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Casey Papovich

Publications and source records attributed to Casey Papovich.

At least 109 records · Page 6Linked to original sources
arXiv2023

The Web Epoch of Reionization Lyman-$α$ Survey (WERLS) I. MOSFIRE Spectroscopy of $\mathbf{z \sim 7-8}$ Lyman-$α$ Emitters

We present the first results from the Web Epoch of Reionization Lyman-$α$ Survey (WERLS), a spectroscopic survey of Lyman-$α$ emission using Keck I/MOSFIRE and LRIS. WERLS targets bright ($J<26$) galaxy candidates with photometric redshifts of $5.5\lesssim z \lesssim 8$ selected from pre-JWST imaging embedded in the Epoch of Reionization (EoR) within three JWST deep fields: CEERS, PRIMER, and COSMOS-Web. Here, we report 11 $z\sim7-8$ Lyman-$α$ emitters (LAEs; 3 secure and 8 tentative candidates) detected in the first five nights of WERLS MOSFIRE data. We estimate our observed LAE yield is $\sim13$%, broadly consistent with expectations assuming some loss from redshift uncertainty, contamination from sky OH lines, and that the Universe is approximately half-ionized at this epoch, whereby observable Lyman-$α$ emission is unlikely for galaxies embedded in a neutral intergalactic medium. Our targets are selected to be UV-bright, and span a range of absolute UV magnitudes with $-23.1 < M_{\text{UV}} < -19.8$. With two LAEs detected at $z=7.68$, we also consider the possibility of an ionized bubble at this redshift. Future synergistic Keck+JWST efforts will provide a powerful tool for pinpointing beacons of reionization and mapping the large scale distribution of mass relative to the ionization state of the Universe.

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arXiv2023

UNCOVER spectroscopy confirms a surprising ubiquity of AGN in red galaxies at $z>5$

JWST is revealing a new population of dust-reddened broad-line active galactic nuclei (AGN) at redshifts $z\gtrsim5$. Here we present deep NIRSpec/Prism spectroscopy from the Cycle 1 Treasury program UNCOVER of 15 AGN candidates selected to be compact, with red continua in the rest-frame optical but with blue slopes in the UV. From NIRCam photometry alone, they could have been dominated by dusty star formation or AGN. Here we show that the majority of the compact red sources in UNCOVER are dust-reddened AGN: $60\%$ show definitive evidence for broad-line H$α$ with FWHM$\, >2000$ km/s, for $20\%$ current data are inconclusive, and $20\%$ are brown dwarf stars. We propose an updated photometric criterion to select red $z>5$ AGN that excludes brown dwarfs and is expected to yield $>80\%$ AGN. Remarkably, among all $z_{\rm phot}>5$ galaxies with F277W$-$F444W$>1$ in UNCOVER at least $33\%$ are AGN regardless of compactness, climbing to at least $80\%$ AGN for sources with F277W$-$F444W$>1.6$. The confirmed AGN have black hole masses of $10^7-10^9$ M$_{\odot}$. While their UV-luminosities ($-16>M_{\rm UV}>-20$ AB mag) are low compared to UV-selected AGN at these epochs, consistent with percent-level scattered AGN light or low levels of unobscured star formation, the inferred bolometric luminosities are typical of $10^7-10^9$ M$_{\odot}$ black holes radiating at $\sim 10-40\%$ of Eddington. The number densities are surprisingly high at $\sim10^{-5}$ Mpc$^{-3}$ mag$^{-1}$, 100 times more common than the faintest UV-selected quasars, while accounting for $\sim1\%$ of the UV-selected galaxies. While their UV-faintness suggest they may not contribute strongly to reionization, their ubiquity poses challenges to models of black hole growth.

