arXiv subjects

Aayush Saxena

Publications and source records attributed to Aayush Saxena.

At least 37 records · Page 2Linked to original sources

arXiv2025

Glimmers in the Cosmic Dawn. II. A variability census of supermassive black holes across the Universe

Understanding the origin and evolution of supermassive black holes (SMBH) stands as one of the most important challenges in astrophysics and cosmology, with little current theoretical consensus. Improved observational constraints on the cosmological evolution of SMBH demographics are needed. Here we report results of a search via photometric variability for SMBHs appearing as active galactic nuclei (AGN) in the cosmological volume defined by the Hubble Ultra Deep Field (HUDF). This work includes particular focus on a new observation carried out in 2023 with the \textit{Hubble Space Telescope (HST)} using the WFC3/IR/F140W, which is compared directly to equivalent data taken 11 years earlier in 2012. Two earlier pairs of observations from 2009 to 2012 with WFC3/IR/F105W and WFC3/IR/F160W are also analysed. We identify 521, 188, and 109 AGN candidates as nuclear sources that exhibit photometric variability at a level of 2, 2.5 and 3~$σ$ in at least one filter. This sample includes 13, 3, and 2 AGN candidates at redshifts $z>6$, when the Universe was $\lesssim900$~Myr old. After variability and luminosity function (down to $M_{\rm UV}=-17\:$mag) completeness corrections, we estimate the co-moving number density of SMBHs, $n_{\rm SMBH}(z)$. At $z \gtrsim 6$, $n_{\rm SMBH}\gtrsim 6\times10^{-3}\:{\rm cMpc^{-3}}$. At low-$z$ our observations are sensitive to AGN fainter than $M_{\rm UV}=-17 \:$mag, and we estimate $n_{\rm SMBH}\gtrsim 10^{-2}\:{\rm cMpc^{-3}}$. We discuss how these results place strong constraints on a variety of SMBH seeding theories.

astro-ph.GA↗

arXiv2025

21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations

We study the physical origin and spectroscopic impact of extreme nebular emission in high-redshift galaxies. The nebular continuum, which can appear during an extreme starburst, is of particular importance as it tends to redden UV slopes and has a significant contribution to the UV luminosities of galaxies. Furthermore, its shape can be used to infer the gas density and temperature of the interstellar medium. First, we provide a theoretical background, showing how different stellar populations (SPS models, initial mass functions (IMFs), and stellar temperatures) and nebular conditions impact observed galaxy spectra. We demonstrate that, for systems with strong nebular continuum emission, 1) UV fluxes can increase by up to 0.7~mag (or more in the case of hot/massive stars) above the stellar continuum, which may help reconcile the surprising abundance of bright high-redshift galaxies and the elevated UV luminosity density at $z>10$, 2) at high gas densities, UV slopes can redden from $β<-2.5$ to $β\sim-1$, 3) observational measurements of $ξ_{\rm ion}$ are gross underestimates, and 4) UV downturns from two-photon emission can masquerade as damped Ly$α$ systems. Second, we present a dataset of 58 galaxies observed with NIRSpec on JWST at $2.5 10$ galaxies, finding that UV slopes and UV downturns are in some cases redder or steeper than expected from SPS models, which may hint at more exotic (e.g. hotter/more massive stars or AGN) ionizing sources.

