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With JWST, new opportunities to study the evolution of galaxies in the early Universe are emerging. Spitzer constraints on rest-optical properties of $z\gtrsim7$ galaxies demonstrated the power of using galaxy stellar masses and star formation histories (SFHs) to indirectly infer the cosmic star formation history. However, only the brightest individual $z\gtrsim8$ objects could be detected with Spitzer, making it difficult to robustly constrain activity at $z\gtrsim10$. Here, we leverage the greatly improved rest-optical sensitivity of JWST at $z\gtrsim8$ to constrain the ages of seven UV-bright ($M_{UV}\lesssim-19.5$) galaxies selected to lie at $z\sim8.5-11$, then investigate implications for $z\gtrsim15$ star formation. We infer the properties of individual objects with two spectral energy distribution modelling codes, then infer a distribution of ages for bright $z\sim8.5-11$ galaxies. We find a median age of $\sim20$ Myr, younger than that inferred at $z\sim7$ with a similar analysis, consistent with an evolution towards larger specific star formation rates at early times. The age distribution suggests that only $\sim3$ percent of bright $z\sim8.5-11$ galaxies would be similarly luminous at $z\gtrsim15$, implying that the number density of bright galaxies declines by at least an order of magnitude between $z\sim8.5-11$ and $z\sim15$. This evolution is challenging to reconcile with some early JWST results suggesting the abundance of bright galaxies does not significantly decrease towards very early times, but we suggest this tension may be eased if young stellar populations form on top of older stellar components, or if bright $z\sim15$ galaxies are observed during a burst of star formation., 16 pages, 10 figures, 1 appendix, accepted to MNRAS

The onset of the {\it JWST}-era provides a much-improved opportunity to characterize the resolved structure of early star forming systems. Previous {\it Spitzer} observations of $z\gtrsim 6$ galaxies revealed the presence of old stars and luminous HII regions (via [OIII]+H$\beta$ emission), but the poor resolution stunted our ability to map their locations with respect to the star forming regions identified in the rest-UV. In this paper, we investigate the internal structure of 12 of the most luminous $z\simeq 6-8$ galaxies in the EGS field observed with recent {\it JWST}/NIRCam imaging. The systems appear clumpy in the rest-UV, with more than half of the light coming from $\simeq 10^7$ to 10$^{9}$ M$_\odot$ star forming complexes that are $\simeq 150$ - 480 pc in size. The clumps tend to be dominated by young stars (median = 36 Myr), but we also find large variations in clump ages within individual galaxies. The [OIII]+H$\beta$ EW varies significantly across individual galaxies (reflecting differences in stellar and gas properties), but the HII regions largely track the UV-bright complexes. Perhaps surprisingly, the rest-optical continuum is just as clumpy as the UV, and we do not find older (and redder) nuclear stellar components that were previously undetected or faint in the UV. The majority of the stellar mass in bright $6, Comment: main text 12 pages, 7 figures; accepted for publication by MNRAS

Finding and characterising the first galaxies that illuminated the early Universe at cosmic dawn is pivotal to understand the physical conditions and the processes that led to the formation of the first stars. In the first few months of operations, imaging from the James Webb Space Telescope (JWST) have been used to identify tens of candidates of galaxies at redshift (z) greater than 10, less than 450 million years after the Big Bang. However, none of these candidates has yet been confirmed spectroscopically, leaving open the possibility that they are actually low-redshift interlopers. Here we present spectroscopic confirmation and analysis of four galaxies unambiguously detected at redshift 10.3, Comment: 32 pages, 8 figures, accepted to Nature Astronomy

