arXiv subjects

William M. Baker

Publications and source records attributed to William M. Baker.

At least 55 records · Page 3Linked to original sources

arXiv2025

The Stellar Populations and Rest-Frame Colors of Star-Forming Galaxies at $z \approx 8$: Exploring the Impact of Filter Choice and Star Formation History Assumption with JADES

Our understanding of the physical properties of star-forming galaxies during the Epoch of Reionization (EoR, at $z > 6$) suffers from degeneracies among the apparent properties of the stars, the nebular gas, and the dust. These degeneracies are most prominent with photometry, which has insufficient (1) spectral resolution and (2) rest-frame spectral coverage. We explore ways to break these degeneracies with a sample of $N = 22$ high-redshift star-forming galaxies at $7 < z_{\mathrm{phot}} \leq 9$, using some of the deepest existing imaging from JWST/NIRCam and JWST/MIRI with JADES. Key to this study is the imaging from JWST/MIRI at $7.7\ μ\mathrm{m}$, which provides coverage of the rest-frame $I$-band at the observed redshifts. We infer stellar population properties and rest-frame colors using a variety of filter sets and star formation history assumptions to explore the impact of these choices. Evaluating these quantities both with and without the $7.7\ μ\mathrm{m}$ data point shows that dense spectral coverage with JWST/NIRCam (eight or more filters, including at least one medium-band) can compensate for lacking the rest-frame $I$-band coverage for the vast majority ($\approx 80\%$) of our sample. Furthermore, these galaxy properties are most consistently determined by assuming the delayed-tau star formation history, which provides the smallest offsets and scatters around these offsets when including JWST/MIRI. Within extragalactic surveys like JADES and CEERS, our findings suggest that robust characterization of the stellar population properties and rest-frame colors for high-redshift star-forming galaxies is possible with JWST/NIRCam alone at $z \approx 8$.

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arXiv2025

Constraining the major merger history of $z \sim 3-9$ galaxies using JADES: dominant in-situ star formation

We present a comprehensive analysis of galaxy close-pair fractions and major merger rates to evaluate the importance of mergers in the hierarchical growth of galaxies over cosmic time. This study focuses on the previously poorly understood redshift range of $z \approx 3-9$ using JADES observations. Our mass-complete sample includes primary galaxies with stellar masses of ${\rm log}(M_\star/{\rm M_\odot}) = [8, 10]$, having major companions (mass ratio $\geq 1/4$) selected by $5-30$ pkpc projected separation and redshift proximity criteria. Pair fractions are measured using a statistically robust method incorporating photometric redshift posteriors and available spectroscopic data. The pair fraction evolves with redshift and shows dependence on the stellar mass: at ${\rm log}(M_\star/{\rm M_\odot}) = [8.0, 8.5]$ there is an increase up to $z\sim5-6$, followed by a turnover; while at higher stellar masses there is a flattening and weak decline with increasing redshift. Similarly, the derived galaxy major merger rate increases and flattens beyond $z \sim 6$ to $2-8~{\rm Gyr^{-1}}$ per galaxy, showing a weak scaling with stellar mass, driven by the evolution of the galaxy stellar mass function. A comparison between the cumulative mass accretion from major mergers and the mass assembled through star formation indicates that major mergers contribute approximately $3-13\%$ to the total mass growth over the studied redshift range, which is in agreement with the ex-situ mass fraction estimated from our simple numerical model. These results highlight that major mergers contribute little to the direct stellar mass growth compared to in-situ star formation but could still play an indirect role by driving star formation itself.

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arXiv2025

JADES: comprehensive census of broad-line AGN from Reionization to Cosmic Noon revealed by JWST

The depth and coverage of the first years of JWST observations have revealed low luminosity active galactic nuclei (AGN) across a wide redshift range, shedding light on black hole (BH) assembly and feedback. We present our spectroscopic sample of 34 Type 1 AGN obtained from JADES survey data and spanning $1.5 < z < 9$. Our sample of AGN probes a BH mass range of $10^{6-9}$~M$_{\odot}$ at bolometric luminosities down to $10^{43}$~erg~s$^{-1}$, implying generally sub-Eddington ratios of $<0.5L_{\rm Edd}$. Most of these AGN are hosted in low mass ($M_{\star}\sim10^8$~M$_{\odot}$) galaxies and are overmassive relative to the local $M_{BH}-M_{\star}$ relation, while remaining consistent with the local $M_{BH}$-$σ_*$ relation. The wide redshift range provided by our sample allows us to trace the emergence of local $M_{BH}$-$M_*$ scaling relation across the cosmic epoch. Additionally, we explore the capability of narrow-line diagnostics in identifying Type 2 AGN and find that a significant fraction of our AGN would be missed by them due to low metallicity or lack of high energy ionizing photons (potentially due to dust absorption, dense gas blanketing the broad and narrow line regions, or intrinsically soft ionizing spectra). We explore the UV luminosity function of AGN and their hosts and find that it is subject to significant cosmic variance and is also dependent on the AGN bolometric luminosity. Finally, we show that the electron and Balmer scattering scenarios recently proposed to explain the broad components of the Balmer lines are untenable on multiple grounds. There is no evidence that the black hole masses have been overestimated by orders of magnitude as proposed in those scenarios.