astro-ph.GA↗

arXiv2025

Witnessing the onset of reionisation via Lyman-$α$ emission at redshift 13

$\require{mediawiki-texvc}$Cosmic Reionisation commenced when ultraviolet (UV) radiation produced in the first galaxies began illuminating the cold, neutral gas that filled the primordial Universe. Recent James Webb Space Telescope (JWST) observations have shown that surprisingly UV-bright galaxies were in place beyond redshift $z = 14$, when the Universe was less than $300 \, \mathrm{Myr}$ old. Smooth turnovers of their UV continua have been interpreted as damping-wing absorption of Lyman-$α$ (Ly$α$), the principal hydrogen transition. However, spectral signatures encoding crucial properties of these sources, such as their emergent radiation field, largely remain elusive. Here we report spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES) of a galaxy at redshift $z = 13.0$ that reveal a singular, bright emission line unambiguously identified as Ly$α$, in addition to a smooth turnover. We observe an equivalent width of $\text{EW}_\mathrm{Lyα} > 40 \, Å$ (rest frame), previously only seen at $z < 9$ where the intervening intergalactic medium (IGM) becomes increasingly ionised. Together with an extremely blue UV continuum, the unexpected Ly$α$ emission indicates the galaxy is a prolific producer and leaker of ionising photons. This suggests massive, hot stars or an active galactic nucleus (AGN) have created an early reionised region to prevent complete extinction of Ly$α$, thus shedding new light on the nature of the earliest galaxies and the onset of Reionisation only $330 \, \mathrm{Myr}$ after the Big Bang.

astro-ph.GA↗

arXiv2025

GA-NIFS: High number of dual active galactic nuclei at z~3

Merger events can trigger gas accretion onto supermassive black holes (SMBHs) located at the centre of galaxies, and form close pairs of AGN. The fraction of AGN in pairs offers critical insights into the dynamics of galaxy interactions, SMBH growth, and their co-evolution with host galaxies. However, the identification of dual AGN is difficult, as it requires high-quality spatial and spectral data; hence, only very few pairs have been found in the distant Universe so far. This study aims to provide a first observational estimate of the fraction of dual AGN at 2<z<6 by analysing a sample of 16 AGN observed with JWST/NIRSpec IFS, as part of the GA-NIFS survey. For two AGN in our sample, we also incorporate archival VLT/MUSE data to expand the search area. We searched for nearby companion galaxies and emission-line sources within the ~20x20 kpc field of view of the NIRSpec data cubes, extending up to ~50 kpc using the MUSE data cubes. We analysed the spectra of such emitters to determine their physical and kinematic properties. We report the serendipitous discovery of a triple AGN and four dual AGN (two of which considered as candidates), with projected separations in the range 3-28 kpc. Their AGN classification is mainly based on standard optical emission line flux ratios, as observed with JWST/NIRSpec, and is complemented with additional multi-wavelength diagnostics. The identification of these 3-5 multiple AGN out of the 16 AGN systems in GA-NIFS (i.e. ~20-30%) suggests they might be more common than previously thought based on other observational campaigns. Moreover, our inferred fraction of dual AGN moderately exceeds predictions from cosmological simulations that mimic our observational criteria (~10%). This work highlights the exceptional capabilities of NIRSpec for detecting distant dual AGN, and prompts new investigations to constrain their fraction across cosmic time.

astro-ph.GA↗

arXiv2025

The eventful life of a luminous galaxy at z = 14: metal enrichment, feedback, and low gas fraction?

JADES-GS-z14-0 is the most distant spectroscopically confirmed galaxy yet, at $z>14$. With a UV magnitude of -20.81, it is one of the most luminous galaxies at cosmic dawn and its half-light radius of 260 pc means that stars dominate the observed UV emission. We report ALMA detection of [OIII]88$μ$m line emission with a significance of 6.67$σ$ and at a frequency of 223.524~GHz, corresponding to a redshift of $14.1796\pm0.0007$, which is consistent with the candidate CIII] line detected in the NIRSpec spectrum. At this spectroscopic redshift, the Lyman-$α$ break identified with NIRSpec requires a damped Lyman-$α$ absorber with a column density of $\log(N_{\rm HI}/\mathrm{cm}^{-2})=21.96$. The total [O\,{\sc iii}]88$μ$m luminosity (log$(L_{\rm [OIII]}/L_\odot) = 8.3\pm 0.1$) is fully consistent with the local $L_{\rm [OIII]}-SFR$ relation and indicating a gas-phase metallicity $>0.1~{\rm Z_{\rm \odot}}$. Using \texttt{prospector} SED modeling and combining the ALMA data with JWST observations, we find $Z=0.17~{\rm Z_{\rm \odot}}$ and a non-zero escape fraction of ionizing photons ($\sim11\%$), which is necessary by the code to reproduce the UV spectrum. We measure an ${\rm [O III]}5007$Å/[O III]88$μ$m line flux ratio between 1 and 20, resulting in an upper limit to the electron density of roughly 700 cm$^{-3}$ assuming a single-cloud photoionization model. The [OIIII]88$μ$m emission line is spectrally resolved, with a FWHM of 100 km/s, resulting in a dynamical mass of log($M_{\rm dyn}/M_\odot$) = 9.0$\pm0.2$. When compared to the stellar mass, this value represents a conservative upper limit on the gas mass fraction, which ranges from 50\% to 80\%, depending on the assumed star formation history. Past radiation-driven outflows may have cleared the galaxy from the gas, reducing the gas fraction and thus increasing the escape fraction of ionizing photons.

