arXiv subjects

Norbert Pirzkal

Publications and source records attributed to Norbert Pirzkal.

At least 19 recordsLinked to original sources

arXiv2026

Unified modelling of broad and narrow optical-UV emission lines from massive black holes: interpreting high-redshift JWST observations

JWST is uncovering a large population of active galactic nuclei (AGN) at high redshift, motivating the extension of models calibrated on local sources to a broader range of physical conditions. We present a consistent suite of broad- and narrow-line photoionisation models based on ionising spectra that vary with black-hole mass and Eddington ratio, spanning sub- to super-Eddington accretion. For the broad-line region (BLR), we investigate the detectability of low-mass accreting black holes. At z=6, broad Hα may be detectable down to $M_{BH}\approx10^{5.7} M\odot$ for a BH accreting at the Eddington limit under favourable assumptions. We derive line-to-accretion-luminosity corrections, with hydrogen and helium recombination lines providing more robust tracers than metal lines. We also explore which changes in black-hole properties and BLR conditions can contribute to the large Hα equivalent widths, elevated Balmer decrements, and weak high-ionisation lines observed in some high-redshift sources, finding that variations in BLR structure can substantially modify the emergent spectrum. For the narrow-line region (NLR), Hα and [O III] λ5007 are among the most promising tracers of low-mass accreting black holes, probing $M_{BH}\approx10^{5.5} M\odot$ at the Eddington limit. Many classical diagnostic diagrams miss low-metallicity AGN; we identify alternative UV and optical diagnostics that more robustly separate AGN from Pop. II and Pop. III stellar photoionisation over the conditions explored. Nevertheless, NLR line ratios are driven primarily by metallicity and ionisation parameter, with only a weak dependence on black-hole mass and Eddington ratio. Looking ahead, these models are intended for Bayesian interpretation of JWST spectra and connection to cosmological simulations through physically motivated priors.

astro-ph.GA↗

arXiv2026

Breaking the brightness barrier: JWST/NIRCam DHS spectroscopy for high-precision time-series observations

Many of the most scientifically compelling exoplanets orbit bright nearby stars that exceed the brightness limits of existing JWST spectroscopic observing modes. To address this limitation, a new NIRCam Short Wavelength Grism Time-Series mode has been developed by combining the Dispersed Hartmann Sensor (DHS) with a new on-board multistripe detector readout capability. The DHS disperses the incoming light through multiple pupil sub-apertures, reducing the incident flux and providing slitless spectroscopy between approximately 1.0 and 2.3 um. The multistripe readout mode further increases the accessible brightness range (K ~ 2.5 - 5.7 mag, depending on the spectral type of the object), with the previous limit being K ~ 5.7 mag at 1.5 um, by reading only the detector regions containing the DHS spectra, reducing the detector frame time with the standard RAPID readout mode from 10.74 s to 1.36 s over even less when using smaller substripe sizes. Together with the simultaneous long-wavelength grism observations, the new mode provides spectroscopic continuous coverage from approximately 1.0 to 5.0 um for targets as bright as K ~ 2.5 mag. We present the first results of on-orbit commissioning of this new observing mode with the final stage consisting of observations of a full transit of the exoplanet WASP-18b, which allowed us to demonstrate the feasibility of using NIRCam DHS for high-precision time-series observations. The commissioning presented in this paper marks the first deployment of the multistripe detector readout on JWST. Beyond NIRCam DHS, this new capability will be extended to other spectroscopic modes, including NIRISS SOSS and NIRSpec PRISM. Bright nearby stars host many of the highest-priority targets for exoplanet atmospheric characterization, making multistripe an important new capability for maximizing the scientific return of JWST.

astro-ph.IM↗

arXiv2025

Discovery of Multiply Ionized Iron Emission Powered by an Active Galactic Nucleus in a z~7 Little Red Dot

