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Benjamin D. Johnson

Publications and source records attributed to Benjamin D. Johnson.

At least 127 records · Page 7Linked to original sources
arXiv2023

Modeling the High-Energy Ionizing Output from Simple Stellar and X-ray Binary Populations

We present a methodology for modeling the joint ionizing impact due to a "simple X-ray population" (SXP) and its corresponding simple stellar population (SSP), where "simple" refers to a single age and metallicity population. We construct composite spectral energy distributions (SEDs) including contributions from ultra-luminous X-ray sources (ULXs) and stars, with physically meaningful and consistent consideration of the relative contributions of each component as a function of instantaneous burst age and stellar metallicity. These composite SEDs are used as input for photoionization modeling with Cloudy, from which we produce a grid for the time- and metallicity-dependent nebular emission from these composite populations. We make the results from the photoionization simulations publicly available. We find that the addition of the SXP prolongs the high-energy ionizing output from the population, and correspondingly increases the intensity of nebular lines such as He II $λ$1640,4686, [Ne V] $λ$3426,14.3$μ$m, and [O IV] 25.9$μ$m by factors of at least two relative to models without an SXP spectral component. This effect is most pronounced for instantaneous bursts of star formation on timescales $>$ 10 Myr and at low metallicities ($\sim$ 0.1 $Z_{\odot}$), due to the imposed time- and metallicity-dependent behavior of the SXP relative to the SSP. We propose nebular emission line diagnostics accessible with JWST suitable for inferring the presence of a composite SXP + SSP, and discuss how the ionization signatures compare to models for sources such as intermediate mass black holes.

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arXiv2023

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).

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arXiv2023

Detection of Accretion Shelves Out to the Virial Radius of a Low-Mass Galaxy with JWST

We report the serendipitous discovery of an extended stellar halo surrounding the low-mass galaxy Ark 227 ($M_\ast=5\times10^9 M_\odot$; d=35 Mpc) in deep JWST NIRCam imaging from the Blue Jay Survey. The F200W-F444W color provides robust star-galaxy separation, enabling the identification of stars at very low density. By combining resolved stars at large galactocentric distances with diffuse emission from NIRCam and Dragonfly imaging at smaller distances, we trace the surface brightness and color profiles of this galaxy over the entire extent of its predicted dark matter halo, from 0.1-100 kpc. Controlled N-body simulations have predicted that minor mergers create "accretion shelves" in the surface brightness profile at large radius. We observe such a feature in Ark 227 at 10-20 kpc, which, according to models, could be caused by a merger with total mass ratio 1:10. The metallicity declines over this radial range, further supporting the minor merger scenario. There is tentative evidence of a second shelf at $μ_V\approx 35$ mag arcsec$^{-2}$ extending from 50-100 kpc, along with a corresponding drop in metallicity. The stellar mass in this outermost envelope is $\approx10^7M_\odot$. These results suggest that Ark 227 experienced multiple mergers with a spectrum of lower-mass galaxies -- a scenario that is broadly consistent with the hierarchical growth of structure in a cold dark matter-dominated universe. Finally, we identify an ultra-faint dwarf associated with Ark 227 with $M_\ast\approx10^5 M_\odot$ and $μ_{V,e}=28.1$ mag arcsec$^{-2}$, demonstrating that JWST is capable of detecting very low-mass dwarfs to distances of at least ~30 Mpc.

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arXiv2023

JADES: Resolving the Stellar Component and Filamentary Overdense Environment of HST-Dark Submillimeter Galaxy HDF850.1 at $z=5.18$