astro-ph.GA↗

arXiv2025

Dissecting the massive pristine, neutral gas reservoir of a remarkably bright galaxy at z = 14.179

At cosmic dawn, the first stars and galaxies are believed to form from and be deeply embedded in clouds of dense, pristine gas. Here we present a study of the JWST/NIRSpec data of the most distant, spectroscopically confirmed galaxy observed to date, JADES-GS-z14-0 (GS-z14 for short), at $z=14.179$, combined with recent far-infrared measurements of the [OIII]-$88μ$m and [CII]-$158μ$m line transitions and underlying dust-continuum emission. Based on the observed prominent damped Lyman-$α$ (DLA) absorption profile, we determine a substantial neutral atomic hydrogen (HI) column density, $\log (N_{\rm HI} / {\rm cm^{-2}}) = 22.27^{+0.08}_{-0.09}$, consistent with previous estimates though seemingly at odds with the dynamical and gas mass of the galaxy. Using various independent but complementary approaches, considering the implied neutral gas mass from the DLA measurement, the star-formation rate surface density, and the metal abundance, we demonstrate that the total gas mass of GS-z14 is of the order $\log (M_{\rm gas} / M_\odot) = 9.8\pm 0.3$. This implies a substantial gas mass fraction, $f_{\rm gas} \gtrsim 0.9$ and that the bulk of the interstellar medium (ISM) is in the form of HI. We show that the derived gas mass is fully consistent with the non-detection of [CII]-$158μ$m, assuming an appropriate scaling to the neutral gas. The low dust-to-gas ratio, $A_V/N_{\rm HI} = (1.3\pm 0.6)\times 10^{-23}$\,mag\,cm$^2$, derived in the line-of-sight through the DLA further indicates that the absorbing gas is more pristine than the central, star-forming regions probed by the [OIII]-$88μ$m emission. These results highlight the implications for far-infrared line-detection searchers attainable with ALMA and demonstrate that the bright, relatively massive galaxy GS-z14 at $z=14.179$ is deeply embedded in a substantial, pristine HI gas reservoir dominating its baryonic matter content.

astro-ph.GA↗

arXiv2025

Inferring the Ionizing Photon Contributions of High-Redshift Galaxies to Reionization with JWST NIRCam Photometry

JWST is providing constraints on the history of reionization owing to its ability to detect faint galaxies at $z\gg6$. Modeling this history requires understanding both the ionizing photon production rate ($ξ_{\rm ion}$) and the fraction of those photons that escape into the intergalactic medium ($f_{\rm esc}$). Observational estimates of these quantities generally rely on spectroscopy for which large samples with well-defined selection functions are limited. To overcome this challenge, we present and release an implicit likelihood inference pipeline, PHOTONIOn, trained on mock photometry to predict the escaped ionizing luminosity of individual galaxies ($\dot{N}_{\rm ion}$) based on photometric magnitudes and redshifts. We show that PHOTONIOn is able to reliably infer $\dot{N}_{\rm ion}$ from photometry. This is in contrast to traditional SED-fitting approaches which rely on $f_{\rm esc}$ prescriptions that often over-predict $\dot{N}_{\rm ion}$ for LyC-dim galaxies, even when given access to spectroscopic data. We have deployed PHOTONIOn on a sample of 4,559 high-redshift galaxies from the JADES Deep survey, finding gentle redshift evolutions of $\log_{10}(\dot{N}_{\rm ion}) = (0.08\pm0.01)z + (51.60\pm0.06)$ and $\log_{10}(f_{\rm esc}ξ_{\rm ion}) = (0.07\pm0.01)z + (24.12\pm0.07)$. Late-time values for the ionizing photon production rate density are consistent with theoretical models and observations. We measure the evolution of the IGM ionized fraction to find that observed populations of star-forming galaxies are capable of driving reionization in GOODS-S to completion by $z\sim 5.3$ without the need for AGN or other exotic sources, consistent with other studies of the same field. The $20\%$ of UV-brightest galaxies ($M_{\rm UV}<-18.5$) reionize $35\%$ of the survey volume, demonstrating that UV faint LyC emitters are crucial for reionization.