Some of the most puzzling discoveries of NASA's JWST in the early Universe surround the surprising abundance of compact red sources, which show peculiar continuum shapes and broad hydrogen spectral lines. These sources, dubbed ``Little Red Dots'' or LRDs, have been the subject of intense inquiry in the literature. Any of the proposed explanations, from accreting super-massive black holes ensconced in ultra-dense gas to extremely compact star-systems, has significant implications for the earliest phases of galaxy evolution. Part of the difficulty in concretely identifying the physical mechanisms that drive their rest ultra-violet/optical spectral properties is the lack of bona fide signatures -- either star-formation or accreting super-massive black hole, that uniquely discriminate between competing interpretations. In this work, we report the discovery of several spectral features that strongly favor the existence of an accreting super-massive black hole in an LRD witnessed in the first 800 Myr of cosmic time, including several rare iron transitions and a possible [FeVII]. Additionally, we report on the properties of significant Balmer absorption and find that the small widths and relative depths of the absorption feature suggest the source of the absorber is at or beyond the outer edge of the broad-line region and does it fully cover the accreting SMBH in the center of the system. The detection of these iron features, coupled with the properties of the Balmer absorption, unveils an alternative scenario for LRDs -- one where there are direct sight-lines from the accretion disk to gas on scales at (or beyond) the broad-line gas region.

astro-ph.GA↗

arXiv2025

Lonely Little Red Dots: Challenges to the AGN-nature of little red dots through their clustering and spectral energy distributions

Observations with the James Webb Space Telescope (JWST) reveal a previously unseen population of compact red objects, known as ``little red dots`` (LRDs). We study a new photometrically selected sample of 124 LRDs in the redshift range $z$ $\sim$ 3 - 10 selected from NIRCam coverage of the CEERS, NEP-TDF, JADES and JEMS surveys. For JADES, the NEP-TDF and CEERS, we compare SED models with and without AGN components and analyse the impact of an AGN component on the goodness of fit using the Bayesian information criterion (BIC). We find that whilst the $χ^{2}$ of the majority of models containing AGN components is improved compared to models without AGN components, we show that the BIC suggests models without AGN are a more appropriate fit to LRD SEDs, especially when MIRI data is available. We also measure LRD clustering in the CEERS field, JADES field, and NEP-TDF, where we compare the spatial distribution of LRDs and galaxies with Kolmogorov-Smirnov tests of equality of distribution. We find that the neighbourhood of LRDs tends to be less dense compared to galaxies at all selections and masses and at similar redshifts. We further measure upper limit estimates for the halo masses of LRDs using abundance matching. Whilst the population of LRDs could be a mixture of several different inherent populations, as a whole it does appear that these systems are mostly hosting compact galaxies or star clusters in formation.

astro-ph.GA↗

arXiv2025

Explaining JWST counts with galaxy formation models

A distinct power-law break is apparent m_AB approximately 21 in the deep Near-Infrared PEARLS-JWST galaxy counts. The break becomes more pronounced at longer wavelengths, with the counts slope flattening smoothly with apparent magnitude in the shortest band used at 0.9 microns, trending towards an increasingly broken slope by the longest wavelength passband of JWST NIRCam, 4.4 microns. This behaviour is remarkably well predicted by the GALFORM semi-analytical model of galaxy formation. We use the model to diagnose the origin of this behaviour. We find that the features that are responsible for the break are: 1) the inherent break in the luminosity function; 2) the change in the volume element with redshift and 3) the redshift-dependent nature of the k-correction. We study the contribution to these effects by early and late-type galaxies, using as a proxy for morphology the bulge-to-total stellar mass ratio. We find that the way in which ellipticals populate the bright end of the luminosity function while spirals dominate the faint end is preserved in the galaxy number counts, with a characteristic stellar mass at the break of approximately 10^10 M_sun. We also find that the shape of the number counts is mainly driven by galaxies with relatively low redshift (z < 2) for the PEARLS observational limit of m_AB < 28. We give a comprehensive description of why the galaxy number counts in the near-infrared PEARLS-JWST observation look the way they do and which population of galaxies is dominant at each apparent magnitude.