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arXiv2025

The dawn of disks: unveiling the turbulent ionised gas kinematics of the galaxy population at $z\sim4-6$ with JWST/NIRCam grism spectroscopy

Recent studies of gas kinematics at high redshift have reported disky systems which appear to challenge models of galaxy formation, but it is unclear whether they are representative of the underlying galaxy population. We present the first statistical sample of spatially resolved ionised gas kinematics at high redshift, comprised of $272$ H$α$ emitters in GOODS-S and GOODS-N at redshifts $z\approx3.9-6.5$, observed with JWST/NIRCam slitless spectroscopy and imaging from JADES, FRESCO and CONGRESS. The sample probes two orders of magnitude in stellar mass ($\log (M_{\star}[\mathrm{M}_{\odot}])\approx8-10$) and star formation rate ($\text{SFR}\approx0.3-100\thinspace M_{\odot}/$yr), and is representative down to $\log(M_{\star}[\mathrm{M}_{\odot}])\approx 9$. Using a novel inference tool, $\texttt{geko}$, we model the grism data to measure morphological and kinematic properties of the ionised gas, as probed by H$α$. Our results are consistent with a decrease of the rotational support $v/σ_0$\ and increase of the velocity dispersion $σ_0$ with redshift, with $σ_0\approx100$ km/s and $v/σ_0\approx1-2$ at $z\approx3.9-6.5$. We study the relations between $σ_0$, and $v/σ_0$, and different star formation tracers and find a large scatter and diversity, with the strongest correlations between $σ_0$ and SFR and SFR surface density. The fraction of rotationally supported systems ($v/σ_0>1$) slightly increases with cosmic time, from $(36\pm6)\%$ to $(41\pm6)\%$ from $z\sim 5.5$ to $z\sim 4.5$, for galaxies with masses $9<\log(M_{\star}[\mathrm{M}_{\odot}])<10$. Overall, disks do not dominate the turbulent high-redshift galaxy population in the mass range probed by this work. When placed in the context of studies up to cosmic noon, our results are consistent with a significant increase of disk-like systems with cosmic time.

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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.

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arXiv2025

Zapped then Napped? A rapidly quenched remnant leaker candidate with a steep spectroscopic $β_{UV}$ slope at z=8.5

We use NIRSpec MSA spectroscopy and NIRCam Photometry to explore the properties of JADES-GS8-RL-1, a rapidly quenched, $z=8.5$ galaxy with a stellar mass of $10^{8.9}M_\odot$, a steep blue UV slope, a Balmer break, and no sign of strong emission lines. With a $β_{UV}$=-2.8$\pm 0.2$, as measured from the NIRSpec spectrum, JADES-GS8-RL-1 is consistent with negligible dust attenuation and little to no contribution from the nebular continuum alongside a probable high escape fraction. The $β_{UV}$ slope measured from photometry varies from -3.0 in the central regions to -2.2 at the outskirts suggesting possible regional differences in the escape fraction. There are no high-ionisation emission lines, only a tentative 2.9\sig detection of [OII]. Using photometry, this emission appears to be extended, possibly corresponding to weakly ionised gas expelled during or after the quenching process. JADES-GS8-RL-1 is spatially resolved with a half-light radius of 240 pc and has an exponential, disc-like morphology. It appears to have formed all its stars in a short burst within the past 100 Myr with a formation time of $\approx$70 Myr and a quenching time of $\approx$30 Myr. This quenching would have occurred rapidly, making it a more distant example of the kind of low-mass "mini-quenched" galaxies previously observed at high-z. Due to the extremely blue $β_{UV}$ slope, our best-fit model predicts a high value for \fesc of >10\%, consistent with the value derived from the $β_{UV}$ slope, which when combined with our extraordinarily low O32 upper limit suggests JADES-GS8-RL-1 is a fascinating example of a high-z "remnant leaker" in one of its earliest phases, deep in the epoch of reionisation.