We present band 6 ALMA observations of a heavily-obscured radio-loud ($L_{1.4\ \mathrm{GHz}}=10^{25.4}$ W Hz$^{-1}$) AGN candidate at $z_\mathrm{phot}=6.83\pm0.06$ found in the 1.5 deg$^2$ COSMOS field. The ALMA data reveal detections of exceptionally strong [CII]158$\mu$m ($z_\mathrm{[CII]}=6.8532$) and underlying dust continuum emission from this object (COS-87259), where the [CII] line luminosity, line width, and 158$\mu$m continuum luminosity are comparable to that seen from $z\sim7$ sub-mm galaxies and quasar hosts. The 158$\mu$m continuum detection suggests a total infrared luminosity of $9\times10^{12}$ $L_\odot$ with corresponding very large obscured star formation rate (1300 $M_\odot$/yr) and dust mass ($2\times10^9$ $M_\odot$). The strong break seen between the VIRCam and IRAC photometry perhaps suggests that COS-87259 is an extremely massive reionization era galaxy with $M_\ast\approx1.7\times10^{11}$ $M_\odot$. Moreover, the MIPS, PACS, and SPIRE detections imply that this object harbors an AGN that is heavily obscured ($\tau_{_{\mathrm{9.7\mu m}}}=2.3$) with a bolometric luminosity of approximately $5\times10^{13}$ $L_\odot$. Such a very high AGN luminosity suggests this object is powered by an $\approx$1.6 $\times$ 10$^9$ $M_\odot$ black hole if accreting near the Eddington limit, and is effectively a highly-obscured version of an extremely UV-luminous ($M_{1450}\approx-27.3$) $z\sim7$ quasar. Notably, these $z\sim7$ quasars are an exceedingly rare population ($\sim$0.001 deg$^{-2}$) while COS-87259 was identified over a relatively small field. Future very wide-area surveys with, e.g., Roman and Euclid have the potential to identify many more extremely red yet UV-bright $z\gtrsim7$ objects similar to COS-87259, providing richer insight into the occurrence of intense obscured star formation and supermassive black hole growth among this population., Comment: 12 pages, 7 figures, 1 table. Updated to accepted version (MNRAS)

The variety of star formation histories (SFHs) of $z\gtrsim6$ galaxies provides important insights into early star formation, but has been difficult to systematically quantify. Some observations suggest that many $z\sim6-9$ galaxies are dominated by $\gtrsim200$ Myr stellar populations, implying significant star formation at $z\gtrsim9$, while others find that most reionization era galaxies are $\lesssim10$ Myr, consistent with little $z\gtrsim9$ star formation. Here, we quantify the distribution of ages of UV-bright ($-22.5\lesssim M_{UV}\lesssim-21$) galaxies colour-selected to lie at $z\simeq6.6-6.9$, 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 $\sim30-70$ Myr depending on SED model. We quantify the fractions of $\leq10$ Myr and $\geq250$ Myr galaxies, finding that these systems comprise $\sim15-30$ percent and $\sim20-25$ percent 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 $z\gtrsim9$ stellar mass assembly and discuss improvements expected from JWST., Comment: 24 pages, 20 figures, 2 appendices, accepted to MNRAS

The ultraviolet (UV) continuum slope ($\beta~$where$~f_\lambda\propto\lambda^\beta$) of galaxies is sensitive to a variety of properties, from the metallicity and age of the stellar population to dust attenuation throughout the galaxy. Considerable attention has focused on identifying reionization-era galaxies with very blue UV slopes ($\beta7$., Comment: 13 pages, 10 figures, 3 tables; Accepted to ApJ

Recent work has shown that UV-luminous reionization-era galaxies often exhibit strong Lyman-alpha emission despite being situated at redshifts where the IGM is thought to be substantially neutral. It has been argued that this enhanced Ly α transmission reflects the presence of massive galaxies in overdense regions which power large ionized bubbles. An alternative explanation is that massive galaxies shift more of their Ly α profile to large velocities (relative to the systemic redshift) where the IGM damping wing absorption is reduced. Such a mass-dependent trend is seen at lower redshifts, but whether one exists at z ∼ 7 remains unclear owing to the small number of existing systemic redshift measurements in the reionization era. This is now changing with the emergence of [C ii]-based redshifts from ALMA. Here, we report MMT/Binospec Ly α spectroscopy of eight UV-bright (MUV ∼ −22) galaxies at z ≃ 7 selected from the ALMA REBELS survey. We detect Ly α in four of eight galaxies and use the [C ii] systemic redshifts to investigate the Ly α velocity profiles. The Ly α lines are significantly redshifted from systemic (average velocity offset = 223 km s–1) and broad (FWHM ≈ 300–650 km s−1), with two sources showing emission extending to ≈750 km s−1. We find that the broadest Ly α profiles are associated with the largest [C ii] line widths, suggesting a potential link between the Ly α FWHM and the dynamical mass. Since Ly α photons at high velocities transmit efficiently through the z = 7 IGM, our data suggest that velocity profiles play a significant role in boosting the Ly α visibility of the most UV-luminous reionization-era galaxies.