astro-ph.GA↗

arXiv2024

JWST NIRSpec spectroscopy of the triply-lensed $z = 10.17$ galaxy MACS0647$-$JD

We present JWST/NIRSpec prism spectroscopy of MACS0647-JD, the triply-lensed $z \sim 11$ candidate discovered in HST imaging and spatially resolved by JWST imaging into two components A and B. Spectroscopy of component A yields a spectroscopic redshift $z=10.17$ based on 7 detected emission lines: CIII] $λλ$1907,1909, [OII] $λ$3727, [NeIII] $λ$3869, [NeIII] $λ$3968, H$δ$ $λ$4101, H$γ$ $λ$4340, and [OIII] $λ$4363. These are the second-most distant detections of these emission lines to date, in a galaxy observed just 460 million years after the Big Bang. Based on observed and extrapolated line flux ratios we derive a gas-phase metallicity $Z =$ log(O/H) = $7.5 - 8.0$, or $(0.06 - 0.2)$ $Z_\odot$, ionization parameter log($U$) $\sim -1.9\pm0.2$, and an ionizing photon production efficiency ${\rm log}(ξ_{\rm ion})=25.2\pm0.2\,$erg$^{-1}$ Hz. The spectrum has a softened Lyman-$α$ break, evidence for a strong Ly$α$ damping wing, suggesting that MACS0647-JD was unable to ionize its surroundings beyond its immediate vicinity ($R_{\text{HII}} \ll 1$ pMpc). The Ly$α$ damping wing also suppresses the F150W photometry, explaining the slightly overestimated photometric redshift $z = 10.6 \pm 0.3$. MACS0647-JD has a stellar mass log($M/M_\odot$) = $8.1 \pm 0.3$, including $\sim$ 6$\times 10^7 M_\odot$ in component A, most of which formed recently (within $\sim$ 20 Myr) with a star formation rate $2\pm1 M_\odot$ / yr, all within an effective radius $70\pm24\,$pc. The smaller component B ($r \sim 20$) pc is likely older ($\sim$100 Myr) with more dust ($A_V \sim 0.1$ mag), as found previously. Spectroscopy of a fainter companion galaxy C separated by a distance of \about\ 3$\,$kpc reveals a Lyman break consistent with $z = 10.17$. MACS0647-JD is likely the most distant galaxy merger known.

astro-ph.GA↗

arXiv2024

The MUSE Ultra Deep Field (MUDF). V. Characterizing the Mass-Metallicity Relation for Low Mass Galaxies at $z\sim 1$-$2$

Using more than 100 galaxies in the MUSE Ultra Deep Field with spectroscopy from the Hubble Space Telescope's Wide Field Camera 3 and the Very Large Telescope's Multi Unit Spectroscopic Explorer, we extend the gas-phase mass-metallicity relation (MZR) at $z\approx\,$1$\,$-$\,$2 down to stellar masses of M$_{\star}$ $\approx$ 10$^{7.5}$ M$_{\odot}$. The sample reaches six times lower in stellar mass and star formation rate (SFR) than previous HST studies at these redshifts, and we find that galaxy metallicities decrease to log(O/H) + 12 $\approx$ 7.8 $\pm$ 0.1 (15% solar) at log(M$_{\star}$/M$_{\odot}$) $\approx$ 7.5, without evidence of a turnover in the shape of the MZR at low masses. We validate our strong-line metallicities using the direct method for sources with [O III] $λ$4363 and [O III] $λ$1666 detections, and find excellent agreement between the techniques. The [O III] $λ$1666-based metallicities double existing measurements with S/N $\geq$ 5 for unlensed sources at $z~>$ 1, validating the strong-line calibrations up to $z \sim$2.5. We confirm that the MZR resides $\sim$0.3 dex lower in metallicity than local galaxies and is consistent with the fundamental metallicity relation (FMR) if the low mass slope varies with SFR. At lower redshifts ($z\sim$0.5) our sample reaches $\sim$0.5 dex lower in SFR than current calibrations and we find enhanced metallicities that are consistent with extrapolating the MZR to lower SFRs. Finally, we detect only a $\sim$0.1 dex difference in the metallicities of galaxies in groups versus isolated environments. These results are based on robust calibrations and reach the lowest masses and SFRs that are accessible with HST, providing a critical foundation for studies with the Webb and Roman Space Telescopes.

astro-ph.GA↗

arXiv2023

The Complete CEERS Early Universe Galaxy Sample: A Surprisingly Slow Evolution of the Space Density of Bright Galaxies at z ~ 8.5-14.5

