Adarsh Kuruvanthodi

We report the spectroscopic discovery of a massive quiescent galaxy at z _spec = 7.29 ± 0.01, just ∼700 Myr after the big bang. RUBIES-UDS-QG-z7 was selected from public JWST/NIRCam and MIRI imaging from the PRIMER survey and observed with JWST/NIRSpec as part of RUBIES. The NIRSpec/PRISM spectrum reveals one of the strongest Balmer breaks observed thus far at z > 6, with no emission lines but tentative Balmer and Ca absorption features, as well as a Lyman break. Simultaneous modeling of the NIRSpec/PRISM spectrum and NIRCam and MIRI photometry (spanning 0.9–18 μm) shows that the galaxy formed a stellar mass of log ( M * / M ⊙ ) = 10.2 3 − 0.04 + 0.04 before z ∼ 8 and ceased forming stars 50–100 Myr prior to the time of observation, resulting in log ( sSFR/Gyr − 1 ) < − 1 . We measure a small physical size of 20 9 − 24 + 33 pc , which implies a high stellar-mass surface density within the effective radius of log ( Σ * , e / M ⊙ kpc − 2 ) = 10.8 5 − 0.12 + 0.11 , comparable to the highest densities measured in quiescent galaxies at z ∼ 2–5. The 3D stellar-mass density profile of RUBIES-UDS-QG-z7 is remarkably similar to the central densities of local massive ellipticals, suggesting that at least some of their cores may have already been in place at z > 7. The discovery of RUBIES-UDS-QG-z7 has strong implications for galaxy formation models: the estimated number density of quiescent galaxies at z ∼ 7 is >100 × larger than predicted from any model to date, indicating that quiescent galaxies have formed earlier than previously expected.

We report the discovery of robust spectroscopically confirmed Balmer break (BB) galaxies and candidates, with secure spectroscopic redshifts 7.1 ≤ z ≤ 9.6, from publicly available James Webb Space Telescope (JWST) extra-galactic photometric and spectroscopic surveys. To achieve this, we used dedicated filters probing the BB and inspected the objects with NIRSpec spectroscopy. We have recovered the previously known objects with strong BBs and here reveal 10−11 new objects with clear BBs, thus tripling the number of spectroscopically confirmed galaxies with a BB at z > 7. Approximately half of them show a pure BB and no signs of recent star formation, whereas the other half show BB and emission lines, most likely indicating galaxies whose star formation ceased earlier and has restarted recently. Overall, we find that ∼10 − 20% of all galaxies from our sample show signatures of an evolved stellar population. Furthermore, we find that the strength of the BB does not significantly depend on the rest-UV and rest-optical brightness of these sources. In short, our work confirms that photometry alone has the potential to measure BB strengths and to identify evolved stellar populations at high redshift, and that such objects may be more frequent than previously thought. The presence of galaxies with a range of break strengths and the joint presence of BB and emission lines indicate a bursty nature of the star formation in the early Universe.

Context. Recent James Webb Space Telescope (JWST) spectroscopic observations of the z = 10.6 galaxy GN-z11 have revealed a very peculiar UV spectrum exhibiting intense emission lines of nitrogen, which are not typically detected in galaxy spectra. This observation indicates a super-solar N/O abundance ratio at low metallicity, which only resembles the abundances seen in globular cluster (GC) stars. This discovery suggests that we might be seeing proto-GCs in formation or possibly even signatures of supermassive stars. Aims. To examine whether other objects with strong N IV and/or N III emission lines (N-emitters, hereafter) exist and to better understand their origin and nature, we have examined the available JWST spectra and data from the literature. Methods. Using the NIRSpec/JWST observations from CEERS, we found an extreme N-emitter, CEERS-1019 at z = 8.6782, showing intense N IV] λ1486 and N III] λ1750 emission. From the observed rest-UV and optical lines, we conclude that it is compatible with photoionization from stars and we have determined accurate abundances for C, N, O, and Ne, relative to H. We also (re-)analyzed other N-emitters from the literature, including three lensed objects at z = 2.3 − 3.5 (Sunburst cluster, SMACS2031, and Lynx arc) and a low-redshift compact galaxy, Mrk 996. We carried out a comparison among the observed abundance ratios to observations from normal star-forming galaxies, predicted wind yields from massive stars, and predictions from supermassive stars (SMS with ∼10⁴ − 10⁵M_⊙). Results. For CEERS-1019, we find a highly supersolar ratio log(N/O)= − 0.18 ± 0.11, and abundances of log(C/O)= − 0.75 ± 0.11 and log(Ne/O)= − 0.63 ± 0.07, which are normal compared to other galaxies at the low metallicity (12 + log(O/H) = 7.70 ± 0.18) of this galaxy. The three lensed N-emitters also show strongly enhanced N/O ratios and two of them normal C/O. The high N/O abundances can be reproduced by massive star winds assuming a special timing and essentially no dilution with the ambient interstellar medium (ISM). Alternatively, these N/O ratios can be explained by mixing the ejecta of SMS with comparable amounts of unenriched ISM. Massive star ejecta (from WR stars) are needed to explain the galaxies with enhanced C/O (Lynx arc, Mrk 996). On the other hand, a SMS in the “conveyer-belt model” (put forward to explain globular clusters) would predict a high N/O and small changes in C/O, compatible with CEERS-1019, the Sunburst cluster, SMACS2031, and GN-z11. Based on the chemical abundances, possible enrichment scenarios, and other properties (e.g., their compactness and high ISM density), we discuss which objects could contain proto-GCs. We suggest that this is the case for CEERS-1019, SMACS2031, and the Sunburst cluster. Enrichment in the Lynx arc and Mrk 996 is likely due to normal massive stars (WR), which implies that the star-forming regions in these objects cannot become GCs. Finally, we propose that some N-emitters enriched by SMS could also have formed intermediate mass black holes and we suggest that this might be the case for GN-z11. Conclusions. Our observations and analysis reinforce the suggested link between some N-emitters and proto-GC formation, which is supported both by empirical evidence and quantitative models. Furthermore, the observations provide possible evidence for the presence of supermassive stars in the early Universe (z > 8) and at z ∼ 2 − 3. Our analysis also suggests that the origin and nature of the N-emitters is diverse, including objects such as GN-z11, which may possibly host an active galactic nucleus (AGN).