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arXiv2023

Cosmic Evolution Early Release Science Survey (CEERS): Multi-classing Galactic Dwarf Stars in the deep JWST/NIRCam

Low mass (sub)stellar objects represent the low end of the initial mass function, the transition to free-floating planets and a prominent interloper population in the search for high-redshift galaxies. Without proper motions or spectroscopy, can one identify these objects photometrically? JWST/NIRCam has several advantages over HST/WFC3 NIR: more filters, a greater wavelength range, and greater spatial resolution. Here, we present a catalogue of (sub)stellar dwarfs identified in the Cosmic Evolution Early Release Science Survey (CEERS). We identify 518 stellar objects down to $m_F200W \sim 28$ using half-light radius, a full three magnitudes deeper than typical HST/WFC3 images. A kNN nearest neighbour algorithm identifies and types these sources, using four HST/WFC3 and four NIRCam filters, trained on SpeX spectra of nearby brown dwarfs. The kNN with four neighbors classifies well within two subtypes: e.g M2$\pm$2 or T4$\pm$2, achieving $\sim$95% precision and recall. More granular typing results in worse metrics. In CEERS, we find 9 M8$\pm$2, 2 L6$\pm$2, 1 T4$\pm$2, and 15 T8$\pm$2. We compare the observed long wavelength NIRCam colours -- not used in the kNN -- to those expected for brown dwarf atmospheric models. The NIRCam F356W-F444W and F410M-F444W colours are redder by a magnitude for the type assigned by the kNN, hinting at a wider variety of atmospheres for these objects. We find a 300-350pc scale-height for M6$\pm$2 dwarfs plus a second structural component and a 150-200pc scale-height for T6$\pm$2 type dwarfs, consistent with literature values.

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arXiv2023

CEERS Key Paper VIII: Emission Line Ratios from NIRSpec and NIRCam Wide-Field Slitless Spectroscopy at z>2

We use James Webb Space Telescope Near-Infrared Camera Wide Field Slitless Spectroscopy (NIRCam WFSS) and Near-Infrared spectrograph (NIRSpec) in the Cosmic Evolution Early Release survey (CEERS) to measure rest-frame optical emission-line of 155 galaxies at z>2. The blind NIRCam grism observations include a sample of galaxies with bright emission lines that were not observed on the NIRSpec masks. We study the changes of the Ha, [OIII]/Hb, and [NeIII]/[OII] emission lines in terms of redshift by comparing to lower redshift SDSS and CLEAR samples. We find a significant (>3$σ$) correlation between [OIII]/Hb with redshift, while [NeIII]/[OII] has a marginal (2$σ$) correlation with redshift. We compare [OIII]/Hb and [NeIII]/[OII] to stellar mass and Hb SFR. We find that both emission-line ratios have a correlation with Hb SFR and an anti-correlation with stellar mass across the redshifts 0<z<9. Comparison with MAPPINGS~V models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-SFR galaxies. We additionally compare to IllustriousTNG predictions and find that they effectively describe the highest [OIII]/Hb ratios observed in our sample, without the need to invoke MAPPINGS models with significant shock ionizionation components.

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arXiv2023

JWST CEERS probes the role of stellar mass and morphology in obscuring galaxies

In recent years, observations have uncovered a population of massive galaxies that are invisible or very faint in deep optical/near-infrared (near-IR) surveys but brighter at longer wavelengths. However, the nature of these optically dark or faint galaxies (OFGs; one of several names given to these objects) is highly uncertain. In this work, we investigate the drivers of dust attenuation in the JWST era. In particular, we study the role of stellar mass, size, and orientation in obscuring star-forming galaxies (SFGs) at $3 < z < 7.5$, focusing on the question of why OFGs and similar galaxies are so faint at optical/near-IR wavelengths. We find that stellar mass is the primary proxy for dust attenuation, among the properties studied. Effective radius and axis ratio do not show a clear link with dust attenuation, with the effect of orientation being close to random. However, there is a subset of highly dust attenuated ($A_V > 1$, typically) SFGs, of which OFGs are a specific case. For this subset, we find that the key distinctive feature is their compact size (for massive systems with $\log (M_{*}/M_{\odot}) > 10$); OFGs exhibit a 30% smaller effective radius than the average SFG at the same stellar mass and redshift. On the contrary, OFGs do not exhibit a preference for low axis ratios (i.e., edge-on disks). The results in this work show that stellar mass is the primary proxy for dust attenuation and compact stellar light profiles behind the thick dust columns obscuring typical massive SFGs.

