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:

Abstract The UV continuum slope of galaxies, β, is a powerful diagnostic of the metallicity and ages of stars, nebular gas properties, dust content, and the escape of Lyman continuum photons. In this study, we present β measurements for 395 spectroscopically confirmed galaxies at 5 < z < 14.3 selected primarily from JADES, using high quality JWST NIRSpec/PRISM spectra. We find a median β = −2.15, finding a mild increase in blueness of β with increasing redshift and fainter UV magnitudes. Interestingly, we find evidence for reddening of the average β at z > 9.5, deviating from the trend observed at z < 9.5. Using stacked spectra in bins of redshift and β, we derive trends between β and dust attenuation, metallicity, ionization parameter, and stellar age indicators, finding a lack of dust attenuation to be the dominant driver of bluer β values. We further report five galaxies with β ≤ −2.9, which show a range of spectroscopic properties and signs of significant LyC photon leakage. Finally, we show that the redder β values at z > 9.5 may require rapid build-up of dust reservoirs in the very early Universe or a significant contribution from the nebular continuum emission to the observed UV spectra, with the nebular continuum fraction depending on the gas temperatures and densities. We show that in the absence of dust, nebular emission at ne > 10, 000 cm−3 can reproduce the range of red β that we see in our sample. Higher gas densities can also redden the nebular continuum emission, potentially explaining the observed β values.

Abstract:

The discovery of high-redshift galaxies exhibiting a steep spectral UV downturn potentially indicative of two-photon continuum emission marks a turning point in our search for signatures of star formation following a top-heavy IMF in the early Universe. We develop a photometric search method for identifying further nebular-dominated galaxy candidates, whose nebular continuum dominates over the starlight, due to the high ionising photon production efficiencies ξion associated with massive star formation. We utilise the extensive medium-band imaging from JADES, which enables the identification of Balmer jumps across a wide range of redshifts (1.5 < z < 8.5), through the deficit in rest-frame optical continuum level. As Balmer jumps are a general recombination feature of young starbursts (≲ 3 Myr), we further demand a high observed log (ξion, obs/(Hz erg−1)) > 25.60 to power the strong nebular continuum, together with a relatively non-blue UV slope (mF115W − mF200W > −0.4 at z = 6) indicating a lack of stellar continuum emission. Our nebular-dominated candidates, constituting ∼11 per cent of galaxies at z ∼ 6 (decreasing to ∼2 per cent at z ∼ 2, not completeness-corrected) are faint in the rest-frame optical (median Mopt = −17.95) with extreme line emission (median EWHα, rest = 1567 Å, EW[O III] + Hβ, rest = 2292 Å). However, hot H ii region temperatures, collisionally-enhanced two-photon continuum emission, and strong UV lines are expected to accompany top-heavy star formation. Thus nebular-dominated galaxies do not necessarily exhibit the biggest Balmer jumps, nor the largest ξion, obs or reddest UV slopes. Hence continuum spectroscopy is ultimately required to establish the presence of a two-photon downturn in our candidates, thus advancing our understanding of primordial star formation and AGN.

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.