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arXiv2025

The abundance and nature of high-redshift quiescent galaxies from JADES spectroscopy and the FLAMINGO simulations

We use NIRSpec/MSA spectroscopy and NIRCam imaging to study a sample of 18 massive ($\log\; M_{*}/M_{\odot} \gt 10\;$dex), central quiescent galaxies at $2\leq z \leq 5$ in the GOODS fields, to investigate their number density, star-formation histories, quenching timescales, and incidence of AGN. The data depth reaches $\log M_*/M_\odot \approx 9\;$dex, yet the least-massive central quiescent galaxy found has $\log M_*/M_\odot \gt 10\;$dex, suggesting that quenching is regulated by a physical quantity that scales with $M_*$. With spectroscopy, we assess the completeness and purity of photometric samples, finding number densities 10 times higher than predicted by galaxy formation models, confirming earlier photometric studies. We compare our number densities to predictions from FLAMINGO, the largest-box full-hydro simulation suite to date. We rule out cosmic variance at the 3-$σ$ level, providing spectroscopic confirmation that galaxy formation models do not match observations at $z>3$. Using FLAMINGO, we find that the vast majority of quiescent galaxies' stars formed in situ, with these galaxies not having undergone multiple major dry mergers. This is in agreement with the compact observed size of these systems and suggests that major mergers are not a viable channel for quenching most massive galaxies. Several of our observed galaxies are old, with four galaxies displaying 4000-Å breaks with formation and quenching redshifts of $z\geq8$ and $z\geq6$. Using tracers, we find that 8 galaxies host AGN, including old systems suggesting a high AGN duty cycle with a continuing trickle of gas to fuel accretion.

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arXiv2025

JADES: The star-formation and chemical enrichment history of a luminous galaxy at z~9.43 probed by ultra-deep JWST/NIRSpec spectroscopy

We analyse ultra-deep JWST observations of the galaxy JADES-GS-z9-0 at z = 9.4327, and derive detailed stellar and interstellar medium (ISM) properties of this luminous (MUV=-20.43) high-redshift system. Complementary information from NIRCam imaging and NIRSpec (both low- and medium-resolution) spectroscopy reveal a compact system (Re ~110 pc) characterised by a steeply rising star formation history, which is reflected in the inferred young stellar age (t ~ 3 Myr, light-weighted), high star-formation rate surface density (ΣSFR ~ 72 M yr-1 kpc-2), high ionisation parameter (log(U) ~ -1.5), low metallicity (12+log(O/H) ~ 7.5), and low carbon-over-oxygen abundance ([C/O] = -0.64). Leveraging the detection of N iii]1750 we derive nitrogen-over-oxygen abundance ([N/O] ~ 0) higher than the plateau followed by low-redshift galaxies of similar metallicity, possibly revealing the imprint from (very) massive stars on the ISM enrichment and favouring a top-heavy Initial Mass Function (IMF) scenario. Massive stars powering a hard radiation field are also required to explain the rest-frame UV line ratios, though the presence of the high-excitation [Ne v]λ3426 emission line possibly hints at additional ionization from an AGN. We also report the tentative detection of Lyα emission in the G140M spectrum, shifted by ~450 km/s redward of the systemic redshift. Combined with a modelling of the Lyα spectral break, we rule out the presence of very high column densities of neutral gas pertaining to local absorbers, as well as any extended surrounding ionised bubble, suggesting that JADES-GS-z9-0 has not yet significantly contributed to cosmic Reionization.

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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.

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arXiv2024

A dormant, overmassive black hole in the early Universe

Recent observations have found a large number of supermassive black holes already in place in the first few hundred million years after Big Bang. The channels of formation and growth of these early, massive black holes are not clear, with scenarios ranging from heavy seeds to light seeds experiencing bursts of high accretion rate. Here we present the detection, from the JADES survey, of broad Halpha emission in a galaxy at z=6.68, which traces a black hole with mass of ~ 4 * 10^8 Msun and accreting at a rate of only 0.02 times the Eddington limit. The host galaxy has low star formation rate (~ 1 Msun/yr, a factor of 3 below the star forming main sequence). The black hole to stellar mass ratio is ~ 0.4, i.e. about 1,000 times above the local relation, while the system is closer to the local relations in terms of dynamical mass and velocity dispersion of the host galaxy. This object is most likely the tip of the iceberg of a much larger population of dormant black holes around the epoch of reionisation. Its properties are consistent with scenarios in which short bursts of super-Eddington accretion have resulted in black hole overgrowth and massive gas expulsion from the accretion disk; in between bursts, black holes spend most of their life in a dormant state.