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arXiv2023

Searching for Islands of Reionization: A Potential Ionized Bubble Powered by a Spectroscopic Overdensity at z = 8.7

We present the results from a spectroscopic survey using the MOSFIRE near-infrared spectrograph on the 10m Keck telescope to search for Ly$α$ emission from candidate galaxies at z ~9-10 in four of the CANDELS fields (GOODS-N, EGS, UDS, and COSMOS). We observed 11 target galaxies, detecting Ly$α$ from one object in ~8.1 hours of integration, at z = 8.665 +/- 0.001 with an integrated signal-to-noise > 7. This galaxy is in the CANDELS Extended Groth Strip (EGS) field, and lies physically close (3.5 physical Mpc [pMpc]) to another confirmed galaxy in this field with Ly$α$ detected at z=8.683 (Zitrin et al. 2015). The detection of Ly$α$ suggests the existence of large (~1 pMpc) ionized bubbles fairly early in the reionization process. We explore the ionizing output needed to create bubbles of this size at this epoch and find that such a bubble requires more than the ionizing power provided by the full expected population of galaxies (by integrating the UV Luminosity Function down to M_UV = -13). The Ly$α$ we detect would be able to escape the predominantly-neutral intergalactic medium at this epoch if our galaxy is inhabiting an overdensity, which would be consistent with the photometric overdensity previously identified in this region by Finkelstein et al. 2021. This implies that the CANDELS EGS field is hosting an overdensity at z = 8.7 which is powering one (or more) ionized bubbles, a hypothesis that will be imminently testable with forthcoming James Webb Space Telescope observations in this field.

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arXiv2023

A CEERS Discovery of an Accreting Supermassive Black Hole 570 Myr after the Big Bang: Identifying a Progenitor of Massive z > 6 Quasars

We report the discovery of an accreting supermassive black hole at z=8.679, in CEERS_1019, a galaxy previously discovered via a Ly$α$-break by Hubble and with a Ly$α$ redshift from Keck. As part of the Cosmic Evolution Early Release Science (CEERS) survey, we observed this source with JWST/NIRSpec spectroscopy, MIRI and NIRCam imaging, and NIRCam/WFSS slitless spectroscopy. The NIRSpec spectra uncover many emission lines, and the strong [O III] emission line confirms the ground-based Ly$α$ redshift. We detect a significant broad (FWHM~1200 km/s) component in the H$β$ emission line, which we conclude originates in the broad-line region of an active galactic nucleus (AGN), as the lack of a broad component in the forbidden lines rejects an outflow origin. This hypothesis is supported by the presence of high-ionization lines, as well as a spatial point-source component embedded within a smoother surface brightness profile. The mass of the black hole is log($M_{BH}/M_{\odot})=6.95{\pm}0.37$, and we estimate that it is accreting at 1.2 ($\pm$0.5) x the Eddington limit. The 1-8 $μ$m photometric spectral energy distribution (SED) from NIRCam and MIRI shows a continuum dominated by starlight and constrains the host galaxy to be massive (log M/M$_{\odot}$~9.5) and highly star-forming (SFR~30 M$_{\odot}$ yr$^{-1}$). Ratios of the strong emission lines show that the gas in this galaxy is metal-poor (Z/Z$_{\odot}$~0.1), dense (n$_{e}$~10$^{3}$ cm$^{-3}$), and highly ionized (log U~-2.1), consistent with the general galaxy population observed with JWST at high redshifts. We use this presently highest-redshift AGN discovery to place constraints on black hole seeding models and find that a combination of either super-Eddington accretion from stellar seeds or Eddington accretion from massive black hole seeds is required to form this object by the observed epoch.

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arXiv2023

CEERS Key Paper VII: JWST/MIRI Reveals a Faint Population of Galaxies at Cosmic Noon Unseen by Spitzer

The Cosmic Evolution Early Release Science (CEERS) program observed the Extended Groth Strip with the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) in 2022. In this paper, we discuss the four MIRI pointings that observed with longer wavelength filters, including F770W, F1000W, F1280W, F1500W, F1800W, and F2100W. We compare the MIRI galaxies with the Spitzer/MIPS 24$μ$m population in the EGS field. We find that MIRI can observe an order of magnitude deeper than MIPS in significantly shorter integration times, attributable to JWST's much larger aperture and MIRI's improved sensitivity. MIRI is exceptionally good at finding faint ($L_{\rm IR}<10^{10} L_\odot$) galaxies at $z\sim1-2$. We find that a significant portion of MIRI galaxies are "mid-IR weak"--they have strong near-IR emission and relatively weaker mid-IR emission, and most of the star formation is unobscured. We present new IR templates that capture how the mid-IR to near-IR emission changes with increasing infrared luminosity. We present two color-color diagrams to separate mid-IR weak galaxies and active galactic nuclei (AGN) from dusty star-forming galaxies and find that these color diagrams are most effective when used in conjunction with each other. We present the first number counts of 10$μ$m sources and find that there are $\lesssim10$ IR AGN per MIRI pointing, possibly due to the difficulty of distinguishing AGN from intrinsically mid-IR weak galaxies (due to low metallicities or low dust content). We conclude that MIRI is most effective at observing moderate luminosity ($L_{\rm IR}=10^9-10^{10}L_\odot$) galaxies at $z=1-2$, and that photometry alone is not effective at identifying AGN within this faint population.

