galaxies into early-type ellipticals.

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 (M) over dot(BH)=0.22 +/- 0.03M(circle dot)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 +/- 0.2) x 10(11)M(circle dot)) and quiescent (sSFR<10(-1)0yr(-1)) as reported previously, even considering the contribution from AGN emission. Noticeably, COS-XQG1 displays a broad H beta and H alpha emission component with a full width at half-maximum of 4365(-81)(+81)kms(-1) in its NIRSpec spectrum, which is typical of Type 1 AGNs. The line width and luminosity of the broad H alpha emission give a black hole mass of log (M-BH/M-circle dot)=8.43 +/- 0.02(+/- 0.5). With a stellar velocity dispersion measurement (sigma(star)=238 +/- 32kms(-1)), we find that this galaxy is consistent with the local relations in the MBH-sigma(star) and M-BH-M-star planes, which might suggest that massive quiescent galaxies at z >= 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.

We present the analysis of three intermediate-mass quiescent galaxies (QGs) with stellar masses of ∼1010 M at redshifts z ∼ 3–4 using NIRSpec low-resolution spectroscopy. Utilizing the spectral energy distribution fitting code BAGPIPES, we confirm these target galaxies are consistent with quiescent population, with their specific star formation rates falling below 2 dex the star-forming main sequence at the same redshifts. Additionally, we identify these QGs to be less massive than those discovered in previous works, particularly prior to the JWST era. Two of our target galaxies exhibit the potentially blended H α + [N II] emission line within their spectra with signal-to-noise ratio >5. We discuss whether this feature comes from an active galactic nucleus (AGN) or star formation, although future high-resolution spectroscopy is required to reach a conclusion. One of the target galaxies is covered by JWST/NIRCam imaging of the Public Release IMaging for Extragalactic Research survey. Using the 2D profile fitting code GALFIT, we examine its morphology, revealing a disc-like profile with a Sérsic index of n = 1.1 ± 0.1. On the size–mass relation, we find a potential distinction between less massive (log10 (M∗/M) 10 (M∗/M) > 10.3) QGs in their evolutionary pathways. The derived quenching time-scales for our targets are less than 1 Gyr. This may result from these galaxies being quenched by AGN feedback, supporting the AGN scenario of the emission line features. © 2024 The Author(s)

We present the results of a systematic study of the rest-frame optical morphology of quiescent galaxies at z >= 3 using the Near-Infrared Camera (NIRCam) onboard JWST. Based on a sample selected by UVJ color or NUVUVJ color, we focus on 26 quiescent galaxies with 9.8= 3. The analytical fit for them at log(M-star/M-circle dot)>10.3 implies that our size - stellar mass relations are below those at lower redshifts, with the amplitude of similar to 0.6kpc at M-star=5x10(10)M(circle dot). This value agrees with the extrapolation from the size evolution of quiescent galaxies at z<3 in the literature, implying that the size of quiescent galaxies increases monotonically from z similar to 3-5. Our sample is mainly composed of galaxies with bulge-like structures according to their median S & eacute;rsic index and axis ratio of n similar to 3-4 and q similar to 0.6-0.8, respectively. On the other hand, there is a trend of increasing fraction of galaxies with low S & eacute;rsic index, suggesting 3 ...