Dr Harley Katz

Abstract:

The recent discovery of a large number of massive black holes within the first two billion years after the big bang, as well as their peculiar properties, have been largely unexpected based on the extrapolation of the properties of luminous quasars. These findings have prompted the development of several theoretical models for the early formation and growth of black holes, which are, however, difficult to differentiate. We report the metallicity measurement around a gravitationally lensed massive black hole at redshift 7.04 (classified as a Little Red Dot), hosted in a galaxy with very low dynamical mass. The weakness of the [O iii]5007 emission line relative to the narrow H emission indicates extremely low metallicity, about solar, and even more metal poor in the surrounding few 100 pc. We argue that such properties cannot be uncommon among accreting black holes around this early cosmic epoch. Explaining such a low chemical enrichment in a system that has developed a massive black hole is challenging for most theories. Models assuming heavy black hole seeds (such as Direct Collapse Black Holes) or super-Eddington accretion scenarios struggle to explain the observations, although they can potentially reproduce the observed properties in some cases. Models invoking ‘primordial black holes’ (i.e. putative black holes formed shortly after the big bang) may potentially explain the low chemical enrichment associated with this black hole, although this class of models also requires further developments for proper testing.

Abstract:

Aims. The escape fraction of Lyman continuum photons ( fesc(LyC)) is the last key unknown in our understanding of cosmic reionization. Directly estimating the escape fraction ( fesc) of ionizing photons in the epoch of reionization (EoR) is impossible, due to the opacity of the intergalactic medium (IGM). However, a high fesc leaves clear imprints in the spectrum of a galaxy, due to reduced nebular line and continuum emission, which also leads to bluer UV continuum slopes (βUV). Methods. In this work, we exploited the large archive of deep JamesWebb Space Telescope (JWST) NIRSpec spectra from the DAWN JWST archive to analyze over 1400 galaxies at 5 < zspec < 10 and constrain their fesc based on spectral-energy-distribution fitting enhanced with a picket-fence model. We identify 71 high-confidence sources with significant fesc based on Bayes-factor analysis strongly favoring fesc > 0 over fesc= 0 solutions. We compare the characteristics of this high-escape subset against both the parent sample and established diagnostics including βUV slope, O32, and SFR surface density (ΣSFR). Results. For the overall sample, we find that most sources have a low escape fraction (<1%); however, a small subset of sources seems to emit a large number of their ionizing photons into the IGM, such that the average fesc is found to be ∼10%, as needed for galaxies to drive reionization. Conclusions. Although uncertainties remain regarding recent burstiness and the intrinsic stellar ionizing-photon output at low metallicities, our results demonstrate the unique capability of JWST/NIRSpec to identify individual LyC leakers, measure average fesc, and thus constrain the drivers of cosmic reionization.

Abstract:

The Hα emission line in galaxies is a powerful tracer of their recent star formation activity. With the advent of JWST, we are now able to routinely observe Hα in galaxies at high redshift (z ≳ 3) and thus measure their star formation rates (SFRs). However, using classical SFR(Hα) calibrations to derive the SFRs leads to biased results because high-redshift galaxies are commonly characterized by low metallicities and bursty star formation histories, affecting the conversion factor between the Hα luminosity (LHα ) and the SFR. We developed a set of new SFR(Hα) calibrations that allowed us to predict the SFRs of Hα-emitters at z ≳ 3 with very little error. We used the SPHINX cosmological simulations to select a sample of star-forming galaxies representative of the Hα-emitter population observed with JWST. We then derived linear corrections to the classical SFR(Hα) calibrations that took variations in the physical properties (e.g., stellar metallicities) among individual galaxies into account. We obtained two new SFR(Hα) calibrations that compared to the classical calibrations reduce the root mean squared error (RMSE) in the predicted SFRs by ΔRMSE ≈ 0.04 dex and ΔRMSE ≈ 0.06 dex, respectively. Using the recent JWST NIRCam/grism observations of Hα-emitters at z ∼ 6, we show that the new calibrations affect the high-redshift galaxy population statistics: (i) the estimated cosmic SFR density decreases by ΔρSFR ≈ 12%, and (ii) the observed slope of the star formation main sequence increases by Δ∂ log SFR/∂ log M★ = 0.08 ± 0.02.

Abstract:

Numerous high-z galaxies have recently been observed with the James Webb Space Telescope (JWST), providing new insights into early galaxy evolution. Their physical properties are typically derived through spectral energy distribution (SED) fitting, but the reliability of this approach remains uncertain owing to limited constraints on star formation histories (SFHs) and on the contribution from emission for such early systems. Applying BAGPIPES on simulated SEDs with SFR10 > 0.3 M ⊙ yr⁻¹ at z = 6 from the SPHINX ²⁰ cosmological simulation, we examine the uncertainties related to the recovery of stellar masses, star formation rates (SFR10), and stellar metallicities from mock JWST/Near-Infrared Camera photometry, spanning F115W–F444W. Even without dust or emission lines, fitting the intrinsic stellar continuum overestimates the stellar mass by about 60%, on average (and by up to a factor of five for low-mass galaxies with recent starbursts). It also underestimates the SFR10 by a factor of 2, due to inaccurate SFHs and age–metallicity degeneracies. In full SED-fitting models that include dust attenuation and nebular emission, stellar mass estimates are primarily affected by age–metallicity degeneracy and emission lines. Short-term SFRs are most sensitive to dust attenuation and nebular emission, while long-term SFRs additionally depend on the assumed SFHs. Incorporating bands that are free of strong emission lines, such as F410M, helps mitigate stellar mass overestimation by disentangling line emission from older stellar populations. We also find that best fit or likelihood-weighted estimates are generally more accurate than median posterior values. Although stellar mass functions are reproduced reasonably well (particularly when the minimum-χ ² estimates are used), the slope of the main sequence of star formation acutely depends on the adopted fitting model. Overall, these results underscore the importance of careful modelling when interpreting high-z photometry, particularly for galaxies with recent star formation burst and/or strong emission lines, to minimise systematic biases in derived physical properties.