Amanda Stoffers

Understanding the sources and evolution of cosmic reionization remains a central challenge in astrophysics, with the escape of ionizing Lyman-continuum (LyC) photons from early galaxies representing a major uncertainty. In this work, we use more than 35,000 galaxy realisations from the thesan-zoom cosmological radiation-hydrodynamic simulations to identify indirect diagnostics of the LyC photon escape fraction (fesc) and the LyC photon escape rate ($\dot{N}_\mathrm{ion,esc}$) across the redshift range z = 3 − 16. We train random forest regression models using these diagnostics to predict both quantities. We present four models: two trained with the full set of simulation-derived indicators to predict fesc and $\dot{N}_\mathrm{ion,esc}$, and two restricted to observables accessible to JWST photometric surveys. We find the 10-to-100 Myr star-formation rate ratio (SFR10/SFR100) and the gas-to-stellar mass ratio (Mgas/M*) to be the strongest diagnostics of fesc, suggesting a strong relationship between ionizing photon escape and gas clearing through bursty star formation. In contrast, rest-frame UV (1500 Å) absolute magnitude (MUV) dominates $\dot{N}_\mathrm{ion,esc}$ prediction. Motivated by the strong predictive power of MUV, we combine observed UV luminosity functions with derived $\dot{N}_\mathrm{ion,esc} - M_\mathrm{UV}$ relations to construct histories of reionization. These are consistent with observational constraints, avoiding the recently reported crisis in the ionizing photon budget. Our analysis suggests that the bulk of reionization occurred rapidly after z ≈ 8, driven by UV-bright galaxies, with the MUV < −17 populations providing the dominant contribution.

A notable achievement of the first generation of JWST surveys was the discovery of an abundance of faint high-redshift (z > 4) broad-line active galactic nuclei (AGN) in the Lbol < 1045 erg s−1 regime, previously only accessible at z < 1. The high prevalence of absorption features in their broad hydrogen lines is one of the peculiarities of a significant fraction of this new population. In this paper, we conduct a broad census of Balmer absorption in a sample of 47 Type-1 AGN spanning 2 < z < 7. Accounting for incompleteness of JWST spectroscopic data, we estimate that $44_{-6}^{+21}$ % of Little Red Dots (LRDs) have absorption in Hα while the incidence rate is <25 % (at 2σ) in Little Blue Dots (LBDs). Additionally, R1000 JWST/NIRSpec data is strongly resolution limited, implying an inability to disentangle Doppler broadening in the absorption from optical depth effects. This is alleviated with R2700 observations, although some degeneracies remain. We find a significant correlation between the velocity of the Balmer absorption and the [OIII]5007 narrow line luminosity, suggesting a common driving mechanism. We do not find any other significant correlations (and do not confirm previously claimed correlations) between Balmer absorption and other spectral properties of LRDs (except for the expected correlation between Hα and Hβ absorption velocities). Comparing LRDs, quasars and stellar Hα absorbers, we establish that absorption in both LRDs and broad absorption line quasars is consistent with radiatively driven outflows, echoing the physics of variable stars yet occurring at vastly different scales.

Direct observations of Lyman continuum (LyC) radiation from galaxies during the epoch of reionization (EoR) are impeded by absorption in the intergalactic medium, requiring indirect methods to infer the escape fraction of ionizing photons ($f_\mathrm{esc}^{\mathrm{LyC}}$). One approach is to develop and validate such methods on local analogues of the high-redshift galaxies with directly detected LyC leakage. In this study, we constrain $f_\mathrm{esc}^\mathrm{LyC}$ using a Bayesian spectral energy distribution (SED) fitting framework built in prospector, which incorporates a non-parametric star formation history, a flexible dust attenuation curve, self-consistent nebular emission, and fibre aperture-loss corrections. Our methodology jointly fits broad-band photometry and emission-line fluxes. We apply six models to the Low-redshift LyC Survey (LzLCS), a sample of local galaxies with physical properties comparable to EoR galaxies, and evaluate them based on their ability to recover the observed flux and their relative Bayesian evidence. The best-performing model is further assessed through a parameter recovery test, demonstrating that $f_\mathrm{esc}^\mathrm{LyC}$ can be recovered within uncertainties. Building on these results, we present updated $f_\mathrm{esc}^\mathrm{LyC}$ estimates for the LzLCS sample, with a median of 4 per cent, and values reaching as high as 51 per cent, with 26 of 64 galaxies having a cosmologically relevant $f_\mathrm{esc}^\mathrm{LyC}$ ($\gtrsim 5~{{\ \rm per\ cent}}$). Additionally, we present a revised ultraviolet $\beta$-slope versus $\log _{10}(f_\mathrm{esc}^\mathrm{LyC})$ relation, derived using symbolic regression with pysr trained on a synthetic data set generated with prospector: $\log _{10}(f_\mathrm{esc}^\mathrm{LyC}) = (-2.30 \pm 1.28)\beta - (6.26 \pm 2.91)$, ($\sigma = 0.43$ dex). The relation successfully reproduces the $f_\mathrm{esc}^\mathrm{LyC}$ obtained from full SED fitting of the LzLCS sample within $1\sigma$.