Dr Harley Katz

Abstract:

The nature of little red dots (LRDs) has largely been investigated through their continuum emission, with lines assumed to arise from a broad-line region. In this paper, we instead use recombination lines to infer the intrinsic properties of the central engine. Our analysis first reveals a tension between the ionizing properties implied from Hα and He ii λ4686. The high Hα EWs require copious H-ionizing photons, more than the bluest active galactic nucleus (AGN) ionizing spectra can provide. In contrast, He ii emission is marginally detected, and its low EW is, at most, consistent with the softest AGN spectra. The low He ii/Hβ (∼10⁻², <20×  local AGN median) further points to an unusually soft ionizing spectrum. We extend our analysis to dense gas envelopes (quasi-star/black-hole star) and find that hydrogen recombination lines become optically thick and lose diagnostic power, but He ii remains optically thin and a robust tracer. Photoionization modeling with Cloudy rules out standard AGN accretion disk spectra. Alternative explanations include exotic AGN with red rest-optical emission, high average optical depth (>10) from gas/dust, and soft ionizing spectra with abundant H-ionizing photons, consistent with, e.g., a cold accretion disk or a composite of AGN and stars. The latter is an intriguing scenario since high hydrogen densities are highly conducive for star formation, and nuclear star clusters are found in the vicinity of local massive black holes. While previous studies have mostly focused on features dominated by the absorbing hydrogen cloud, the He ii-based diagnostic proposed here represents a crucial step toward understanding the central engine of LRDs.

Abstract:

As observations have yet to constrain the ionizing properties of the faintest (MUV ≳ −16) galaxies, their contribution to cosmic reionization remains unclear. The rest-frame ultraviolet (UV) continuum slope (β) is a powerful diagnostic of stellar populations and one of the few feasible indicators of the escape fraction of ionizing photons (fesc) for such faint galaxies at high redshift. Leveraging ultradeep JWST/NIRCam GLIMPSE imaging of the strong lensing field Abell S1063, we estimate the UV continuum slopes of 553 galaxies at z > 6 with absolute magnitudes down to MUV ≃ −12.5. We find a modest evolution of β with redshift and a flattening in the β–MUV relation such that galaxies fainter than MUV ∼ −16.5 no longer exhibit the bluest UV slopes. The 136 ultrafaint galaxies with MUV > −16 are a diverse population encompassing dusty (30%), old (15%), and low-mass (50%) galaxies. We apply the empirical β–fesc relation from local Lyman continuum leakers, finding the mean fesc peaks at ∼20% at MUV = −16.5 and declines towards fainter galaxies, while remaining consistent with fesc = 14% within the uncertainties, in agreement with recent radiative transfer simulations. Incorporating GLIMPSE constraints on the UV luminosity function, ionizing photon production efficiency, and escape fractions produces a reionization history consistent with independent observational constraints. Our results indicate galaxies with an MUV between −18 and −14 supplied ∼60% of the ionizing photons to cosmic reionization, while the lower fesc of fainter galaxies produces a natural cutoff in the ionizing photon production rate density.

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:

We present results of Population III (Pop III) formation in the megatron suite of simulations, which self-consistently follows radiation and non-equilibrium chemistry, and resolves gas at near-pc resolution in a Milky Way-mass progenitor at Cosmic Dawn. While the very first Pop III stars form in haloes with masses well below the atomic cooling limit, the majority of Pop III stars form in more massive systems above the K atomic cooling threshold as a Lyman–Werner (LW) background of is rapidly established. We find that the global Pop III star formation rate stabilizes to a value of at . Among the three processes that quench Pop III star formation in minihaloes, the LW background, gas starvation, and external chemical enrichment, the LW background is most important. A small fraction of haloes undergo multiple episodes of Pop III star formation when the earlier forming stars all directly collapse to black holes. If the haloes become massive enough, they can form up to Pop III stars in a single burst, which may be observable by James Webb Space Telescope with moderate gravitational lensing. Pop III stars form at a wide range of distances from UV-bright galaxies, with only per cent of Pop III stars forming within the virial radius of galaxies with . Finally, by tracking Pop III star remnants down to , we find that per cent reside in the stellar halo of our simulated Milky Way analogue, while the remainder are gravitationally bound to lower mass systems, including satellite haloes.

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.