Dr Harley Katz

Abstract:

The physical processes that led to the formation of billion-solar-mass black holes within the first 700 million years of cosmic time, a period known as cosmic dawn, remain a puzzle¹. Several theoretical scenarios have been proposed to seed and rapidly grow black holes^2-4, but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that exhibits singular properties: among the largest hydrogen Balmer breaks reported at any redshift, broad multi-peaked Hβ emission, and Balmer line absorption in several transitions. We model this source as an enshrouded black hole in which the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free envelope around a supermassive black hole^5,6. This source may provide evidence of an early black hole embedded in dense gas-a theoretical configuration proposed to rapidly grow black holes by super-Eddington accretion^7,8. Radiation from the black hole seems to dominate almost all observed light, leaving limited room for contribution from its host galaxy. If the source merged with its brighter neighbour, it would resemble the recently discovered 'little red dots' with perplexing spectral energy distributions^9-11. The redness of the black hole is due to gas, not dust^12,13, and scattering, not kinematics, gives rise to the complex line shapes and luminosities-black hole masses of these sources may therefore be overestimated by orders of magnitude.

Abstract:

The correlation between galaxy stellar mass and gas-phase metallicity, known as the mass–metallicity relation (MZR), gives key insights into the processes that govern galaxy evolution. However, unquantified observational and selection biases can result in systematic errors in attempts to recover the intrinsic MZR, particularly at higher redshifts. We characterize the MZR at z ∼ 3–6 within a fully Bayesian framework using JWST/NIRSpec spectra of 191 galaxies from the RUBIES survey. We forward model the observed mass–metallicity surface using prospector-generated spectra to account for two selection biases: the survey selection function and the success in observing high signal-to-noise ratio emission lines. We demonstrate that the RUBIES selection function, based on F444W magnitude and F150W – F444W color, has a negligible effect on our measured MZR. A correct treatment of the non-Gaussian metallicity uncertainties from strong-line calibrations lowers the derived MZR normalization by 0.2 dex and flattens the slope by ∼20%; forward modeling the effect of emission line observability steepens the slope by ∼15%. Both of these biases must be taken into account in order to properly measure the intrinsic MZR. This novel forward-modeling process motivates careful consideration of selection functions in future surveys, and paves the way for robust, high-redshift chemical enrichment studies that trace the evolution of the MZR across cosmic time.

Abstract:

We report the discovery of two z ∼ 12 galaxy candidates with unusually red UV slopes (βUV ≳ −1.5), and probe the origin of such colors at cosmic dawn. From Prospector fits to the UNCOVER/MegaScience dataset—deep JWST/NIRCam imaging of A2744 in 20 broad- and medium bands—we identify several new z  >  10 galaxies. Medium-band data improve redshift estimates, revealing two lensed (μ ∼ 3.3) z ∼ 12 galaxies in a close pair with βUV ≳ −1.5 at an UV absolute magnitude of MUV ∼ −19 mag, lying away from typical scatter on previously known MUV–βUV relations. Spectral energy distribution fitting with Prospector, Bagpipes, and EAZY support their high-z nature, with probability of low-z interlopers of p(z < 7) < 10%. The potential low-z interlopers are z ∼ 3 quiescent galaxies (QGs), but unexpected to be detected at the given field of view unless z ∼ 3 QG stellar mass function has a strong turn up at (Formula presented) logM*[M⊙]∼9. Unlike typical blue high-redshift candidates (βUV ≲ −2.0), these red slopes require either dust or nebular continuum reddening. The dust scenario implies AV ∼ 0.8 mag, which is larger than theoretical predictions, but is consistent with a dust-to-stellar mass ratio ( (Formula presented) logMdust/M*∼−3 ). The nebular scenario demands dense gas ( (Formula presented) lognH[cm⁻³]∼4.0 ) around hot stars ( (Formula presented) logTeff[K]∼4.9 ). Spectroscopic follow-up is essential to determine their true nature and reveal missing galaxies at the cosmic dawn.

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:

The ultraviolet (UV) continuum slope of galaxies, , is a powerful diagnostic of the metallicity and ages of stars, nebular gas properties, dust content, and the escape of Lyman continuum (LyC) photons. In this study, we present measurements for 395 spectroscopically confirmed galaxies at selected primarily from JADES, using high-quality JWST (James Webb Space Telescope) NIRSpec/PRISM spectra. We find a median , finding a mild increase in blueness of with increasing redshift and fainter UV magnitudes. Interestingly, we find evidence for reddening of the average at , deviating from the trend observed at . Using stacked spectra in bins of redshift and , we derive trends between and dust attenuation, metallicity, ionization parameter, and stellar age indicators, finding a lack of dust attenuation to be the dominant driver of bluer -values. We further report five galaxies with , which show a range of spectroscopic properties and signs of significant LyC photon leakage. Finally, we show that the redder -values at 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 cm can reproduce the range of red that we see in our sample. Higher gas densities can also redden the nebular continuum emission, potentially explaining the observed -values.