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arXiv2023

Confirmation and refutation of very luminous galaxies in the early universe

During the first 500 million years of cosmic history, the first stars and galaxies formed, seeding the Universe with heavy elements and eventually reionizing the intergalactic medium. Observations with JWST have uncovered a surprisingly high abundance of candidates for early star-forming galaxies, with distances (redshifts, $z$), estimated from multi-band photometry, as large as $z\approx 16$, far beyond pre-JWST limits. While generally robust, such photometric redshifts can suffer from degeneracies and occasionally catastrophic errors. Spectroscopic measurement is required to validate these sources and to reliably quantify physical properties that can constrain galaxy formation models and cosmology. Here we present JWST spectroscopy that confirms redshifts for two very luminous galaxies with $z > 11$, but also demonstrates that another candidate with suggested $z\approx 16$ instead has $z = 4.9$, with an unusual combination of nebular line emission and dust reddening that mimics the colors expected for much more distant objects. These results reinforce evidence for the early, rapid formation of remarkably luminous galaxies, while also highlighting the necessity of spectroscopic verification. The large abundance of bright, early galaxies may indicate shortcomings in current galaxy formation models, or deviation from physical properties (such as the stellar initial mass function) that are generally believed to hold at later times.

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arXiv2023

ALMA FIR View of Ultra High-redshift Galaxy Candidates at $z\sim$ 11-17: Blue Monsters or Low-$z$ Red Interlopers?

We present ALMA Band~7 observations of a remarkably bright galaxy candidate at $z_{\rm phot}$=$16.7^{+1.9}_{-0.3}$ ($M_{\rm UV}$=$-21.6$), S5-z17-1, identified in JWST Early Release Observation data of Stephen's Quintet. We do not detect the dust continuum at 866~$μ$m, ruling out the possibility that \targb\ is a low-$z$ dusty starburst with a star-formation rate of $\gtrsim 30$~$M_{\odot}$~yr$^{-1}$. We detect a 5.1$σ$ line feature at $338.726\pm0.007$~GHz exactly coinciding with the JWST source position, with a 2\% likelihood of the signal being spurious. The most likely line identification would be [OIII]52$μ$m at $z=16.01$ or [CII]158$μ$m at $z=4.61$, whose line luminosities do not violate the non-detection of the dust continuum in both cases. Together with three other $z\gtrsim$ 11--13 candidate galaxies recently observed with ALMA, we conduct a joint ALMA and JWST spectral energy distribution (SED) analysis and find that the high-$z$ solution at $z\sim$11--17 is favored in every candidate as a very blue (UV continuum slope of $\simeq-2.3$) and luminous ($M_{\rm UV}\simeq[-$24:$-21]$) system. Still, we find in several candidates that reasonable SED fits ($Δ$ $χ^{2}\lesssim4$) are reproduced by type-II quasar and/or quiescent galaxy templates with strong emission lines at $z\sim3$--5, where such populations predicted from their luminosity functions and EW([OIII]+H$β$) distributions are abundant in survey volumes used for the identification of the $z\sim$11--17 candidates. While these recent ALMA observation results have strengthened the likelihood of the high-$z$ solutions, lower-$z$ possibilities are not completely ruled out in several of the $z\sim$11--17 candidates, indicating the need to consider the relative surface densities of the lower-$z$ contaminants in the ultra high-$z$ galaxy search.