HDF850.1 is the brightest submillimeter galaxy (SMG) in the Hubble Deep Field. It is known as a heavily dust-obscured star-forming galaxy embedded in an overdense environment at $z = 5.18$. With nine-band NIRCam images at 0.8-5.0 $μ$m obtained through the JWST Advanced Deep Extragalactic Survey (JADES), we detect and resolve the rest-frame UV-optical counterpart of HDF850.1, which splits into two components because of heavy dust obscuration in the center. The southern component leaks UV and H$α$ photons, bringing the galaxy $\sim$100 times above the empirical relation between infrared excess and UV continuum slope (IRX-$β_\mathrm{UV}$). The northern component is higher in dust attenuation and thus fainter in UV and H$α$ surface brightness. We construct a spatially resolved dust attenuation map from the NIRCam images, well matched with the dust continuum emission obtained through millimeter interferometry. The whole system hosts a stellar mass of $10^{10.8\pm0.1}\,\mathrm{M}_\odot$ and star-formation rate of $10^{2.8\pm0.2}\,\mathrm{M}_\odot\,\mathrm{yr}^{-1}$, placing the galaxy at the massive end of the star-forming main sequence at this epoch. We further confirm that HDF850.1 resides in a complex overdense environment at $z=5.17-5.30$, which hosts another luminous SMG at $z=5.30$ (GN10). The filamentary structures of the overdensity are characterized by 109 H$α$-emitting galaxies confirmed through NIRCam slitless spectroscopy at 3.9-5 $μ$m, of which only eight were known before the JWST observations. Given the existence of a similar galaxy overdensity in the GOODS-S field, our results suggest that $50\pm20$% of the cosmic star formation at $z=5.1-5.5$ occur in protocluster environments.

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arXiv2023

FRESCO: An extended, massive, rapidly rotating galaxy at z=5.3

With the remarkable sensitivity and resolution of JWST in the infrared, measuring rest-optical kinematics of galaxies at $z>5$ has become possible for the first time. This study pilots a new method for measuring galaxy dynamics for highly multiplexed, unbiased samples by combining FRESCO NIRCam grism spectroscopy and JADES medium-band imaging. Here we present one of the first JWST kinematic measurements for a galaxy at $z>5$. We find a significant velocity gradient, which, if interpreted as rotation yields $V_{rot} = 240\pm50$km/s and we hence refer to this galaxy as Twister-z5. With a rest-frame optical effective radius of $r_e=2.25$kpc, the high rotation velocity in this galaxy is not due to a compact size as may be expected in the early universe but rather a high total mass, ${\rm log(M}_{dyn}/{\rm M}_\odot)=11.0\pm0.2$. This is a factor of roughly 4x higher than the stellar mass within the effective radius. We also observe that the radial H$α$ equivalent width profile and the specific star formation rate map from resolved stellar population modeling is centrally depressed by a factor of $\sim1.5$ from the center to $r_e$. Combined with the morphology of the line-emitting gas in comparison to the continuum, this centrally suppressed star formation is consistent with a star-forming disk surrounding a bulge growing inside-out. While large, rapidly rotating disks are common to z~2, the existence of one after only 1Gyr of cosmic time, shown for the first time in ionized gas, adds to the growing evidence that some galaxies matured earlier than expected in the history of the universe.

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arXiv2023

The JWST Advanced Deep Extragalactic Survey: Discovery of an Extreme Galaxy Overdensity at $z = 5.4$ with JWST/NIRCam in GOODS-S

We report the discovery of an extreme galaxy overdensity at $z = 5.4$ in the GOODS-S field using JWST/NIRCam imaging from JADES and JEMS alongside JWST/NIRCam wide field slitless spectroscopy from FRESCO. We identified potential members of the overdensity using HST+JWST photometry spanning $λ= 0.4-5.0\ μ\mathrm{m}$. These data provide accurate and well-constrained photometric redshifts down to $m \approx 29-30\,\mathrm{mag}$. We subsequently confirmed $N = 81$ galaxies at $5.2 < z < 5.5$ using JWST slitless spectroscopy over $λ= 3.9-5.0\ μ\mathrm{m}$ through a targeted line search for $\mathrm{H} α$ around the best-fit photometric redshift. We verified that $N = 42$ of these galaxies reside in the field while $N = 39$ galaxies reside in a density around $\sim 10$ times that of a random volume. Stellar populations for these galaxies were inferred from the photometry and used to construct the star-forming main sequence, where protocluster members appeared more massive and exhibited earlier star formation (and thus older stellar populations) when compared to their field galaxy counterparts. We estimate the total halo mass of this large-scale structure to be $12.6 \lesssim \mathrm{log}_{10} \left( M_{\mathrm{halo}}/M_{\odot} \right) \lesssim 12.8$ using an empirical stellar mass to halo mass relation, which is likely an underestimate as a result of incompleteness. Our discovery demonstrates the power of JWST at constraining dark matter halo assembly and galaxy formation at very early cosmic times.