astro-ph.GA↗

arXiv2025

Photometric detection at $7.7\ μ\mathrm{m}$ of a galaxy beyond redshift $14$ with JWST/MIRI

The James Webb Space Telescope (JWST) has spectroscopically confirmed numerous galaxies at $z > 10$. While weak rest-ultraviolet emission lines have only been seen in a handful of sources, the stronger rest-optical emission lines are highly diagnostic and accessible at mid-infrared wavelengths with the Mid-Infrared Instrument (MIRI) of JWST. We report the photometric detection of the distant spectroscopically confirmed galaxy JADES-GS-z14-0 at $z = 14.32^{+0.08}_{-0.20}$ with MIRI at $7.7\ μ\mathrm{m}$. The most plausible solution for the stellar population properties is that this galaxy contains half a billion solar masses in stars with a strong burst of star formation in the most recent few million years. For this model, at least one-third of the flux at $7.7\ μ\mathrm{m}$ comes from the rest-optical emission lines $\mathrm{H}β$ and/or $\mathrm{[OIII]}λ\lambda4959,5007$. The inferred properties of JADES-GS-z14-0 suggest rapid mass assembly and metal enrichment during the earliest phases of galaxy formation. This work demonstrates the unique power of mid-infrared observations in understanding galaxies at the redshift frontier.

astro-ph.GA↗

arXiv2024

Hitting the slopes: A spectroscopic view of UV continuum slopes of galaxies reveals a reddening at z > 9.5

The UV continuum slope of galaxies, $β$, is a powerful diagnostic. Understanding the redshift evolution of $β$ and its dependence on key galaxy properties can shed light on the evolution of galaxy physical properties over cosmic time. In this study, we present $β$ measurements for 295 spectroscopically confirmed galaxies at $5.5 9.5$ to begin to redden, deviating from the trend observed at $z < 9.5$. By producing stacked spectra in bins of redshift and $β$, we derive trends between $β$ and dust attenuation, metallicity, ionization parameter, and stellar age indicators directly from spectra, finding a lack of dust attenuation to be the dominant driver of bluer $β$ values. We further report six galaxies with $β<-3.0$, which show a range of spectroscopic properties and signs of significant LyC photon leakage. Finally, we show that the redder $β$ values at $z > 9.5$ may require rapid build-up of dust reservoirs in the very early Universe or a significant contribution from the nebular continuum emission to the observed UV spectra, with the nebular continuum fraction depending on the gas temperatures and densities. Our modeling shows that in the absence of dust, nebular emission at $T > 15,000$ K can reproduce the range of $β$ that we see in our sample. Higher gas temperatures driven by hot, massive stars can boost the fraction of nebular continuum emission, potentially explaining the observed $β$ values as well as bright UV magnitudes seen across galaxies at $z > 10$.

astro-ph.GA↗

arXiv2024

Efficient Ionizers with Low H$\boldsymbolβ$+[OIII] Equivalent Widths: JADES Spectroscopy of a Peculiar High-z Population

Early JWST photometric studies discovered a population of UV faint ($\rm 700$Å) exclude the most metal-poor efficient ionizers and favor 1) more chemically enriched systems with comparable extreme radiation fields and 2) older starbursting systems. In contrast, metallicity degeneracies are reduced in H$α$ space, enabling the identification of these metal-poor efficient ionizers by their specific star-formation rate.