We present a sample of 88 candidate z~8.5-14.5 galaxies selected from the completed NIRCam imaging from the Cosmic Evolution Early Release Science (CEERS) survey. These data cover ~90 arcmin^2 (10 NIRCam pointings) in six broad-band and one medium-band imaging filter. With this sample we confirm at higher confidence early JWST conclusions that bright galaxies in this epoch are more abundant than predicted by most theoretical models. We construct the rest-frame ultraviolet luminosity functions at z~9, 11 and 14, and show that the space density of bright (M_UV=-20) galaxies changes only modestly from z~14 to z~9, compared to a steeper increase from z~8 to z~4. While our candidates are photometrically selected, spectroscopic followup has now confirmed 13 of them, with only one significant interloper, implying that the fidelity of this sample is high. Successfully explaining the evidence for a flatter evolution in the number densities of UV-bright z>10 galaxies may thus require changes to the dominant physical processes regulating star formation. While our results indicate that significant variations of dust attenuation with redshift are unlikely to be the dominant factor at these high redshifts, they are consistent with predictions from models which naturally have enhanced star-formation efficiency and/or stochasticity. An evolving stellar initial mass function could also bring model predictions into better agreement with our results. Deep spectroscopic followup of a large sample of early galaxies can distinguish between these competing scenarios.

astro-ph.GA↗

arXiv2023

Reaching for the stars -- JWST/NIRSpec spectroscopy of a lensed star candidate at $z=4.76$

We present JWST/NIRSpec observations of a highly magnified star candidate at a photometric redshift of $z_{\mathrm{phot}}\simeq4.8$, previously detected in JWST/NIRCam imaging of the strong lensing (SL) cluster MACS J0647+7015 ($z=0.591$). The spectroscopic observation allows us to precisely measure the redshift of the host arc at $z_{\mathrm{spec}}=4.758\pm0.004$, and the star's spectrum displays clear Lyman- and Balmer-breaks commensurate with this redshift. A fit to the spectrum suggests a B-type super-giant star of surface temperature $T_{\mathrm{eff,B}}\simeq15000$ K with either a redder F-type companion ($T_{\mathrm{eff,F}}\simeq6250$K) or significant dust attenuation ($A_V\simeq0.82$) along the line of sight. We also investigate the possibility that this object is a magnified young globular cluster rather than a single star. We show that the spectrum is in principle consistent with a star cluster, which could also accommodate the lack of flux variability between the two epochs. However, the lack of a counter image and the strong upper limit on the size of the object from lensing symmetry, $r\lesssim0.5$ pc, could indicate that this scenario is somewhat less likely -- albeit not completely ruled out by the current data. The presented spectrum seen at a time when the Universe was only $\sim1.2$ Gyr old showcases the ability of JWST to study early stars through extreme lensing.

astro-ph.GA↗

arXiv2023

CEERS MIRI Imaging: Data Reduction and Quality Assessment

The Cosmic Evolution Early Release Science Survey (CEERS), targeting the Extended Groth Strip extragalactic field, is one of the JWST Director's Discretionary Early Release Science programs. To date, all observations have been executed and include NIRCam/MIRI imaging and NIRSpec/NIRCam spectroscopic exposures. Here, we discuss the MIRI imaging, which includes eight pointings, four of which provide deep imaging with the bluer bands (F560W, F770W) and four with contiguous wavelength coverage in F1000W, F1280W, F1500W, and F1800W, where two of these also include coverage in F770W and F2100W. We present a summary of the data, the data quality, and data reduction. The data reduction is based on the JWST Calibration Pipeline combined with custom modifications and additional steps designed to enhance the output quality, including improvements in astrometry and the removal of detector artifacts. We estimate the image depth of the reduced mosaics, and show that these generally agree with expectations from the Exposure Time Calculator. We compare the MIRI F560W and F770W flux densities for bright sources to measurements from Spitzer/IRAC Ch3 (5.8 $μ$m) and Ch4 (8.0 $μ$m), and we find that they agree with systematic differences of $<0.1$ mag. For the redder MIRI bands, we assess their quality by studying the spectral energy distributions (SEDs) of Galactic stars. The SEDs are consistent with the expected Rayleigh-Jeans law with a deviation $\sim 0.03$ mag, indicating that the MIRI colors are reliable. We also discuss all publicly released data products (images and source catalogs), which are available on the CEERS website (https://ceers.github.io/).