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arXiv2023

JADES: Discovery of extremely high equivalent width Lyman-alpha emission from a faint galaxy within an ionized bubble at z=7.3

We report the discovery of a remarkable Ly$α$ emitting galaxy at z = 7.2782, JADES-GS-z7-LA, with EW$_0$(Ly$α$) $= 388.0 \pm 88.8$Åand UV magnitude -17.0. The spectroscopic redshift is confirmed via rest-frame optical lines [O II], H$β$ and [O III] in its JWST/NIRSpec Micro-Shutter Assembly (MSA) spectrum. The Ly$α$ line is detected in both lower resolution PRISM as well as medium resolution G140M grating spectra. The LSF-deconvolved Ly$α$ FWHM in the grating is $383.9 \pm 56.2$ km/s and the Ly$α$ velocity offset compared to the systemic redshift is $113.3 \pm 80.0$ km/s, indicative of very little neutral gas or dust within the galaxy. We estimate the Ly$α$ escape fraction to be >70%. JADES-GS-z7-LA has a O32 ratio of $11.1 \pm 2.2$ and a R23 ratio of $11.2 \pm 2.6$, consistent with low metallicity and high ionization parameters. Deep NIRCam imaging also revealed a close companion source (separated by 0.23"), which exhibits similar photometry to that of JADES-GS-z7-LA, with a photometric excess in the F410M NIRCam image consistent with [O III]+H$β$ emission at the same redshift. The spectral energy distribution of JADES-GS-z7-LA indicates a "bursty" star formation history, with a low stellar mass of $\approx 10^7$ $M_\odot$. Assuming that the Ly$α$ transmission through the intergalactic medium is the same as its measured escape fraction, an ionized region of size > 1.5 pMpc is needed to explain the high Ly$α$ EW and low velocity offset compared to systemic seen in JADES-GS-z7-LA. Owing to its UV-faintness, we show that it is incapable of single-handedly ionizing a region large enough to explain its Ly$α$ emission. Therefore, we suggest that JADES-GS-z7-LA (and possibly the companion source) may be a part of a larger overdensity, presenting direct evidence of overlapping ionized bubbles at $z>7$.

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arXiv2023

JADES Initial Data Release for the Hubble Ultra Deep Field: Revealing the Faint Infrared Sky with Deep JWST NIRCam Imaging

JWST has revolutionized the field of extragalactic astronomy with its sensitive and high-resolution infrared view of the distant universe. Adding to the new legacy of JWST observations, we present the first NIRCam imaging data release from the JWST Advanced Deep Extragalactic Survey (JADES) providing 9 filters of infrared imaging of $\sim$25 arcmin$^2$ covering the Hubble Ultra Deep Field and portions of Great Observatories Origins Deep Survey (GOODS) South. Utilizing 87 on-sky dual-filter hours of exposure time, these images reveal the deepest ever near-infrared view of this iconic field. We supply carefully constructed 9-band mosaics of the JADES bands, as well as matching reductions of 5 additional bands from the JWST Extragalactic Medium-band Survey (JEMS). Combining with existing HST imaging, we provide 23-band space-based photometric catalogs and photometric redshifts for $\approx47,500$ sources. To promote broad engagement with the JADES survey, we have created an interactive {\tt FitsMap} website to provide an interface for professional researchers and the public to experience these JWST datasets. Combined with the first JADES NIRSpec data release, these public JADES imaging and spectroscopic datasets provide a new foundation for discoveries of the infrared universe by the worldwide scientific community.