astro-ph.GA↗

arXiv2024

JADES: Measuring reionization properties using Lyman-alpha emission

Ly$α$ is the transition to the ground state from the first excited state of hydrogen (the most common element). Resonant scattering of this line by neutral hydrogen greatly impedes its emergence from galaxies, so the fraction of galaxies emitting Ly$α$ is a tracer of the neutral fraction of the intergalactic medium (IGM), and thus the history of reionisation. In previous works, we used early JWST/NIRSpec data from the JWST Advanced Deep Extragalactic Survey (JADES) to classify and characterise Ly$α$ emitting galaxies (LAEs). This survey is approaching completion, and the current sample is nearly an order of magnitude larger. From a sample of 795 galaxies in JADES at $4.0<z<14.3$, we find evidence for Ly$α$ emission in 150sources. We reproduce the previously found correlation between Ly$α$ escape fraction ($f_{esc}^{Lyα}$) - Ly$α$ rest-frame equivalent width ($REW_{Lyα}$) and the negative correlation between Ly$α$ velocity offset - $f_{esc}^{Lyα}$. Both $f_{esc}^{Lyα}$ and $REW_{Lyα}$ decrease with redshift ($z\gtrsim5.5$), indicating the progression of reionisation on a population scale. Our data are used to demonstrate an increasing IGM transmission of Ly$α$ from $z\sim14-6$. We measure the completeness-corrected fraction of LAEs (\xlya) from $z=4-9.5$. An application of these \xlya values to the results of previously utilised semi-analytical models suggests a high neutral fraction at $z=7$ ($X_{HI}\sim0.8-0.9$). Using an updated fit to the intrinsic distribution of $REW_{Lyα}$ results in a lower value in agreement with current works ($X_{HI}=0.64_{-0.21}^{+0.13}$). This sample of LAEs will be paramount for unbiased population studies of galaxies in the EoR.

astro-ph.GA↗

arXiv2024

JADES: Primaeval Lyman-$\mathrmα$ emitting galaxies reveal early sites of reionisation out to redshift $z \sim 9$

$\require{mediawiki-texvc}$Given the sensitivity of the resonant Lyman-$\mathrmα$ (Ly$\mathrmα$) transition to absorption by neutral hydrogen, observations of Ly$\mathrmα$ emitting galaxies (LAEs) have been widely used to probe the ionising capabilities of reionisation-era galaxies and their impact on the intergalactic medium (IGM). However, prior to JWST our understanding of the contribution of fainter sources and of ionised `bubbles' at earlier stages of reionisation remained uncertain. Here, we present the characterisation of three exceptionally distant LAEs at $z>8$, newly discovered by JWST/NIRSpec in the JADES survey. These three similarly bright ($M_\text{UV} \approx -20\,\mathrm{mag}$) LAEs exhibit small Ly$\mathrmα$ velocity offsets from the systemic redshift, $Δv_\mathrm{Lyα} \lesssim 200\,\mathrm{km\,s^{-1}}$, yet span a range of Ly$\mathrmα$ equivalent widths ($15\,Å$, $31\,Å$, and $132\,Å$). The former two show moderate Ly$\mathrmα$ escape fractions ($f_\mathrm{esc,Lyα} \approx 10\%$), whereas Ly$\mathrmα$ escapes remarkably efficiently from the third ($f_\mathrm{esc,Lyα} \approx 72\%$), which moreover is very compact (half-light radius of $90\pm10\,\mathrm{pc}$). We find these LAEs are low-mass galaxies dominated by very recent, vigorous bursts of star formation accompanied by strong nebular emission from metal-poor gas. We infer the two LAEs with modest $f_\mathrm{esc,Lyα}$, one of which reveals evidence for ionisation by an active galactic nucleus, may have reasonably produced small ionised bubbles preventing complete IGM absorption of Ly$\mathrmα$. The third, however, requires a $\sim 3\,\text{physical Mpc}$ bubble, indicating faint galaxies have contributed significantly. The most distant LAEs thus continue to be powerful observational probes into the earlier stages of reionisation.