We present specific star-formation rates for 40 UV-bright galaxies at $z\sim7-8$ observed as part of the Reionization Era Bright Emission Line Survey (REBELS) ALMA large program. The sSFRs are derived using improved measures of SFR and stellar masses, made possible by measurements of far-infrared (FIR) continuum emission and [CII]-based spectroscopic redshifts. For each source in the sample, we derive stellar masses from SED fitting and total SFRs from calibrations of the UV and FIR emission. The median sSFR is $18_{-5}^{+7}$ Gyr$^{-1}$, significantly larger than literature measurements lacking constraints in the FIR. The increase in sSFR reflects the larger obscured SFRs we derive from the dust continuum relative to that implied by the UV+optical SED. We suggest that such differences may reflect spatial variations in dust across these luminous galaxies, with the component dominating the FIR distinct from that dominating the UV. We demonstrate that the inferred stellar masses (and hence sSFRs) are strongly-dependent on the assumed star formation history (SFH) in reionization-era galaxies. When large sSFR galaxies are modeled with non-parametric SFHs, the derived stellar masses can increase by an order of magnitude relative to constant star formation models, owing to the presence of a significant old stellar population that is outshined by the recent burst. The [CII] line widths in the largest sSFR systems are often very broad, suggesting dynamical masses that are easily able to accommodate the dominant old stellar population suggested by non-parametric models. Regardless of these systematic uncertainties in the derived parameters, we find that the sSFR increases rapidly toward higher redshifts for massive galaxies ($9.6, 18 pages, 10 figures, submitted to MNRAS

We report the identification of radio (0.144-3 GHz), mid-IR, far-IR, and sub-mm (24-850$\mu$m) emission at the position of one of 41 UV-bright (M$_\mathrm{UV}^{}\lesssim-21.25$) $z\simeq6.6-6.9$ Lyman-break galaxy candidates in the 1.5 deg$^2$ COSMOS field. This source, COS-87259, exhibits a sharp flux discontinuity (factor $>$3) between two narrow/intermediate bands at 9450 and 9700 Angstroms and is undetected in all nine bands blueward of 9600 Angstroms, as expected from a Lyman-alpha break at $z\simeq6.8$. The full multi-wavelength (X-ray through radio) data of COS-87529 can be self-consistently explained by a very massive (M$_{\ast}=10^{10.8}$ M$_{\odot}$) and extremely red (rest-UV slope $\beta=-0.59$) $z\simeq6.8$ galaxy with hyperluminous infrared emission (L$_{\mathrm{IR}}=10^{13.6}$ L$_{\odot}$) powered by both an intense burst of highly-obscured star formation (SFR$\approx$1800 M$_{\odot}$ yr$^{-1}$) and an obscured ($\tau_{\mathrm{9.7\mu m}}=7.7\pm2.5$) radio-loud (L$_{\mathrm{1.4\ GHz}}\approx10^{25.4}$ W Hz$^{-1}$) AGN. The radio emission is compact (1.04$\pm$0.12 arcsec) and exhibits an ultra-steep spectrum between 1.32-3 GHz ($\alpha=-1.57^{+0.22}_{-0.21}$) that flattens at lower frequencies ($\alpha=-0.86^{+0.22}_{-0.16}$ between 0.144-1.32 GHz), consistent with known $z>4$ radio galaxies. We also demonstrate that COS-87259 may reside in a significant (11$\times$) galaxy overdensity at $z\simeq6.6-6.9$, as common for systems hosting radio-loud AGN. Nonetheless, a spectroscopic redshift will ultimately be required to establish the true nature of COS-87259 as we cannot yet completely rule out low-redshift solutions. If confirmed to lie at $z\simeq6.8$, the properties of COS-87259 would be consistent with a picture wherein AGN and highly-obscured star formation activity are fairly common among very massive (M$_{\ast}>10^{10}$ M$_{\odot}$) reionization-era galaxies., Comment: 14 pages, 6 figures. Accepted in MNRAS with minor revisions. This accepted version considers very recent data from LOFAR and MeerKAT which improve our analyses on the radio slope and luminosity