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arXiv2023

Minor merger growth in action: JWST detects faint blue companions around massive quiescent galaxies at 0.5 < z < 3

Minor mergers are thought to drive the structural evolution of massive quiescent galaxies; however, existing HST imaging is primarily sensitive to stellar mass ratios >1:10. Here, we report the discovery of a large population of low-mass companions within 35 kpc of known logM*/Msun > 10.5 quiescent galaxies at 0.5 < z < 3. While massive companions like those identified by HST are rare, JWST imaging from JADES reveals that the average massive quiescent galaxy hosts ~5 nearby companions with stellar mass ratios <1:10. Despite a median stellar mass ratio of just 1:900, these tiny companions are so numerous that they represent at least 30\% of the total mass being added to quiescent galaxies via minor mergers. While relatively massive companions have colors similar to their hosts, companions with mass ratios <1:10 typically have bluer colors and lower mass-to-light ratios than their host galaxies at similar radii. The accretion of these tiny companions is likely to drive evolution in the color gradients and stellar population properties of the host galaxies. Our results suggest that the well-established ``minor merger growth" model for quiescent galaxies extends down to very low mass ratios of <1:100, and demonstrates the power of JWST to constrain both the spatially-resolved properties of massive galaxies and the properties of low-mass companions beyond the local universe.

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arXiv2023

Carbonaceous dust grains seen in the first billion years of cosmic time

Large dust reservoirs (up to $\sim 10^8 \, \mathrm{M_\odot}$) have been detected in galaxies out to redshift $z \sim 8$, when the age of the universe was only about 600 Myr. Generating significant amounts of dust within such a short timescale has proven challenging for theories of dust formation and has prompted the revision of the modelling of potential sites of dust production such as the atmospheres of asymptotic giant branch (AGB) stars in low-metallicity environments, supernovae (SNe) ejecta, and the accelerated growth of grains in the interstellar medium (ISM). However, degeneracies between different evolutionary pathways remain when the total dust mass of galaxies is the only available observable. Here we report observations of the $2175 \, \mathring{\rm A}$ dust attenuation feature, well known in the Milky Way (MW) and galaxies at $z \lesssim 3$, in the near-infrared spectra of galaxies up to $z \sim 7$, corresponding to the first billion years of cosmic time. The relatively short timescale implied for the formation of carbonaceous grains giving rise to this feature suggests a rapid production process, likely in Wolf-Rayet (WR) stars or SN ejecta.

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arXiv2023

JADES: deep spectroscopy of a low-mass galaxy at redshift 2.3 quenched by environment

We report the discovery of a quiescent galaxy at $z=2.34$ with a stellar mass of only $M_\star = 9.5^{+1.8}_{-1.2} \times 10^{8} \mathrm{M}_\odot$, based on deep JWST/NIRSpec spectroscopy. This is the least massive quiescent galaxy found so far at high redshift. We use a Bayesian approach to model the spectrum and photometry, and find the target to have been quiescent for 0.6 Gyr with a mass-weighted average stellar age of 0.8-1.7 Gyr (dominated by systematics). The galaxy displays an inverse colour gradient with radius, consistent with environment-driven quenching. Based on a combination of spectroscopic and robust (medium- and broad-band) photometric redshifts, we identify a galaxy overdensity near the location of the target (5-$σ$ above the background level at this redshift). We stress that had we been specifically targetting galaxies within overdensities, the main target would not have been selected on photometry alone; therefore, environment studies based on photometric redshifts are biased against low-mass quiescent galaxies. The overdensity contains three spectroscopically confirmed, massive, old galaxies ($M_\star = 8-17 \times 10^{10} \mathrm{M}_\odot$). The presence of these evolved systems points to accelerated galaxy evolution in overdensities at redshifts z > 2, in agreement with previous works. In projection, our target lies only 35 pkpc away from the most massive galaxy in this overdensity (spectroscopic redshift z = 2.349) which is located close to overdensity's centre. This suggests the low-mass galaxy was quenched by environment, making it possibly the earliest evidence for environment-driven quenching to date.

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arXiv2023

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

The power-law slope of the rest-UV continuum ($f_λ\proptoλ^β$) 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 $β=-2.4$. 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 $β<-2.8$, 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.