astro-ph.GA↗

arXiv2024

Monster radio jet (>66 kpc) observed in quasar at z$\sim$5

We present the discovery of a large extended radio jet associated with the extremely radio-loud quasar J1601+3102 at $z\sim5$ from sub-arcsecond resolution imaging at 144 MHz with the LOFAR International Telescope. These large radio lobes have been argued to remain elusive at $z>4$ due to energy losses in the synchrotron emitting plasma as a result of scattering of the strong CMB at these high redshifts. Nonetheless, the 0.3" resolution radio image of J1601+3102 reveals a Northern and Southern radio lobe located at 9 and 57 kpc from the optical quasar, respectively. The measured jet size of 66 kpc makes J1601+3102 the largest extended radio jet at $z>4$ to date. However, it is expected to have an even larger physical size in reality due to projection effects brought about by the viewing angle. Furthermore, we observe the rest-frame UV spectrum of J1601+3102 with Gemini/GNIRS to examine its black hole properties, which results in a mass of 4.5$\times$10$^{8}$ M$_{\odot}$ with an Eddington luminosity ratio of 0.45. The BH mass is relatively low compared to the known high-$z$ quasar population, which suggests that a high BH mass is not strictly necessary to generate a powerful jet. This discovery of the first $\sim100$ kpc radio jet at $z>4$ shows that these objects exist despite energy losses from Inverse Compton scattering and can put invaluable constraints on the formation of the first radio-loud sources in the early Universe.

astro-ph.GA↗

arXiv2024

Extreme emission line galaxies detected in JADES JWST/NIRSpec I: inferred galaxy properties

Extreme emission line galaxies (EELGs) exhibit large equivalent widths (EW) in their rest-optical emission lines ([OIII]$\lambda5007$ or H$α$ rest-frame EW$ > 750Å$) which can be tied to a recent upturn in star formation rate, due to the sensitivity of the nebular line emission and the rest-optical continuum to young ($<10$Myr) and evolved stellar populations, respectively. By studying a sample of 85 star forming galaxies (SFGs), spanning the redshift and magnitude interval $3 $ M$_{UV}>-21$, in the JWST Advanced Deep Extragalactic Survey (JADES) with NIRSpec/prism spectroscopy, we determine that SFGs initiate an EELG phase when entering a significant burst of star formation, with the highest EWs observed in EELGs with the youngest luminosity-weighted ages ($<5$ Myr old) and the highest burst intensity (those with the greatest excess between their current and long-term average SFR). We spectroscopically confirm that a greater proportion of SFGs are in an EELG phase at high redshift in our UV-selected sample ($61\pm4\%$ in our $z>5.7$ high-redshift bin, compared to $23^{+4}_{-1}\%$ in our lowest-redshift bin $3 5.7$ have observed Lyman-$α$ emission, potentially lying within large ionised regions. The high detection rate of Lyman-$α$ emitters in our EELG selection suggests that the physical conditions associated with entering an EELG phase also promote the escape of Lyman-$α$ photons.

astro-ph.GA↗

arXiv2024

The NIRSpec Wide GTO Survey

The Near-infrared Spectrograph (NIRSpec) on the James Webb Space Telescope is uniquely suited to studying galaxies in the distant Universe with its combination of multi-object capabilities and sensitivity over a large range in wavelength (0.6-5.3 microns). Here we present the NIRSpec Wide survey, part of the NIRSpec Instrument Science Team's Guaranteed Time Observations, using NIRSpec's microshutter array to obtain spectra of more than 3200 galaxies at $z>1$ at both low- and high-resolution ($R\approx100$ and 2700) for a total of 105 hours. With 31 pointings covering $\approx$320 arcmin$^2$ across the five CANDELS fields with exquisite ancillary photometry from the Hubble Space Telescope, the NIRSpec Wide survey represents a fast and efficient way of using JWST to probe galaxies in the early Universe. Pointing centers are determined to maximize the observability of the rarest, high-value sources. Subsequently, the microshutter configurations are optimized to observe the maximum number of "census" galaxies with a selection function based primarily on HST/F160W magnitude, photometric/slitless grism redshift, and predicted \ha\ flux tracing the bulk of the galaxy population at cosmic noon ($z_{\rm med}=2.0$). We present details on the survey strategy, the target selection, an outline of the motivating science cases, and discuss upcoming public data releases to the community.

