On the ages of bright galaxies ∼500\sim 500 Myr after the Big Bang: insights into star formation activity at z≳15z \gtrsim 15 with JWST

With JWST, new opportunities to study the formation and evolution of galaxies in the early Universe are now emerging. Spitzer constraints on rest-optical properties of $z \gtrsim 7$ galaxies demonstrated the power of using stellar masses and star formation histories (SFHs) of galaxies to indirectly infer the star formation history of the Universe. However, only the brightest individual objects at $z \gtrsim 8$ could be detected with Spitzer, making it difficult to robustly constrain past activity at $z \gtrsim 10$. Here, we leverage the greatly improved rest-optical sensitivity of JWST at $z \gtrsim 8$ to constrain the ages and SFHs of eleven UV-bright ($M_\text{UV} \lesssim -19.5$) galaxies selected to lie at $z \sim 8.5 - 11$, then investigate implications for star formation activity at $z \gtrsim 15$. We infer the properties of individual objects in our sample with two spectral energy distribution modelling codes, then infer a distribution of ages for bright $z \sim 8.5 - 11$ galaxies. We find a median age of $\sim 30$ Myr, younger than that inferred at $z \sim 7$ with a similar analysis, which is consistent with an evolution towards larger specific star formation rates at early times. The age distribution suggests that only $\sim 9$ percent of bright $z \sim 8.5 - 11$ galaxies would be similarly luminous at $z \gtrsim 15$, implying that the number density of bright galaxies declines by approximately an order of magnitude between $z \sim 8.5 - 11$ and $z \sim 15$. This evolution is challenging to reconcile with some early JWST results suggesting that 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 galaxies at $z \sim 15$ are observed during a burst of star formation.Comment: 13 pages, 9 figure

Searching for Extremely Blue UV Continuum Slopes at z=7−11z=7-11 in JWST/NIRCam Imaging: Implications for Stellar Metallicity and Ionizing Photon Escape in Early Galaxies

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 the attenuation from dust through the galaxy. Considerable attention has focused on identifying reionization-era galaxies with very blue UV slopes ($\beta<-3$). Not only do such systems provide a signpost of low metallicity stars, but they also identify galaxies that likely have ionizing photons leaking from their HII regions as such blue UV slopes can only be seen if the reddening effect of nebular continuum has been diminished. In this paper we present a search for reionization-era galaxies with very blue UV colors in recent JWST/NIRCam imaging of the EGS field. We characterize UV slopes for a large sample of $z\simeq 7-11$ galaxies, finding a median value of $\beta =-2.1$. Three of the lower luminosity (M$_{\rm{UV}}\simeq -19.5$) and lower stellar mass (5-6$\times10^7$M$_\odot$) systems exhibit both extremely blue UV slopes ($\beta=-3.1$ to $-3.2$) and rest-optical photometry indicating weak nebular line emission. Each system is very compact (r$_e<$260 pc) with very high star formation rate surface densities. We model the SEDs with a suite of BEAGLE models with varying levels of ionizing photon escape. The SEDs cannot be reproduced with our fiducial (f$_{\rm{esc,HII}}$=0) or alpha enhanced (Z$_*<Z_{\rm{ISM}}$) models. The combined blue UV slopes and weak nebular emission are best-fit by models with significant ionizing photon escape from HII regions (f$_{\rm{esc,HII}}$=0.6-0.8) and extremely low metallicity massive stars (Z$_*$=0.01-0.06 Z$_\odot$). The discovery of these galaxies highlights the potential for JWST to identify large numbers of candidate Lyman Continuum leaking galaxies in the reionization era and suggests low metallicity stellar populations may be veryComment: 11 pages, 8 figures, 2 tables; Submitted to Ap

A JWST/NIRCam Study of Key Contributors to Reionization: The Star-forming and Ionizing Properties of UV-faint z∼7−8z\sim7-8 Galaxies