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2023

JWST reveals a possible $z \sim 11$ galaxy merger in triply-lensed MACS0647$-$JD

MACS0647$-$JD is a triply-lensed $z\sim11$ galaxy originally discovered with the Hubble Space Telescope. Here we report new JWST imaging, which clearly resolves MACS0647$-$JD as having two components that are either merging galaxies or stellar complexes within a single galaxy. Both are very small, with stellar masses $\sim10^8\,M_\odot$ and radii $r<100\,\rm pc$. The brighter larger component "A" is intrinsically very blue ($β\sim-2.6$), likely due to very recent star formation and no dust, and is spatially extended with an effective radius $\sim70\,\rm pc$. The smaller component "B" appears redder ($β\sim-2$), likely because it is older ($100-200\,\rm Myr$) with mild dust extinction ($A_V\sim0.1\,\rm mag$), and a smaller radius $\sim20\,\rm pc$. We identify galaxies with similar colors in a high-redshift simulation, finding their star formation histories to be out of phase. With an estimated stellar mass ratio of roughly 2:1 and physical projected separation $\sim400\,\rm pc$, we may be witnessing a galaxy merger 400 million years after the Big Bang. We also identify a candidate companion galaxy C $\sim3\,{\rm kpc}$ away, likely destined to merge with galaxies A and B. The combined light from galaxies A+B is magnified by factors of $\sim$8, 5, and 2 in three lensed images JD1, 2, and 3 with F356W fluxes $\sim322$, $203$, $86\,\rm nJy$ (AB mag 25.1, 25.6, 26.6). MACS0647$-$JD is significantly brighter than other galaxies recently discovered at similar redshifts with JWST. Without magnification, it would have AB mag 27.3 ($M_{UV}=-20.4$). With a high confidence level, we obtain a photometric redshift of $z=10.6\pm0.3$ based on photometry measured in 6 NIRCam filters spanning $1-5\rmμm$, out to $4300\,Å$ rest-frame. JWST NIRSpec observations planned for January 2023 will deliver a spectroscopic redshift and a more detailed study of the physical properties of MACS0647$-$JD.

astro-ph.GA↗

arXiv2023

The MUSE Ultra Deep Field (MUDF). III. Hubble Space Telescope WFC3 Grism Spectroscopy and Imaging

We present extremely deep Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) observations of the MUSE Ultra Deep Field (MUDF). This unique region of the sky contains two quasars at $z \approx$ 3.22 that are separated by only $\sim$500 kpc, providing a stereoscopic view of gas and galaxies in emission and absorption across $\sim$10 billion years of cosmic time. We have obtained 90 orbits of HST WFC3 G141 near-infrared grism spectroscopy of this field in a single pointing, as well as 142 hours of optical spectroscopy with the Very Large Telescope (VLT) Multi Unit Spectroscopic Explorer (MUSE). The WFC3 (F140W, F125W, and F336W) and archival WFPC2 (F702W and F450W) imaging provides five-filter photometry that we use to detect 3,375 sources between $z \approx$ 0 - 6, including 1,536 objects in a deep central pointing with both spectroscopic and photometric coverage. The F140W and F336W mosaics reach exceptional depths of $m_\mathrm{AB}\approx$ 28 and 29, respectively, providing near-infrared and rest-frame ultraviolet information for 1,580 sources, and we reach 5$σ$ continuum detections for objects as faint as $m_\mathrm{AB}\approx$ 27 in the grism spectra. The extensive wavelength coverage of MUSE and WFC3 allows us to measure spectroscopic redshifts for 419 sources, down to galaxy stellar masses of log(M/M$_{\odot}$) $\approx$ 7 at $z \approx$ 1 - 2. In this publication, we provide the calibrated HST data and source catalogs as High Level Science Products for use by the community, which includes photometry, morphology, and redshift measurements that enable a variety of studies aimed at advancing our models of galaxy formation and evolution in different environments.