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arXiv2023

Spectroscopic Confirmation of CEERS NIRCam-selected Galaxies at $\boldsymbol{z \simeq 8-10}$

We present JWST/NIRSpec prism spectroscopy of seven galaxies selected from the Cosmic Evolution Early Release Science Survey (CEERS) NIRCam imaging with photometric redshifts z_phot>8. We measure emission line redshifts of z=7.65 and 8.64 for two galaxies, and z=9.77(+0.37,-0.29) and 10.01(+0.14,-0.19) for two others via the detection of continuum breaks consistent with Lyman-alpha opacity from a mostly neutral intergalactic medium. The presence (absense) of strong breaks (strong emission lines) give high confidence that these two galaxies are at z>9.6, but the break-derived redshifts have large uncertainties given the low spectral resolution and relatively low signal-to-noise of the CEERS NIRSpec prism data. The two z~10 sources are relatively luminous (M_UV<-20), with blue continua (-2.3 8 candidates with CEERS NIRSpec spectroscopy do not have secure redshifts, but the absence of emission lines in their spectra is consistent with redshifts z>9.6. We find that z>8 photometric redshifts are generally in agreement (within uncertainties) with the spectroscopic values. However, the photometric redshifts tend to be slightly overestimated (average Delta(z)=0.50+/-0.12), suggesting that current templates do not fully describe the spectra of very high-z sources. Overall, our results solidifies photometric evidence for a high space density of bright galaxies at z>8 compared to theoretical model predictions, and further disfavors an accelerated decline in the integrated UV luminosity density at z>8.

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arXiv2023

Using [Ne V]/[Ne III] to Understand the Nature of Extreme-Ionization Galaxies

Spectroscopic studies of extreme-ionization galaxies (EIGs) are critical to our understanding of exotic systems throughout cosmic time. These EIGs exhibit spectral features requiring >54.42 eV photons: the energy needed to fully ionize helium into He2+ and emit He II recombination lines. They are likely key contributors to reionization, and they can also probe exotic stellar populations or accretion onto massive black holes. To facilitate the use of EIGs as probes of high ionization, we focus on ratios constructed from strong rest-frame UV/optical emission lines, specifically [O III] 5008, H-beta, [Ne III] 3870, [O II] 3727,3729, and [Ne V] 3427. These lines probe the relative intensity at energies of 35.12, 13.62, 40.96, 13.62 eV, and 97.12, respectively, covering a wider range of ionization than traced by other common rest-frame UV/optical techniques. We use ratios of these lines ([Ne V]/[Ne III] = Ne53 and [Ne III]/[O II]), which are closely separated in wavelength, and mitigates effects of dust attenuation and uncertainties in flux calibration. We make predictions from photoionization models constructed from Cloudy that use a broad range of stellar populations and black hole accretion models to explore the sensitivity of these line ratios to changes in the ionizing spectrum. We compare our models to observations from the Hubble Space Telescope and James Webb Space Telescope of galaxies with strong high-ionization emission lines at z ~ 0, z ~ 2, and z ~ 7. We show that the Ne53 ratio can separate galaxies with ionization from 'normal' stellar populations from those with AGN and even 'exotic' Population III models. We introduce new selection methods to identify galaxies with photoionization driven by Population III stars or intermediate-mass black hole accretion disks that could be identified in upcoming high-redshift spectroscopic surveys.

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arXiv2023

UNCOVER: Candidate Red Active Galactic Nuclei at 3<z<7 with JWST and ALMA

The James Webb Space Telescope (JWST) is revolutionizing our knowledge of $z>5$ galaxies and their actively accreting black holes. Using the JWST Cycle 1 Treasury program Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) in the lensing field Abell 2744, we report the identification of a sample of little red dots at $3 < z_{\rm{phot}} < 7$ that likely contain highly-reddened accreting supermassive black holes. Using a NIRCam-only selection to F444W$<27.7$ mag, we find 26 sources over the $\sim45$ arcmin$^{2}$ field that are blue in F115W$-$F200W$\sim0$ (or $β_{\rm UV}\sim-2.0$ for $f_λ \propto λ^β$), red in F200W$-$F444W = $1-4$ ($β_{\rm opt} \sim +2.0$), and are dominated by a point-source like central component. Of the 20 sources with deep ALMA 1.2-mm coverage, none are detected individually or in a stack. For the majority of the sample, SED fits to the JWST+ALMA observations prefer models with hot dust rather than obscured star-formation to reproduce the red NIRCam colors and ALMA 1.2-mm non-detections. While compact dusty star formation can not be ruled out, the combination of extremely small sizes ($\langle r_e \rangle\approx50$ pc after correction for magnification), red rest-frame optical slopes, and hot dust can by explained by reddened broad-line active galactic nuclei (AGNs). Our targets have faint $M_{\rm 1450} \approx -14\ \, {\rm to} -18$ mag but inferred bolometric luminosities of $L_{\rm bol} = 10^{43}-10^{46}$ erg/s, reflecting their obscured nature. If the candidates are confirmed as AGNs with upcoming UNCOVER spectroscopy, then we have found an abundant population of reddened luminous AGN that are at least ten times more numerous than UV-luminous AGN at the same intrinsic bolometric luminosity.