Spitzer/IRAC imaging has revealed that the brightest $z\sim7-8$ galaxies often exhibit young ages and strong nebular line emission, hinting at high ionizing efficiency among early galaxies. However, IRAC's limited sensitivity has long hindered efforts to study the fainter, more numerous population often thought largely responsible for reionization. Here we use CEERS JWST/NIRCam data to characterize 116 UV-faint (median M$_{UV}=-19.5$) $z\sim6.5-8$ galaxies. The SEDs are typically dominated by young ($\sim$10-50 Myr), low-mass ($M_\ast\sim10^8\ M_\odot$) stellar populations, and we find no need for extremely high stellar masses ($\sim10^{11} M_\odot$). Considering previous studies of UV-bright (M$_{UV}\sim-22$) $z\sim7-8$ galaxies, we find evidence for a strong (5-10$\times$) increase in specific star formation rate toward lower luminosities (median sSFR=103 Gyr$^{-1}$ in CEERS). The larger sSFRs imply a more dominant contribution from OB stars in the relatively numerous UV-faint population, perhaps suggesting that these galaxies are very efficient ionizing agents (median $\xi_{ion}=10^{25.7}$ erg$^{-1}$ Hz). In spite of their much larger sSFRs, we find no significant increase in [OIII]$+$H$\beta$ EWs towards fainter M$_{UV}$ (median $\approx$780 $\mathring{A}$). If confirmed, this may indicate that a substantial fraction of our CEERS galaxies possess extremely low metallicities ($\lesssim$3% $Z_\odot$) where [OIII] emission is suppressed. Alternatively, high ionizing photon escape fractions or bursty star formation histories can also weaken the nebular lines in a subset of our CEERS galaxies. While the majority of our objects are very blue (median $\beta=-2.0$), we identify a significant tail of very dusty galaxies ($\beta\sim-1$) at $\approx$0.5$L_{UV}^\ast$ which may contribute significantly to the $z\sim7-8$ star formation rate density.Comment: Accepted in MNRAS. Updated to use the most recent NIRCam zeropoints. There are no significant changes to the conclusions relative to v

JADES: Using NIRCam Photometry to Investigate the Dependence of Stellar Mass Inferences on the IMF in the Early Universe

The detection of numerous and relatively bright galaxies at redshifts z > 9 has prompted new investigations into the star-forming properties of high-redshift galaxies. Using local forms of the initial mass function (IMF) to estimate stellar masses of these galaxies from their light output leads to galaxy masses that are at the limit allowed for the state of the LambdaCDM Universe at their redshift. We explore how varying the IMF assumed in studies of galaxies in the early universe changes the inferred values for the stellar masses of these galaxies. We infer galaxy properties with the SED fitting code Prospector using varying IMF parameterizations for a sample of 102 galaxies from the JWST Advanced Deep Extragalactic Survey (JADES) spectroscopically confirmed to be at z > 6.7, with additional photometry from the JWST Extragalactic Medium Band Survey (JEMS) for twenty-one galaxies. We demonstrate that models with stellar masses reduced by a factor of three or more do not affect the modeled spectral energy distribution (SED).Comment: The Significance statement is required for PNAS submissio

The UV Continuum Slopes of Early Star-Forming Galaxies in JADES

The power-law slope of the rest-UV continuum ($f_{\lambda}\propto\lambda^{\beta}$) 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 JADES to investigate the UV slopes of 179 $z>9$ galaxies with apparent magnitudes of $m_{\rm F200W}=26-31$, which display a median UV slope of $\beta=-2.4$. We compare to a statistical sample of $z=5-9$ galaxies, finding a shift toward bluer rest-UV colors at all $\rm~M_{UV}$. The most UV-luminous $z>9$ galaxies are significantly bluer than their lower-redshift counterparts, representing a dearth of moderately-red galaxies in the first $500~$Myr. At yet earlier times, the $z>11$ galaxy population exhibits very blue UV slopes, implying very low attenuation from dust. We identify a robust sample of 44 galaxies with $\beta<-2.8$, which have SEDs requiring models of density-bounded HII regions and median ionizing photon escape fractions of $0.51$ to reproduce. Their rest-optical colors imply that this sample has weaker emission lines (median $m_{\rm F356W}-m_{\rm F444W}=0.19$ mag) than typical galaxies (median $m_{\rm F356W}-m_{\rm F444W}=0.39$ mag), consistent with the inferred escape fractions. This sample has relatively low stellar masses (median $\log(M/M_{\odot})=7.5$), and specific star-formation rates (median$=79\rm/Gyr$) nearly twice that of our full sample (median$=44\rm/Gyr$), suggesting they 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 colors, making distinct predictions for the rest-optical emission of these galaxies. Future spectroscopy will be required to distinguish between these physical pictures.Comment: 17 pages, 13 figures; submitted to MNRA

