Dr Harley Katz

Abstract:

Finding the first (Population III or Pop III) stars is one of the fundamental quests of astronomy, aiming to deliver the missing link in how stars form at early cosmic times. Yet their initial mass function, formation sites and feedback remain highly uncertain, as well as the timing and topology of the transition to metal-enriched star formation. The observability of their peculiar spectral features is also debated, due to their short lifetime and faintness. This review summarizes current theoretical expectations for Pop III star formation, and the main observational strategies that have been adopted to constrain their properties across cosmic time, including near-field cosmology studies, direct searches for extremely metal-poor star-forming complexes and/or hard-ionizing spectral signatures at high and intermediate redshifts, and prospects for identifying Pop III activity up to Cosmic Dawn. The combination of JWST spectroscopy, time-domain searches, lensing surveys, stellar archaeology, absorption-line studies, as well as improved simulations, is yielding a growing number of observational candidates and narrowing the allowed parameter space for the first stars, setting the stage for a ‘’golden era’’ of Pop III searches.

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:

We construct and characterize a sample of 146 little red dots (LRDs) across (Formula presented), selecting all sources with v-shaped ultraviolet (UV)–optical continua from James Webb Space Telescope (JWST) NIRSpec/PRISM spectra and compact morphologies in NIRCam/F444W imaging. We show that LRD continuum spectra are well described by modified blackbodies across (Formula presented), with typical (Formula presented)  K ((Formula presented) ) across 2 dex in luminosity, and a tail toward (Formula presented)  K. LRDs therefore trace a locus in the Hertzsprung–Russell diagram that is analogous to stars on the Hayashi track, strongly supporting the picture that LRDs are active galactic nucleus (AGN) embedded in optically thick dense gas envelopes. Hotter LRDs with (Formula presented) typically have strong Balmer breaks, redder UV slopes, and high optical luminosities; other LRDs show weak or no Balmer breaks, and wide variety in (Formula presented) and (Formula presented). Crucially, we demonstrate that the UV–optical continua are strongly linked to the H(Formula presented), H(Formula presented), [O iii], and O i line properties. There is a tight linear relation between the H(Formula presented) and optical continuum luminosities, as well as H(Formula presented) and (Formula presented), indicating that Balmer, O i, and optical emission must primarily be powered by the same source. The Balmer decrement increases strongly toward higher (Formula presented), (Formula presented), and Balmer break strength, providing key evidence for luminosity-dependent effects of collisional (de-)excitation and resonant scattering in the gaseous envelopes. In contrast, we show that [O iii] emission likely originates from star-forming host galaxies, with strong variation in the AGN-to-host ratio among the LRD population. Our work presents an empirical description of the nature and structure of LRDs, defining a new benchmark for ongoing LRD model developments.