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arXiv2023

NGDEEP Epoch 1: The Faint-End of the Luminosity Function at $z \sim$ 9-12 from Ultra-Deep JWST Imaging

We present a robust sample of very high-redshift galaxy candidates from the first epoch of {\it JWST}/NIRCam imaging from the Next Generation Extragalactic Exploratory Deep (NGDEEP) Survey. The NGDEEP NIRCam imaging in the Hubble Ultra Deep Field Parallel Field 2 (HUDF-Par2) reaches $m=30.4$ (5$σ$, point-source) in F277W, making it the deepest public {\it JWST} GO imaging dataset to date. We describe our detailed data reduction process of the six-filter broad-band {\it JWST}/NIRCam imaging, incorporating custom corrections for systematic effects to produce high-quality calibrated images. Using robust photometric redshift selection criteria, we identify a sample of 38 $z \gtrsim 9$ galaxy candidates. These objects span a redshift range of $z=8.5-15.8$, and apparent magnitudes of $m_\mathrm{F277W} = 27-30.5$ AB mag, reaching $\sim 1.5$ mag deeper than previous public {\it JWST} imaging surveys. We calculate the rest-frame ultraviolet (UV) luminosity function at $z \sim$ 9 and 11, and present a new measurement of the luminosity function faint-end slope at $z \sim 11$. There is no significant evolution in the faint-end slope and number density from $z=9$ to 11. Comparing our results with theoretical predictions, we find that some models produce better agreement at the faint end than the bright end. These results will help to constrain how stellar feedback impacts star formation at these early epochs.

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arXiv2023

Efficient NIRCam Selection of Quiescent Galaxies at 3 < z < 6 in CEERS

Substantial populations of massive quiescent galaxies at $z\ge3$ challenge our understanding of rapid galaxy growth and quenching over short timescales. In order to piece together this evolutionary puzzle, more statistical samples of these objects are required. Established techniques for identifying massive quiescent galaxies are increasingly inefficient and unconstrained at $z>3$. As a result, studies report that as much as 70\% of quiescent galaxies at $z>3$ may be missed from existing surveys. In this work, we propose a new empirical color selection technique designed to select massive quiescent galaxies at $3\lesssim z \lesssim 6$ using JWST NIRCam imaging data. We use empirically-constrained galaxy SED templates to define a region in the $F277W-F444W$ vs. $F150W-F277W$ color plane that captures quiescent galaxies at $z>3$. We apply this color selection criteria to the Cosmic Evolution Early Release Science (CEERS) Survey and identify 44 candidate $z\gtrsim3$ quiescent galaxies. Over half of these sources are newly discovered and, on average, exhibit specific star formation rates of post-starburst galaxies. We derive volume density estimates of $n\sim1-4\times10^{-5}$\,Mpc$^{-3}$ at $3< z <5$, finding excellent agreement with existing reports on similar populations in the CEERS field. Thanks to NIRCam's wavelength coverage and sensitivity, this technique provides an efficient tool to search for large samples of these rare galaxies.