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arXiv2023

JADES: Probing interstellar medium conditions at $z\sim5.5-9.5$ with ultra-deep JWST/NIRSpec spectroscopy

We present emission line ratios from a sample of 26 Lyman break galaxies from $z\sim5.5-9.5$ with $-17.0 5.5$ spectra. We find that the emission line ratios exhibited by these $z\sim5.5-9.5$ galaxies occupy clearly distinct regions of line-ratio space compared to typical z~0-3 galaxies, instead being more consistent with extreme populations of lower-redshift galaxies. This is best illustrated by the [OIII]/[OII] ratio, tracing interstellar medium (ISM) ionisation, in which we observe more than half of our sample to have [OIII]/[OII]>10. Our high signal-to-noise spectra reveal more than an order of magnitude of scatter in line ratios such as [OII]/H$β$ and [OIII]/[OII], indicating significant diversity in the ISM conditions within the sample. We find no convincing detections of [NII] in our sample, either in individual galaxies, or a stack of all G395M/F290LP spectra. The emission line ratios observed in our sample are generally consistent with galaxies with extremely high ionisation parameters (log $U\sim-1.5$), and a range of metallicities spanning from $\sim0.1\times Z_\odot$ to higher than $\sim0.3\times Z_\odot$, suggesting we are probing low-metallicity systems undergoing periods of rapid star-formation, driving strong radiation fields. These results highlight the value of deep observations in constraining the properties of individual galaxies, and hence probing diversity within galaxy population.

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arXiv2023

Distant Echoes of the Milky Way's Last Major Merger

The majority of the Milky Way's stellar halo consists of debris from our Galaxy's last major merger, the Gaia-Sausage-Enceladus (GSE). In the past few years, stars from GSE have been kinematically and chemically studied in the inner $30$ kpc of our Galaxy. However, simulations predict that accreted debris could lie at greater distances, forming substructures in the outer halo. Here we derive metallicities and distances using Gaia DR3 XP spectra for an all-sky sample of luminous red giant stars, and map the outer halo with kinematics and metallicities out to $100$ kpc. We obtain follow-up spectra of stars in two strong overdensities - including the previously identified Outer Virgo Overdensity - and find them to be relatively metal-rich and on predominantly retrograde orbits, matching predictions from simulations of the GSE merger. We argue that these are apocentric shells of GSE debris, forming $60-90$ kpc counterparts to the $15-20$ kpc shells that are known to dominate the inner stellar halo. Extending our search across the sky with literature radial velocities, we find evidence for a coherent stream of retrograde stars encircling the Milky Way from $50-100$ kpc, in the same plane as the Sagittarius stream but moving in the opposite direction. These are the first discoveries of distant and structured imprints from the GSE merger, cementing the picture of an inclined and retrograde collision that built up our Galaxy's stellar halo.

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arXiv2023

As Simple as Possible but No Simpler: Optimizing the Performance of Neural Net Emulators for Galaxy SED Fitting

Artificial neural network emulators have been demonstrated to be a very computationally efficient method to rapidly generate galaxy spectral energy distributions (SEDs), for parameter inference or otherwise. Using a highly flexible and fast mathematical structure, they can learn the nontrivial relationship between input galaxy parameters and output observables. However, they do so imperfectly, and small errors in flux prediction can yield large differences in recovered parameters. In this work, we investigate the relationship between an emulator's execution time, uncertainties, correlated errors, and ability to recover accurate posteriors. We show that emulators can recover consistent results to traditional fits, with precision of $25\!-\!40\%$ in posterior medians for stellar mass, stellar metallicity, star formation rate, and stellar age. We find that emulation uncertainties scale with an emulator's width $N$ as $\propto N^{-1}$ while execution time scales as $\propto N^2$, resulting in an inherent tradeoff between execution time and emulation uncertainties. We also find that emulators with uncertainties smaller than observational uncertaities are able to recover accurate posteriors for most parameters without a significant increase in catastrophic outliers. Furthermore, we demonstrate that small architectures can produce flux residuals that have significant correlations, which can create dangerous systematic errors in colors. Finally, we show that the distributions chosen for generating training sets can have a large effect on emulators' ability to accurately fit rare objects. Selecting the optimal architecture and training set for an emulator will minimize the computational requirements for fitting near-future large-scale galaxy surveys.