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arXiv2022

SKYSURF-3: Testing Crowded Object Catalogs in the Hubble eXtreme Deep Field Mosaics to Study Sample Incompleteness from an Extragalactic Background Light Perspective

Extragalactic Background Light (EBL) studies have revealed a significant discrepancy between direct measurements -- via instruments measuring "bare" sky from which Zodiacal and Galactic light models are subtracted -- and measurements of the Integrated Galaxy Light (IGL). This discrepancy could lie in either method, whether it be an incomplete Zodiacal model or missed faint galaxies in the IGL calculations. It has been proposed that the discrepancy is due to deep galaxy surveys, such as those with the Hubble Space Telescope (HST), missing up to half of the faint galaxies with $24 \le m_{AB} \le 29$ mag. We address this possibility by simulating higher number densities of galaxies, and so assess incompleteness due to object overlap, with three replications of the Hubble UltraDeep Field (HUDF). SourceExtractor is used to compare the recovered counts and photometry to the original HUDF, allowing us to assess how many galaxies may have been missed due to confusion, i.e., due to blending with neighboring faint galaxies. This exercise reveals that, while up to 50% of faint galaxies with $28 \le m_{AB} \le 29$ mag were missed or blended with neighboring objects in certain filters, not enough were missed to account for the EBL discrepancy alone in any of the replications.

astro-ph.CO↗

arXiv2022

CLEAR: The Ionization and Chemical-Enrichment Properties of Galaxies at 1.1 < z < 2.3

We use deep spectroscopy from the Hubble Space Telescope (HST) Wide-Field-Camera 3 (WFC3) IR grisms combined with broad-band photometry to study the stellar populations, gas ionization and chemical abundances in star-forming galaxies at $z\sim 1.1-2.3$. The data stem from the CANDELS Lyman-$α$ Emission At Reionization (CLEAR) survey. At these redshifts the grism spectroscopy measure the [OII] 3727, 3729, [OIII] 4959, 5008, H-$β$ strong emission features, which constrain the ionization parameter and oxygen abundance of the nebular gas. We compare the line flux measurements to predictions from updated photoionization models (MAPPINGS (Kewley et al. 2019), which include an updated treatment of nebular gas pressure, log P/k = $n_e T_e$. Compared to low-redshift samples ($z\sim 0.2$) at fixed stellar mass, llog M / M$_\odot$ = 9.4-9.8, the CLEAR galaxies at z=1.35 (z=1.90) have lower gas-phase metallicity, $Δ$(log Z) = 0.25 (0.35) dex, and higher ionization parameters, $Δ$(log q) = 0.25 (0.35) dex, where U = q/c. We provide updated analytic calibrations between the [OIII], [OII], and H-$β$ emission line ratios, metallicity, and ionization parameter. The CLEAR galaxies show that at fixed stellar mass, the gas ionization parameter is correlated with the galaxy specific star-formation rates (sSFRs), where $Δ$ log q = 0.4 $Δ$(log sSFR), derived from changes in the strength of galaxy H-$β$ equivalent width. We interpret this as a consequence of higher gas densities, lower gas covering fractions, combined with higher escape fraction of H-ionizing photons. We discuss both tests to confirm these assertions and implications this has for future observations of galaxies at higher redshifts.

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arXiv2022

On the Stellar Populations of Galaxies at z=9-11: The Growth of Metals and Stellar Mass at Early Times

