Dr Harley Katz

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:

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.

Abstract:

The discovery of high-redshift galaxies exhibiting a steep spectral ultraviolet (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 initial mass function 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 ionizing photon production efficiencies associated with massive star formation. We utilize the extensive medium-band imaging from the JWST Advanced Deep Extragalactic Survey (JADES), which enables the identification of Balmer jumps across a wide range of redshifts (), through the deficit in rest-frame optical continuum level. As Balmer jumps are a general recombination feature of young starbursts ( Myr), we further demand a high observed to power the strong nebular continuum, together with a relatively non-blue UV slope ( at ) indicating a lack of stellar continuum emission. Our nebular-dominated candidates, constituting per cent of galaxies at (decreasing to per cent at , not completeness-corrected) are faint in the rest-frame optical (median ) with extreme line emission (median Å, 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 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 active galactic nucleus.

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.