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Nearly a decade ago, we began to see indications that reionization-era galaxies power hard radiation fields rarely seen at lower redshift. Most striking were detections of nebular C iv emission in what appeared to be typical low-mass galaxies, requiring an ample supply of 48 eV photons to triply ionize carbon. We have obtained deep JWST/NIRSpec R = 1000 spectroscopy of the two z > 6 C iv-emitting galaxies known prior to JWST. Here, we present a rest-UV to optical spectrum of one of these two systems, the multiply-imaged z = 6.1 lensed galaxy RXCJ2248-ID. NIRCam imaging reveals two compact (<22 pc) clumps separated by 220 pc, with one comprising a dense concentration of massive stars (>10 400 M⊙ yr−1 kpc−2) formed in a recent burst. We stack spectra of 3 images of the galaxy (J = 24.8–25.9), yielding a very deep spectrum providing a high-S/N template of strong emission line sources at z > 6. The spectrum reveals narrow high-ionization lines (He ii, C iv, N iv]) with line ratios consistent with powering by massive stars. The rest-optical spectrum is dominated by very strong emission lines ([O iii] EW = 2800 Å), albeit with weak emission from low-ionization transitions ([O iii]/[O ii] = 184). The electron density is found to be very high (6.4–31.0 × 104 cm−3) based on three UV transitions. The ionized gas is metal poor ($12+\log (\rm O/H)=7.43^{+0.17}_{-0.09}$), yet highly enriched in nitrogen ($\log (\rm N/O)=-0.39^{+0.11}_{-0.10}$). The spectrum appears broadly similar to that of GNz11 at z = 10.6, without showing the same AGN signatures. We suggest that the hard radiation field and rapid nitrogen enrichment may be a short-lived phase that many z > 6 galaxies go through as they undergo strong bursts of star formation. We comment on the potential link of such spectra to globular cluster formation.

 

The power-law slope of the rest-ultraviolet (UV) continuum (fλ ∝ λβ) is a key metric of early star-forming galaxies, providing one of our only windows into the stellar populations and physical conditions of z ≳ 10 galaxies. Expanding upon previous studies with limited sample sizes, we leverage deep imaging from the JWST Advanced Deep Extragalactic Survey (JADES) to investigate the UV slopes of 179 z ≳ 9 galaxies with apparent magnitudes of mF200W ≃ 26–31, which display a median UV slope of β = −2.4. We compare to a statistical sample of z ≃ 5–9 galaxies, finding a shift towards bluer rest-UV colours at all $M_{\rm UV}$. The most UV-luminous z ≳ 9 galaxies are significantly bluer than their lower redshift counterparts, representing a dearth of moderately red galaxies within the first 500 Myr. At yet earlier times, the z ≳ 11 galaxy population exhibits very blue UV slopes, implying very low impact from dust attenuation. We identify a robust sample of 44 galaxies with β ≲ −2.8, which have spectral energy distributions requiring models of density-bounded H ii regions and median ionizing photon escape fractions of 0.51 to reproduce. Their rest-optical colours imply that this sample has weaker emission lines (median mF356W − mF444W = 0.19 mag) than typical galaxies (median mF356W − mF444W = 0.39 mag), consistent with the inferred escape fractions. This sample consists of relatively low stellar masses (median $\log (M/{\rm M}_{\odot })=7.5\pm 0.2$), and specific star formation rates (sSFRs; median $=79 \, \rm Gyr^{-1}$) nearly twice that of our full galaxy sample (median sSFRs $=44 \, \rm Gyr^{-1}$), suggesting these objects are more common among systems experiencing a recent upturn in star formation. We demonstrate that the shutoff of star formation provides an alternative solution for modelling of extremely blue UV colours, making distinct predictions for the rest-optical emission of these galaxies. Future spectroscopy will be required to distinguish between these physical pictures.

 

The variety of star formation histories (SFHs) of z ≳ 6 galaxies provides important insights into early star formation, but has been difficult to systematically quantify. Some observations suggest that many z ∼ 6–9 galaxies are dominated by ≳200 Myr stellar populations, implying significant star formation at z ≳ 9, while others find that most reionization era galaxies are ≲10 Myr, consistent with little z ≳ 9 star formation. Here, we quantify the distribution of ages of UV-bright ($-22.5\lesssim M_{\rm \small UV}\lesssim -21$) galaxies colour-selected to lie at z ≃ 6.6–6.9, an ideal redshift range to systematically study the SFHs of reionization era galaxies with ground-based observatories and Spitzer. We infer galaxy properties with two SED modelling codes and compare results, finding that stellar masses are largely insensitive to the model, but the inferred ages can vary by an order of magnitude. We infer a distribution of ages assuming a simple, parametric SFH model, finding a median age of ∼30–70 Myr depending on SED model. We quantify the fractions of ≤10 and ≥250 Myr galaxies, finding that these systems comprise ∼15–30 per cent and ∼20–25 per cent of the population, respectively. With a flexible SFH model, the shapes of the SFHs are consistent with those implied by the simple model (e.g. young galaxies have rapidly rising SFHs). However, stellar masses can differ significantly, with those of young systems sometimes being more than an order of magnitude larger with the flexible SFH. We quantify the implications of these results for z ≳ 9 stellar mass assembly and discuss improvements expected from JWST.