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arXiv2023

Discovery of the Magellanic Stellar Stream Out to 100 Kiloparsecs

The Magellanic Stream (MS) - an enormous ribbon of gas spanning $140^\circ$ of the southern sky trailing the Magellanic Clouds - has been exquisitely mapped in the five decades since its discovery. However, despite concerted efforts, no stellar counterpart to the MS has been conclusively identified. This stellar stream would reveal the distance and 6D kinematics of the MS, constraining its formation and the past orbital history of the Clouds. We have been conducting a spectroscopic survey of the most distant and luminous red giant stars in the Galactic outskirts. From this dataset, we have discovered a prominent population of 13 stars matching the extreme angular momentum of the Clouds, spanning up to $100^\circ$ along the MS at distances of $60-120$ kpc. Furthermore, these kinemetically-selected stars lie along a [$α$/Fe]-deficient track in chemical space from $-2.5 < \mathrm{[Fe/H]} < -0.5$, consistent with their formation in the Clouds themselves. We identify these stars as high-confidence members of the Magellanic Stellar Stream. Half of these stars are metal-rich and closely follow the gaseous MS, whereas the other half are more scattered and metal-poor. We argue that the metal-rich stream is the recently-formed tidal counterpart to the MS, and speculate that the metal-poor population was thrown out of the SMC outskirts during an earlier interaction between the Clouds. The Magellanic Stellar Stream provides a strong set of constraints - distances, 6D kinematics, and birth locations - that will guide future simulations towards unveiling the detailed history of the Clouds.

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arXiv2023

JADES: Differing assembly histories of galaxies -- Observational evidence for bursty SFHs and (mini-)quenching in the first billion years of the Universe

We use deep NIRSpec spectroscopic data from the JADES survey to derive the star formation histories (SFHs) of a sample of 200 galaxies at 0.6$<$z$<$11 and spanning stellar masses from $\rm 10^6$ to $\rm 10^{9.5}~M_\odot$. We find that galaxies at high-redshift, galaxies above the Main Sequence (MS) and low-mass galaxies tend to host younger stellar populations than their low-redshift, massive, and below the MS counterparts. Interestingly, the correlation between age, M$_*$ and SFR existed even earlier than Cosmic Noon, out to the earliest cosmic epochs. However, these trends have a large scatter. Indeed, there are examples of young stellar populations also below the MS, indicating recent (bursty) star formation in evolved systems. We explore further the burstiness of the SFHs by using the ratio between SFR averaged over the last 10 Myr and averaged between 10 Myr and 100 Myr before the epoch of observation ($\mathrm{SFR_{cont, 10}/SFR_{cont, 90}}$). We find that high-redshift and low-mass galaxies have particularly bursty SFHs, while more massive and lower-redshift systems evolve more steadily. We also present the discovery of another (mini-)quenched galaxy at z = 4.4 (in addition to the one at z=7.3 reported by Looser et al. 2023), which might be only temporarily quiescent as a consequence of the extremely bursty evolution. Finally, we also find a steady decline of dust reddening of the stellar population approaching the earliest cosmic epochs, although some dust reddening is still observed in some of the highest redshift and most star forming systems.

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arXiv2023

JADES: Balmer Decrement Measurements at redshifts 4 < z < 7

We present Balmer decrement H$α$/ H$β$ measurements for a sample of 51 galaxies at redshifts z = 4-7 observed with the JWST/NIRSpec MSA, as part of the JADES survey. Leveraging 28-hour long exposures and the efficiency of the prism/clear configuration (but also using information from the medium-resolution gratings), we are able to probe directly the low-mass end of the galaxy population, reaching stellar masses Mstar as low as 10^7 Msun . We find that the correlation between Balmer decrement and Mstar is already established at these high redshifts, indicating a rapid build up of dust in moderately massive galaxies at such early epochs. The lowest-mass galaxies in our sample (Mstar = 1-3 x 10^7 Msun ) display a remarkably low Balmer decrement of 2.88 $\pm$ 0.08, consistent with case B, suggesting very little dust content. However, we warn that such a low observed Balmer decrement may also partly be a consequence of an intrinsically lower H$α$/ H$β$, resulting from the extreme conditions of the ionized gas in these primeval and unevolved systems. We further compare the Balmer decrement to continuum-derived star-formation rates (SFR), finding tentative evidence of a correlation, which likely traces the underlying connection between SFR and mass of cold gas. However, we note that larger samples are required to distinguish between direct and primary correlations from indirect and secondary dependencies at such high redshifts.

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