Cosmic dust is an essential component shaping both the evolution of galaxies and their observational signatures. How quickly dust builds up in the early Universe remains an open question that requires deep observations at (sub-)millimeter wavelengths to resolve. Here we use Atacama Large Millimeter Array observations of 45 galaxies from the Reionization Era Bright Emission Line Survey (REBELS) and its pilot programs, designed to target [CII] and dust emission in UV-selected galaxies at $z\sim7$, to investigate the dust content of high-redshift galaxies through a stacking analysis. We find that the typical fraction of obscured star formation $f_\mathrm{obs} = \mathrm{SFR}_\mathrm{IR} / \mathrm{SFR}_\mathrm{UV + IR}$ depends on stellar mass, similar to what is observed at lower redshift, and ranges from $f_\mathrm{obs} \approx 0.3 - 0.6$ for galaxies with $\log_{10}\left(M_\star / M_\odot\right) = 9.4 - 10.4$. We further adopt the $z\sim7$ stellar mass function from the literature to extract the obscured cosmic star formation rate density (SFRD) from the REBELS survey. Our results suggest only a modest decrease in the SFRD between $3\lesssim z \lesssim 7$, with dust-obscured star formation still contributing $\sim30\%$ at $z\sim7$. While we extensively discuss potential caveats, our analysis highlights the continued importance of dust-obscured star formation even well into the epoch of reionization., Comment: MNRAS Accepted (01 Nov 2022)

Our understanding of reionization has advanced considerably over the past decade, with several results now demonstrating that the IGM transitioned from substantially neutral at $z=7$ to largely reionized at $z=6$. However, little remains known about the sizes of ionized bubbles at $z\gtrsim7$ as well as the galaxy overdensities which drive their growth. Fortunately, rest-UV spectroscopic observations offer a pathway towards characterizing these ionized bubbles thanks to the resonant nature of Lyman-alpha photons. In a previous work, we presented Ly$\alpha$ detections from three closely-separated Lyman-break galaxies at $z\simeq6.8$, suggesting the presence of a large ($R>1$ physical Mpc) ionized bubble in the 1.5 deg$^2$ COSMOS field. Here, we present new deep Ly$\alpha$ spectra of ten UV-bright ($\mathrm{M}_{\mathrm{UV}}^{} \leq -20.4$) $z\simeq6.6-6.9$ galaxies in the surrounding area, enabling us to better characterize this potential ionized bubble. We confidently detect (S/N$>$7) Ly$\alpha$ emission at $z=6.701-6.882$ in nine of ten observed galaxies, revealing that the large-scale volume spanned by these sources (characteristic radius $R = 3.2$ physical Mpc) traces a strong galaxy overdensity ($N/\langle N\rangle \gtrsim 3$). Our data additionally confirm that the Ly$\alpha$ emission of UV-bright galaxies in this volume is significantly enhanced, with 40% (4/10) showing strong Ly$\alpha$ emission (equivalent width$>$25 $\mathrm{\mathring{A}}$) compared to the 8$-$9% found on average at $z\sim7$. The median Ly$\alpha$ equivalent width of our observed galaxies is also $\approx$2$\times$ that typical at $z\sim7$, consistent with expectations if a very large ($R\sim3$ physical Mpc) ionized bubble is allowing the Ly$\alpha$ photons to cosmologically redshift far into the damping wing before encountering HI., Comment: 13 pages, 3 figures, 3 tables. Accepted in MNRAS with minor revisions