 

The first deep-field observations of the JWST have immediately yielded a surprisingly large number of very high redshift candidates, pushing the frontier of observability well beyond z ≳ 10. We here present a detailed SED-fitting analysis of the 10 gravitationally lensed z ∼ 9–16 galaxy candidates detected behind the galaxy cluster SMACS J0723.3−7327 in a previous paper using the BEAGLE tool. Our analysis makes use of dynamical considerations to place limits on the ages of these galaxies and of all three published SL models of the cluster to account for lensing systematics. We find the majority of these galaxies to have relatively low stellar masses $M_{\star }\sim 10^7-10^8\, \mathrm{M}_{\odot }$ and young ages tage ∼ 10–100 Myr but with a few higher mass exceptions ($M_{\star }\sim 10^9\rm{-}10^{10}\, \mathrm{M}_{\odot }$) due to Balmer-break detections at z ∼ 9–10. Because of their very blue UV-slopes, down to β ∼ −3, all of the galaxies in our sample have extremely low dust attenuations AV ≲ 0.02. Placing the measured parameters into relation, we find a very shallow M⋆ − MUV-slope and high sSFRs above the main sequence of star formation with no significant redshift-evolution in either relation. This is in agreement with the bright UV luminosities measured for these objects and indicates that we are naturally selecting UV-bright galaxies that are undergoing intense star formation at the time they are observed. Finally, we discuss the robustness of our high-redshift galaxy sample regarding low-redshift interlopers and conclude that low-redshift solutions can safely be ruled out for roughly half of the sample, including the highest redshift galaxies at z ∼ 12–16. These objects represent compelling targets for spectroscopic follow-up observations with JWST and ALMA.

 

One of the main goals of the JWST is to study the first galaxies in the Universe. We present a systematic photometric analysis of very distant galaxies in the first JWST deep field towards the massive lensing cluster SMACS0723. As a result, we report the discovery of two galaxy candidates at z ∼ 16, only 250 million years after the big bang. We also identify two candidates at z ∼ 12 and six candidates at z ∼ 9−11. Our search extended out to z ≲ 21 by combining colour information across seven near-infrared camera and near-infrared imager and slitless spectrograph filters. By modelling the Spectral Energy Distributions (SEDs) with EAZY and BEAGLE, we test the robustness of the photometric redshift estimates. While their intrinsic (unlensed) luminosity is typical of the characteristic luminosity L* at z > 10, our high-redshift galaxies typically show small sizes and their morphologies are consistent with disks in some cases. The highest-redshift candidates have extremely blue ultraviolet-continuum slopes −3 < β < −2.4, young ages ∼10−100 Myr, and stellar masses around log (M⋆/M⊙) = 8.8 inferred from their spectral energy distribution modelling, which indicate a rapid build-up of their stellar mass. Our search clearly demonstrates the capabilities of JWST to uncover robust photometric candidates up to very high redshifts and peer into the formation epoch of the first galaxies.

 

We present a catalog of 717 candidate galaxies atz> 8 selected from 125 square arcmin of NIRCam imaging as part of the JWST Advanced Deep Extragalactic Survey (JADES). We combine the full JADES imaging data set with data from the JWST Extragalactic Medium Survey and First Reionization Epoch Spectroscopic COmplete Survey (FRESCO) along with extremely deep existing observations from Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) for a final filter set that includes 15 JWST/NIRCam filters and five HST/ACS filters. The high-redshift galaxy candidates were selected from their estimated photometric redshifts calculated using a template-fitting approach, followed by visual inspection from seven independent reviewers. We explore these candidates in detail, highlighting interesting resolved or extended sources, sources with very red long-wavelength slopes, and our highest-redshift candidates, which extend toz_phot∼ 18. Over 93% of the sources are newly identified from our deep JADES imaging, including 31 new galaxy candidates atz_phot> 12. We also investigate potential contamination by stellar objects, and do not find strong evidence from spectral energy distribution fitting that these faint high-redshift galaxy candidates are low-mass stars. Using 42 sources in our sample with measured spectroscopic redshifts from NIRSpec and FRESCO, we find excellent agreement to our photometric redshift estimates, with no catastrophic outliers and an average difference of 〈Δz=z_phot−z_spec〉 = 0.26. These sources comprise one of the most robust samples for probing the early buildup of galaxies within the first few hundred million years of the Universe’s history.