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arXiv2024

The Relation between AGN and Host Galaxy Properties in the JWST Era: I. Seyferts at Cosmic Noon are Obscured and Disturbed

The morphology of a galaxy reflects the mix of physical processes occurring within and around it, offering indirect clues to its formation and evolution. We apply both visual classification and computer vision to test the suspected connection between galaxy mergers and AGN activity, as evidenced by a close/merging galaxy pair, or tidal features surrounding an apparently singular system. We use JADES JWST/NIRCam imagery of a complete, mutliwavelength AGN sample recently expanded with JWST/MIRI photometry. This 0.9-25 $μ$m dataset enables constraints on the host galaxy morphologies of a broad range of AGN beyond z$\sim$1, including heavily obscured examples missing from previous studies. Our primary AGN sample consists of 243 lightly to highly obscured X-ray-selected AGN and 138 presumed Compton-thick, mid-infrared-bright/X-ray-faint AGN revealed by MIRI. Utilizing the shape asymmetry morphology indicator, $A_S$, as the metric for disturbance, we find that 88% of the Seyferts sampled are strongly spatially disturbed ($A_S>0.2$). The experimental design we employ reveals a $\gtrsim 3σ$ obscuration-merger ($N_H$-$A_S$) correlation at $0.6<z<2.4$, and also recovers a physical distinction between the X-ray- and mid-IR-detected AGN suggestive of their link to a common evolutionary scenario. Placing the observed pattern of disturbances in the context of the other average host galaxy properties, we conclude that mergers are common amongst obscured AGN. This finding presents tension with the leading model on AGN fueling that requires Seyfert AGN with sub-quasar luminosities ($L_{bol} < 10^{45}$ ergs/s) to evolve only through non-merger mechanisms.

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arXiv2024

JADES + JEMS: A Detailed Look at the Buildup of Central Stellar Cores and Suppression of Star Formation in Galaxies at Redshifts 3 < z < 4.5

We present a spatially resolved study of stellar populations in 6 galaxies with stellar masses $M_*\sim10^{10}M_\odot$ at $z\sim3.7$ using 14-filter JWST/NIRCam imaging from the JADES and JEMS surveys. The 6 galaxies are visually selected to have clumpy substructures with distinct colors over rest-frame $3600-4100Å$, including a red, dominant stellar core that is close to their stellar-light centroids. With 23-filter photometry from HST to JWST, we measure the stellar-population properties of individual structural components via SED fitting using Prospector. We find that the central stellar cores are $\gtrsim2$ times more massive than the Toomre mass, indicating they may not form via single in-situ fragmentation. The stellar cores have stellar ages of $0.4-0.7$ Gyr that are similar to the timescale of clump inward migration due to dynamical friction, suggesting that they likely instead formed through the coalescence of giant stellar clumps. While they have not yet quenched, the 6 galaxies are below the star-forming main sequence by $0.2-0.7$ dex. Within each galaxy, we find that the specific star formation rate is lower in the central stellar core, and the stellar-mass surface density of the core is already similar to quenched galaxies of the same masses and redshifts. Meanwhile, the stellar ages of the cores are either comparable to or younger than the extended, smooth parts of the galaxies. Our findings are consistent with model predictions of the gas-rich compaction scenario for the buildup of galaxies' central regions at high redshifts. We are likely witnessing the coeval formation of dense central cores, along with the onset of galaxy-wide quenching at $z>3$.

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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.

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arXiv2024

A core in a star-forming disc as evidence of inside-out growth in the early Universe

The physical processes that establish the morphological evolution and the structural diversity of galaxies are key unknowns in extragalactic astrophysics. Here we report the finding of the morphologically-mature galaxy JADES-GS+53.18343-27.79097, which existed within the first 700 million years of the Universe's history. This star-forming galaxy with a stellar mass of 400 million solar masses consists of three components, a highly-compact core with a half-light radius of less than 100 pc, an actively star-forming disc with a radius of about 400 pc, and a star-forming clump, which all show distinctive star-formation histories. The central stellar mass density of this galaxy is within a factor of two of the most massive present-day ellipticals, while being globally 1000 times less massive. The radial profile of the specific star-formation rate is rising toward the outskirts. This evidence suggests the first detection of inside-out growth of a galaxy as a proto-bulge and a star-forming disc in the Epoch of Reionization.