We present a detailed stellar population analysis of 11 bright ($H<26.6$) galaxies at $z=9-11$ (three spectroscopically confirmed) to constrain the chemical enrichment and growth of stellar mass of early galaxies. We use the flexible Bayesian spectral energy distribution (SED) fitting code Prospector with a range of star-formation histories (SFHs), a flexible dust attenuation law and a self-consistent modeling of emission lines. This approach allows us to assess how different priors affect our results, and how well we can break degeneracies between dust attenuation, stellar ages, metallicity and emission lines using data which probe only the rest-frame ultraviolet to optical wavelengths. We measure a median observed ultraviolet spectral slope $β=-1.87_{-0.43}^{+0.35}$ for relatively massive star-forming galaxies ($9<\log(M_{\star}/M_{\odot})<10$), consistent with no change from $z=4$ to $z=9-10$ at these stellar masses, implying rapid enrichment. Our SED-fitting results are consistent with a star-forming main sequence with sub-linear slope ($0.7\pm0.2$) and specific star-formation rates of $3-10~\mathrm{Gyr}^{-1}$. However, the stellar ages and SFHs are less well constrained. Using different SFH priors, we cannot distinguish between median mass-weighted ages of $\sim50-150$ Myr, which corresponds to 50\% formation redshifts of $z_{50}\sim10-12$ at $z\sim9$ and is of the order of the dynamical timescales of these systems. Importantly, the models with different SFH priors are able to fit the data equally well. We conclude that the current observational data cannot tightly constrain the mass-buildup timescales of these $z=9-11$ galaxies, with our results consistent with SFHs implying both a shallow and steep increase of the cosmic SFR density with time at $z>10$.

astro-ph.GA↗

arXiv2021

A Census of the Bright z=8.5-11 Universe with the Hubble and Spitzer Space Telescopes in the CANDELS Fields

We present the results from a new search for candidate galaxies at z ~ 8.5-11 discovered over the 850 arcmin^2 area probed by the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS). We use a photometric redshift selection including both Hubble and Spitzer Space Telescope photometry to robustly identify galaxies in this epoch at F160W < 26.6. We use a detailed vetting procedure, including screening for persistence, stellar contamination, inclusion of ground-based imaging, and followup space-based imaging to build a robust sample of 11 candidate galaxies, three presented here for the first time. The inclusion of Spitzer/IRAC photometry in the selection process reduces contamination, and yields more robust redshift estimates than Hubble alone. We constrain the evolution of the rest-frame ultraviolet luminosity function via a new method of calculating the observed number densities without choosing a prior magnitude bin size. We find that the abundance at our brightest probed luminosities (M_UV=-22.3) is consistent with predictions from simulations which assume that galaxies in this epoch have gas depletion times at least as short as those in nearby starburst galaxies. Due to large Poisson and cosmic variance uncertainties we cannot conclusively rule out either a smooth evolution of the luminosity function continued from z=4-8, or an accelerate decline at z > 8. We calculate that the presence of seven galaxies in a single field (EGS) is an outlier at the 2-sigma significance level, implying the discovery of a significant overdensity. These scenarios will be imminently testable to high confidence within the first year of observations of the James Webb Space Telescope.

astro-ph.GA↗

arXiv2021

The mass-metallicity relation at z~1-2 and its dependence on star formation rate

We present a new measurement of the gas-phase mass-metallicity relation (MZR), and its dependence on star formation rates (SFRs) at 1.3 < z < 2.3. Our sample comprises 1056 galaxies with a mean redshift of z = 1.9, identified from the Hubble Space Telescope Wide Field Camera 3 (WFC3) grism spectroscopy in the Cosmic Assembly Near-Infrared Deep Extragalactic Survey (CANDELS) and the WFC3 Infrared Spectroscopic Parallel Survey (WISP). This sample is four times larger than previous metallicity surveys at z ~ 2, and reaches an order of magnitude lower in stellar mass (10^8 M_sun). Using stacked spectra, we find that the MZR evolves by 0.3 dex relative to z ~ 0.1. Additionally, we identify a subset of 49 galaxies with high signal-to-noise (SNR) spectra and redshifts between 1.3 < z < 1.5, where H-alpha emission is observed along with [OIII] and [OII]. With accurate measurements of SFR in these objects, we confirm the existence of a mass-metallicity-SFR (M-Z-SFR) relation at high redshifts. These galaxies show systematic differences from the local M-Z-SFR relation, which vary depending on the adopted measurement of the local relation. However, it remains difficult to ascertain whether these differences could be due to redshift evolution, as the local M-Z-SFR relation is poorly constrained at the masses and SFRs of our sample. Lastly, we reproduced our sample selection in the IllustrisTNG hydrodynamical simulation, demonstrating that our line flux limit lowers the normalization of the simulated MZR by 0.2 dex. We show that the M-Z-SFR relation in IllustrisTNG has an SFR dependence that is too steep by a factor of around three.

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