 

We report the discovery of a triply imaged active galactic nucleus (AGN), lensed by the galaxy cluster MACS J0035.4−2015 (zd = 0.352). The object is detected in Hubble Space Telescope imaging taken for the RELICS program. It appears to have a quasi-stellar nucleus consistent with a point-source, with a de-magnified radius of re ≲ 100 pc. The object is spectroscopically confirmed to be an AGN at zspec = 2.063 ± 0.005 showing broad rest-frame UV emission lines, and detected in both X-ray observations with Chandra and in ALCS ALMA band 6 (1.2 mm) imaging. It has a relatively faint rest-frame UV luminosity for a quasar-like object, MUV, 1450 = −19.7 ± 0.2. The object adds to just a few quasars or other X-ray sources known to be multiply lensed by a galaxy cluster. Some diffuse emission from the host galaxy is faintly seen around the nucleus, and there is a faint object nearby sharing the same multiple-imaging symmetry and geometric redshift, possibly an interacting galaxy or a star-forming knot in the host. We present an accompanying lens model, calculate the magnifications and time delays, and infer the physical properties of the source. We find the rest-frame UV continuum and emission lines to be dominated by the AGN, and the optical emission to be dominated by the host galaxy of modest stellar mass $M_{\star }\simeq 10^{9.2}\, \mathrm{M}_{\odot }$ . We also observe some variation in the AGN emission with time, which may suggest that the AGN used to be more active. This object adds a low-redshift counterpart to several relatively faint AGN recently uncovered at high redshifts with HST and JWST.

 

ABSTRACT Reionization-era galaxies tend to exhibit weak Ly α emission, likely reflecting attenuation from an increasingly neutral IGM. Recent observations have begun to reveal exceptions to this picture, with strong Ly α emission now known in four of the most massive z = 7–9 galaxies in the CANDELS fields, all of which also exhibit intense [O iii]+H β emission (EW > 800 Å). To better understand why Ly α is anomalously strong in a subset of massive z ≃ 7–9 galaxies, we have initiated an MMT/Binospec survey targeting a larger sample (N = 22) of similarly luminous (≃1–6 L$^{\ast }_{\mathrm{UV}}$) z ≃ 7 galaxies selected over very wide-area fields (∼3 deg2). We confidently (>7σ) detect Ly α in 78 per cent (7/9) of galaxies with strong [O iii]+H β emission (EW > 800 Å) as opposed to only 8 per cent (1/12) of galaxies with more moderate (EW = 200–800 Å) [O iii]+H β. We argue that the higher Ly α EWs of the strong [O iii]+H β population likely reflect enhanced ionizing photon production efficiency owing to their large sSFRs (≳30 Gyr−1). We also find evidence that Ly α transmission from massive galaxies declines less rapidly over 6 < z < 7 than in low-mass lensed systems. In particular, our data suggest no strong evolution in Ly α transmission, consistent with a picture wherein massive z ≃ 7 galaxies often reside in large ionized regions. We detect three closely separated (R = 1.7 physical Mpc) z ≃ 7 Ly α emitters in our sample, conceivably tracing a large ionized structure that is consistent with this picture. We detect tentative evidence for an overdensity in this region, implying a large ionizing photon budget in the surrounding volume. 

Abstract Giant, star-forming clumps are a common feature prevalent among high-redshift star-forming galaxies and play a critical role in shaping their chaotic morphologies and yet, their nature and role in galaxy evolution remains to be fully understood. A majority of the effort to study clumps has been focused at high redshifts, and local clump studies have often suffered from small sample sizes. In this work, we present an analysis of clump properties in the local universe, and for the first time, performed with a statistically significant sample. With the help of the citizen science-powered Galaxy Zoo: Hubble project, we select a sample of 92 z < 0.06 clumpy galaxies in Sloan Digital Sky Survey Stripe 82 galaxies. Within this sample, we identify 543 clumps using a contrast-based image analysis algorithm and perform photometry as well as estimate their stellar population properties. The overall properties of our z < 0.06 clump sample are comparable to the high-redshift clumps. However, contrary to the high-redshift studies, we find no evidence of a gradient in clump ages or masses as a function of their galactocentric distances. Our results challenge the inward migration scenario for clump evolution for the local universe, potentially suggesting a larger contribution of ex situ clumps and/or longer clump migration timescales. 

Far-ultraviolet (FUV; ∼1200–2000 Å) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of the James Webb Space Telescope will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before; however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of ample wavelength coverage, signal-to-noise, spectral-resolution, and sample diversity that has not yet been achieved by any FUV spectral database. We present the Cosmic Origins Spectrograph Legacy Spectroscopic Survey (CLASSY) treasury and its first high-level science product, the CLASSY atlas. CLASSY builds on the Hubble Space Telescope (HST) archive to construct the first high-quality (S/N_{1500 Å}≳ 5/resel), high-resolution (R∼ 15,000) FUV spectral database of 45 nearby (0.002 <z< 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY sample covers a broad range of properties including stellar mass (6.2 < logM_⋆(M_⊙) < 10.1), star formation rate (−2.0 < log SFR (M_⊙yr⁻¹) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O₃₂< 38.0), reddening (0.02 <E(B−V) < 0.67), and nebular density (10 <n_e(cm⁻³) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with thez∼ 0 mass–metallicity relationship, but is offset to higher star formation rates by roughly 2 dex, similar toz≳ 2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.