Context.Supermassive stars (SMSs) with massesM ≳ 10³ − 10⁴M_⊙formed by runaway collisions in young, massive, and dense star clusters have been invoked as a possible solution to the problem of the presence of multiple stellar populations and peculiar abundance patterns observed in globular clusters (GCs). However, no such objects have been observed so far.Aims.We aim to develop observational strategies to search for SMSs hosted within young massive clusters (thought to be the precursors of GCs) using both photometric and spectroscopic observations. Such strategies could be applicable in a relatively general fashion.Methods.We used theoretical predictions of the spectra of SMSs and SMS-hosting clusters, together with predictions from standard simple stellar populations to examine their impact on color–color diagrams and on individual optical spectral lines (primarily hydrogen emission and absorption lines). As a first step, we applied our search strategies to a sample of about 3000 young star clusters (YSCs) from two nearby galaxies with multiband observations from the HST and optical integral-field spectroscopy obtained with MUSE on the Very Large Telescope.Results.We focus on models for SMSs with large radii (corresponding toT_eff ≲ 7000 K), which predict strong Balmer breaks, and construct proper color–color diagrams to select the corresponding SMS-hosting cluster candidates. We show that the spectrophotometric properties of these latter are similar to those of normal clusters with ages of a few hundred million years. However, the cluster SEDs show signs of composite stellar populations due to the presence of nebular lines (Hαand others). Examining the photometry, overall SEDs, and the spectra of approximately 100 clusters with strong Balmer breaks, we find several objects with peculiar SEDs, the presence of emission lines, or other peculiar signatures. After careful inspection of the available data, we do not find good candidates of SMS-hosting clusters. In most cases, the composite spectra can be explained by multiple clusters or H IIregions inside the aperture covered by the spectra, by contamination from a planetary nebula or diffuse gas, or by improper background subtraction. Furthermore, most of our candidate clusters are too faint to host SMSs.Conclusions.We demonstrate a strategy to search for SMSs by applying it to a sample of YSCs in two nearby galaxies. Our method can be applied to larger samples and also extended to higher redshifts with existing and upcoming telescopes, and therefore should provide an important test for GC-formation scenarios invoking such extreme stars.

Unusually high N/O abundance ratios were recently reported for a very compact, intensively star-forming object GN-z11 at z = 10.6 from JWST/NIRSpec observations. We present an empirical comparison with the C, N, and O abundance ratios in Galactic globular clusters (GCs) over a large metallicity range. We show that hot hydrogen-burning nucleosynthesis within supermassive stars (SMS) formed through runaway collisions can consistently explain the observed abundances ratio in GN-z11 and in GCs. This suggests that a proto-globular cluster hosting a SMS could be at the origin of the strong N-enrichment in GN-z11. Our model predicts the behavior of N/O, C/O, and Ne/O ratios as a function of metallicity, which can be tested if high-z objects similar to GN-z11 are detected with JWST in the future. Further studies and statistics will help differentiate the proto-GC scenario from the Wolf-Rayet scenario that we quantify with a population synthesis model, and shed more light on this peculiar object.