The UV continuum slopes of early star-forming galaxies in JADES

© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/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.Peer reviewe

The Star-forming and Ionizing Properties of Dwarf z~6-9 Galaxies in JADES: Insights on Bursty Star Formation and Ionized Bubble Growth

Reionization is thought to be driven by faint star-forming galaxies, but characterizing this population in detail has long remained very challenging. Here we utilize deep nine-band NIRCam imaging from JADES to study the star-forming and ionizing properties of 756 $z\sim6-9$ galaxies, including hundreds of very UV-faint objects ($M_\mathrm{UV}>-18$). The faintest ($m\sim30$) galaxies in our sample typically have stellar masses of $M_\ast\sim(1-3)\times10^7$ $M_\odot$ and young light-weighted ages ($\sim$50 Myr), though some show strong Balmer breaks implying much older ages ($\sim$500 Myr). We find no evidence for extremely massive galaxies ($>3\times10^{10}$ $M_\odot$) in our sample. We infer a strong (factor $>$2) decline in the typical [OIII]$+$H$\beta$ EWs towards very faint $z\sim6-9$ galaxies, yet a weak UV luminosity dependence on the H$\alpha$ EWs at $z\sim6$. We demonstrate that these EW trends can be explained if fainter galaxies have systematically lower metallicities as well as more recently-declining star formation histories relative to the most UV-luminous galaxies in our sample. Our data provide evidence that the brightest galaxies are frequently experiencing a recent strong upturn in SFR. We also discuss how the EW trends may be influenced by a strong correlation between $M_\mathrm{UV}$ and Lyman continuum escape fraction. This alternative explanation has dramatically different implications for the contribution of galaxies along the luminosity function to cosmic reionization, highlighting the need for deep spectroscopic follow-up. Finally, we quantify the photometric overdensities around two $z>7$ strong Ly$\alpha$ emitters in the JADES footprint. One Ly$\alpha$ emitter lies close to a strong photometric overdensity while the other shows no significant nearby overdensity, perhaps implying that not all strong $z>7$ Ly$\alpha$ emitters reside in large ionized bubbles.Comment: 29 pages, 16 figures. Submitted to MNRAS. Comments welcom

The Cosmos in its Infancy: JADES Galaxy Candidates at z > 8 in GOODS-S and GOODS-N

© 2024. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/We present a catalog of 717 candidate galaxies at z > 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 to z phot ∼ 18. Over 93% of the sources are newly identified from our deep JADES imaging, including 31 new galaxy candidates at z 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.Peer reviewe

The Cosmos in its Infancy: JADES Galaxy Candidates at z > 8 in GOODS-S and GOODS-N

We present a catalog of 717 candidate galaxies at $z > 8$ selected from 125 square arcminutes of NIRCam imaging as part of the JWST Advanced Deep Extragalactic Survey (JADES). We combine the full JADES imaging dataset with data from the JEMS and FRESCO JWST surveys along with extremely deep existing observations from HST/ACS for a final filter set that includes fifteen 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 to $z_{phot} = 18$. We also investigate potential contamination by stellar objects, and do not find strong evidence from SED fitting that these faint high-redshift galaxy candidates are low-mass stars. Over 93\% of the sources are newly identified from our deep JADES imaging, including 31 new galaxy candidates at $z_{phot} > 12$. 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 $\langle \Delta z = z_{phot}- z_{spec} \rangle= 0.26$. 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.Comment: v2: 40 pages, 18 figures, submitted to AAS Journals, online data catalog (JADES Deep only) found at https://doi.org/10.5281/zenodo.809252