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arXiv2023

A z=1.85 galaxy group in CEERS: evolved, dustless, massive intra-halo light and a brightest group galaxy in the making

We present CEERS JWST/NIRCam imaging of a massive galaxy group at z=1.85, to explore the early JWST view on massive group formation in the distant Universe. The group contains >16 members (including 6 spectros. confirmations) down to log10(Mstar/Msun)=8.5, including the brightest group galaxy (BGG) in the process of actively assembling at this redshift. The BGG is comprised of multiple merging components extending ~3.6" (30kpc) across the sky. The BGG contributes 69% of the group's total galactic stellar mass, with one of the merging components containing 76% of the total mass of the BGG and a SFR>1810Msun/yr. Most importantly, we detect intra-halo light (IHL) in several HST and JWST/NIRCam bands, allowing us to construct a state-of-the-art rest-frame UV-NIR Spectral Energy Distribution of the IHL for the first time at this high redshift. This allows stellar population characterisation of both the IHL and member galaxies, as well as the morphology distribution of group galaxies vs. their star-formation activity when coupled with Herschel data. We create a stacked image of the IHL, giving us a sensitivity to extended emission of 28.5 mag/arcsec2 at rest-frame 1um. We find that the IHL is extremely dust poor (Av~0), containing an evolved stellar population of log10(t50/yr)=8.8, corresponding to a formation epoch for 50% of the stellar material 0.63Gyr before z=1.85. There is no evidence of ongoing star-formation in the IHL. The IHL in this group at z=1.85 contributes ~10% of the total stellar mass, comparable with what is observed in local clusters. This suggests that the evolution of the IHL fraction is more self-similar with redshift than predicted by some models, challenging our understanding of IHL formation during the assembly of high-redshift clusters. JWST is unveiling a new side of group formation at this redshift, which will evolve into Virgo-like structures in the local Universe.

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arXiv2023

CEERS Spectroscopic Confirmation of NIRCam-Selected z > 8 Galaxy Candidates with JWST/NIRSpec: Initial Characterization of their Properties

We present JWST NIRSpec spectroscopy for 11 galaxy candidates with photometric redshifts of $z\simeq9-13$ and $M_{\rm\,UV} \in[-21,-18]$ newly identified in NIRCam images in the Cosmic Evolution Early Release Science (CEERS) Survey. We confirm emission line redshifts for 7 galaxies at $z=7.762-8.998$ using spectra at $\sim1-5μ$m either with the NIRSpec prism or its three medium resolution gratings. For $z\simeq9$ photometric candidates, we achieve a high confirmation rate of $\simeq$90\%, which validates the classical dropout selection from NIRCam photometry. No robust emission lines are identified in three galaxy candidates at $z>10$, where the strong [OIII] and H$β$ lines would be redshifted beyond the wavelength range observed by NIRSpec, and the Lyman-$α$ continuum break is not detected with the current sensitivity. Compared with HST-selected bright galaxies ($M_{\rm\,UV}\simeq-22$) that are similarly spectroscopically confirmed at $z\gtrsim8$, these NIRCam-selected galaxies are characterized by lower star formation rates (SFR$\simeq4\,M_{\odot}$~yr$^{-1}$) and lower stellar masses ($\simeq10^{8}\,M_{\odot}$), but with higher [OIII]+H$β$ equivalent widths ($\simeq$1100$Å$), and elevated production efficiency of ionizing photons ($\log(ξ_{\rm\,ion}/{\rm\,Hz\,erg}^{-1})\simeq25.8$) induced by young stellar populations ($<10$~Myrs) accounting for $\simeq20\%$ of the galaxy mass, highlighting the key contribution of faint galaxies to cosmic reionization. Taking advantage of the homogeneous selection and sensitivity, we also investigate metallicity and ISM conditions with empirical calibrations using the [OIII]/H$β$ ratio. We find that galaxies at $z\sim8-9$ have higher SFRs and lower metallicities than galaxies at similar stellar masses at $z\sim2-6$, which is generally consistent with the current galaxy formation and evolution models.

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arXiv2023

Extremely red galaxies at $z=5-9$ with MIRI and NIRSpec: dusty galaxies or obscured AGNs?

We study a new population of extremely red objects (EROs) recently discovered by JWST based on their NIRCam colors F277W$-$F444W $>1.5$ mag. We find 37 EROs in the CEERS field with F444W $<28$ mag and photometric redshifts between $5 7$ by up to a factor $\sim$60. Similarly, if they are OSOs with luminosities in the L$_{\rm bol}>10^{46-47}$ erg s$^{-1}$ range, their number would exceed that of bright blue QSOs by more than two orders of magnitude. Additional photometry at mid-IR wavelengths will reveal the true nature of the red continuum emission in these EROs and will place this puzzling population in the right context of galaxy evolution.

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