astro-ph.GA↗

arXiv2024

Searching for Emission Lines at $z>11$: The Role of Damped Lyman-$α$ and Hints About the Escape of Ionizing Photons

We describe new ultra-deep James Webb Space Telescope (JWST) NIRSpec PRISM and grating spectra for the galaxies JADES-GS-z11-0 ($z_{\mathrm{spec}} = 11.122^{+0.005}_{-0.003}$) and JADES-GS-z13-0 ($z_{\mathrm{spec}} = 13.20^{+0.03}_{-0.04}$), the most distant spectroscopically-confirmed galaxy discovered in the first year of JWST observations. The extraordinary depth of these observations (75 hours and 56 hours, respectively) provides a unique opportunity to explore the redshifts, stellar properties, UV magnitudes, and slopes for these two sources. For JADES-GS-z11-0, we find evidence for multiple emission lines, including [\ion{O}{2}]$λ\lambda3726,3729$Åand [\ion{Ne}{3}$]\lambda3869$Å, resulting in a spectroscopic redshift we determine with 94\% confidence. We present stringent upper limits on the emission line fluxes and line equivalent widths for JADES-GS-z13-0. At this spectroscopic redshift, the Lyman-$α$ break in JADES-GS-z11-0 can be fit with a damped Lyman-$α$ absorber with $\log{(N_\mathrm{HI}/\mathrm{cm}^{-2})} = 22.42^{+0.093}_{-0.120}$. These results demonstrate how neutral hydrogen fraction and Lyman-damping wings may impact the recovery of spectroscopic redshifts for sources like these, providing insight into the overprediction of the photometric redshifts seen for distant galaxies observed with JWST. In addition, we analyze updated NIRCam photometry to calculate the morphological properties of these resolved sources, and find a secondary source $0.3^{\prime\prime}$ south of JADES-GS-z11-0 at a similar photometric redshift, hinting at how galaxies grow through interactions in the early Universe.

astro-ph.GA↗

arXiv2024

Spectroscopic confirmation of two luminous galaxies at $z\sim14$

The first observations of JWST have revolutionized our understanding of the Universe by identifying for the first time galaxies at $z\sim13$. In addition, the discovery of many luminous galaxies at Cosmic Dawn ($z>10$) has suggested that galaxies developed rapidly, in apparent tension with many standard models. However, most of these galaxies lack spectroscopic confirmation, so their distances and properties are uncertain. We present JADES JWST/NIRSpec spectroscopic confirmation of two luminous galaxies at redshifts of $z=14.32^{+0.08}_{-0.20}$ and $z=13.90\pm0.17$. The spectra reveal ultraviolet continua with prominent Lyman-$α$ breaks but no detected emission lines. This discovery proves that luminous galaxies were already in place 300~million years after the Big Bang and are more common than what was expected before JWST. The most distant of the two galaxies is unexpectedly luminous and is spatially resolved with a radius of 260 parsecs. Considering also the very steep ultraviolet slope of the second galaxy, we conclude that both are dominated by stellar continuum emission, showing that the excess of luminous galaxies in the early Universe cannot be entirely explained by accretion onto black holes. Galaxy formation models will need to address the existence of such large and luminous galaxies so early in cosmic history.

astro-ph.GA↗

arXiv2024

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 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}$ $M_\odot$). We infer a strong (factor $>$2) decline in the typical [OIII]$+$H$β$ EWs towards very faint $z\sim6-9$ galaxies, yet a weak UV luminosity dependence on the H$α$ 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. Finally, we quantify the photometric overdensities around two $z>7$ strong Ly$α$ emitters. One Ly$α$ 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$α$ emitters reside in large ionized bubbles.

astro-ph.GA↗