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arXiv2024

Different regulation of stellar metallicities between star-forming and quiescent galaxies -- Insights into galaxy quenching

One of the most important questions in astrophysics is what causes galaxies to stop forming stars. Previous studies have shown a tight link between quiescence and black hole mass. Other studies have revealed that quiescence is also associated with 'starvation', the halting of gas inflows, which results in the remaining gas being used up by star formation and in rapid chemical enrichment. In this work, we find the missing link between these two findings. Using a large sample of galaxies, we uncover the intrinsic dependencies of the stellar metallicity on galaxy properties. In the case of star-forming galaxies, stellar metallicity is primarily driven by stellar mass. However, for passive galaxies, the stellar metallicity is primarily driven by the stellar velocity dispersion. The latter is known to be tightly correlated with black hole mass. This result can be seen as connecting previous studies, where the integrated effect of black hole feedback (i.e. black hole mass, traced by the velocity dispersion) prevents gas inflows, starving the galaxy, which is seen by the rapid increase in the stellar metallicity, and leading to the galaxy becoming passive.

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arXiv2024

JADES Ultra-red Flattened Objects: Morphologies and Spatial Gradients in Color and Stellar Populations

One of the more surprising findings after the first year of JWST observations is the large number of spatially extended galaxies (ultra-red flattened objects, or UFOs) among the optically-faint galaxy population otherwise thought to be compact. Leveraging the depth and survey area of the JADES survey, we extend observations of the optically-faint galaxy population to an additional 112 objects, 56 of which are well-resolved in F444W with effective sizes, $R_e > 0.25''$, more than tripling previous UFO counts. These galaxies have redshifts around $2 < z < 4$, high stellar masses ($\mathrm{log(M_*/M_{\odot})} \sim 10-11$), and star-formation rates around $\sim 100-1000 \mathrm{M_{\odot}/yr}$. Surprisingly, UFOs are red across their entire extents which spatially resolved analysis of their stellar populations shows is due to large values of dust attenuation (typically $A_V > 2$ mag even at large radii). Morphologically, the majority of our UFO sample tends to have low Sérsic indices ($n \sim 1$) suggesting these large, massive, optically faint galaxies have little contribution from a bulge in F444W. Further, a majority have axis-ratios between $0.2 < q < 0.4$, which Bayesian modeling suggests that their intrinsic shapes are consistent with being a mixture of inclined disks and prolate objects with little to no contribution from spheroids. While kinematic constraints will be needed to determine the true intrinsic shapes of UFOs, it is clear that an unexpected population of large, disky or prolate objects contributes significantly to the population of optically faint galaxies.

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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.

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arXiv2024

Identification of High-Redshift Galaxy Overdensities in GOODS-N and GOODS-S

We conduct a systematic search for high-redshift galaxy overdensities at $4.9 < z_{\,\mathrm{spec}} < 8.9$ in both the GOODS-N and GOODS-S fields using JWST/NIRCam imaging from JADES and JEMS in addition to JWST/NIRCam wide field slitless spectroscopy from FRESCO. High-redshift galaxy candidates are identified using HST+JWST photometry spanning $λ= 0.4-5.0\ μ\mathrm{m}$. We confirmed the redshifts for roughly a third of these galaxies using JWST/FRESCO spectroscopy over $λ= 3.9-5.0\ μ\mathrm{m}$ through identification of either $\mathrm{H} α$ or $\left[\mathrm{OIII}\right]\lambda5008$ around the best-fit photometric redshift. The rest-UV magnitudes and continuum slopes of these galaxies were inferred from the photometry: the brightest and reddest objects appear in more dense environments and thus are surrounded by more galaxy neighbors than their fainter and bluer counterparts, suggesting accelerated galaxy evolution within overdense environments. We find $17$ significant ($δ_{\mathrm{gal}} \geq 3.04$, $N_{\mathrm{galaxies}} \geq 4$) galaxy overdensities across both fields ($7$ in GOODS-N and $10$ in GOODS-S), including the two highest redshift spectroscopically confirmed galaxy overdensities to date at $\left< z_{\mathrm{\,spec}} \right> = 7.954$ and $\left< z_{\mathrm{\,spec}} \right> = 8.222$ (representing densities around $\sim 6$ and $\sim 12$ times that of a random volume). We estimate the total halo mass of these large-scale structures to be $11.5 \leq \mathrm{log}_{10}\left(M_{\mathrm{halo}}/M_{\odot}\right) \leq 13.4$ using an empirical stellar mass to halo mass relation, which are likely underestimates as a result of incompleteness. These protocluster candidates are expected to evolve into massive galaxy clusters with $\mathrm{log}_{10}\left(M_{\mathrm{halo}}/M_{\odot}\right) \gtrsim 14$ by $z = 0$.

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