Dr Harley Katz

Abstract:

The dimensionless ionization parameter, U ≡ q/c, where q is the ratio of the local ionizing photon flux to the local hydrogen density, is a key metric to parameterize nebular conditions. Prior to JWST, the rest-frame optical emission lines and their ratios which trace the ionization parameter (e.g., O32≡[O III]/[O II]) were inaccessible at high redshifts. Here we quantify, for the first time, the evolution of the ionization parameter in galaxies across the last 13 billion years of cosmic time by comparing JWST/NIRSpec PRISM and G395M spectroscopy of 434 galaxies at 3 ≲ z ≲ 9 from the RUBIES survey with z < 3 samples from SDSS, LEGA-C, and KBSS. We leverage a large suite of Cloudy photoionization models to infer the ionization parameter from [O III] and [O II]. We find that U increases with redshift and specific star formation rate (sSFR), and decreases with stellar mass. Crucially, and in contrast to previous linear best-fit calibrations, our inference results in a systematic uncertainty in log U of ∼ 0.3 dex at zero measurement uncertainty due to the wide range of photoionization models that predict the same O32 ratio without informative priors. We compare to SPHINX²⁰ and LUMEN simulations and find that the simulated galaxies exhibit higher O32 ratios at fixed redshift and stellar mass compared to RUBIES observations. Finally, we combine the predictive power of observed and physical quantities with multivariate relations to estimate U from redshift, stellar mass, and sSFR for use in cases where O32 is not available from spectroscopy. We find that the ionization parameter increases at fixed stellar mass and sSFR by a factor of ∼ 4 from z = 2 to z = 6, demonstrating that the redshift evolution encapsulates physics beyond that traced by stellar mass and sSFR alone. Finally, we show that a toy model with the first order assumption that H II region volume is proportional to galaxy volume can explain the excess redshift dependence of the ionization parameter as being consistent with observed evolution in galaxy sizes.

Abstract:

We introduce MEGATRON, a new galaxy formation model for cosmological radiation hydrodynamics simulations of high-redshift galaxies. The model accounts for the non-equilibrium chemistry and heating/cooling processes of ≥ 80 atoms, ions, and molecules, coupled to on-the-fly radiation transfer. We apply the model in a cosmological setting to the formation of a 10⁹ M⊙halo at z = 6, and run 25 realizations at pc-scale resolution, varying numerous parameters associated with our state-of-the-art star formation, stellar feedback, and chemical enrichment models. We show that the overall budget of feedback energy is the key parameter that controls star formation regulation at high redshift, with other numerical parameters (e.g. supernova clustering, star formation conditions) having a more limited impact. As a similar feedback model has been shown to produce realistic z = 0 galaxies, our work demonstrates that calibration at z = 0 does not guarantee strong regulation of star formation at high-redshift. Interestingly, we find that subgrid model variations that have little impact on the final z = 6 stellar mass can lead to substantial changes on the observable properties of high-redshift galaxies. For example, different star formation models based on, e.g. density thresholds or turbulence inspired criteria, lead to fundamentally distinct nebular emission line ratios across the interstellar medium (ISM). These results highlight the ISM as an important resource for constraining models of star formation, feedback, and galaxy formation in the JWST era, where emission line measurements for > 1, 000 high-redshift galaxies are now available. Subject headings: high-redshift galaxies, ISM, galaxy formation.

Abstract:

Among the most puzzling early discoveries of JWST are “little red dots” (LRDs), compact red sources that host broad Balmer emission lines, and in many cases exhibit a “V-shaped” change in slope in the rest-optical. The physical properties of LRDs currently have order-of-magnitude uncertainties, because models to explain the continuum of these sources differ immensely. Here, we leverage the complete selection of red sources in the RUBIES program, supplemented with public PRISM spectra, to study the origin of this V shape. By fitting a broken power law with a flexible inflection point, we find that a large fraction of red Hα emitters at 2 < z < 6 exhibit a strong change in slope, and that all strong inflections appear associated with the Balmer limit (0.3645 μm). Using a simple model of a reddened active galactic nucleus (AGN) with an unobscured scattered-light component, we demonstrate that the observed V shape in LRDs is unlikely to occur at any specific wavelength if the entire continuum is dominated by light from a power-law AGN continuum. In contrast, models with an intrinsic feature at the Balmer limit, such as those that are dominated by an evolved stellar population, can produce the observed spectral shapes, provided that a reddened component picks up sufficiently redward of the break. While no model can comfortably explain the full LRD spectral energy distribution, the common inflection location suggests that a single component consistently dominates the rest-frame UV optical in LRDs, and that this component is associated with T ∼ 10⁴ K hydrogen.

Abstract:

We present VLT/X-shooter spectroscopy of the extremely metal-poor starburst galaxy SBS 0335‑052E, a nearby (D∼54 Mpc) analogue of high-redshift systems, probing its outflow up to a distance of ∼2.6 kpc. Using direct-method oxygen abundances, we find a complex metallicity profile that generally declines with distance, decreasing by 0.37 dex from the galaxy centre out to 2.4 kpc into the outflow. This implies a metal-loading factor roughly an order of magnitude lower than predictions based on the mass–metallicity relation for low-mass galaxies. We also detect extended He II emission, including a broad, redshifted component beyond 2 kpc, which is distinct from the narrow emission associated with star clusters. No Hβ emission associated with the broad redshifted component is detected, which implies extremely high He II /Hβ ratios (0.7 - 4.8). Such extreme values, combined with the decreasing metallicity, challenge our current models of stellar feedback. The contribution of an intermediate-mass black hole could simultaneously account for the declining metallicity and the unusually extended He II emission.

Abstract:

We present a JWST and Atacama Large Millimeter/submillimeter Array (ALMA) detailed study of the interstellar medium properties of high-redshift galaxies. Our JWST/NIRSpec integral field unit spectroscopy targeting three galaxies at z = 6-7 detects key rest-frame optical emission lines, allowing us to derive [O ii] λλ3726, 3729-based electron densities of ne,optical ∼ 1000 cm⁻³ on average and [O iii] λ4363-based metallicities of 12 + log ( O / H ) = 8.0 - 8.2 in two galaxies. New ALMA Band 9 and 10 observations detect the [O iii] 52 μm line in one galaxy but do not in the others, resulting in far-infrared (FIR)-based densities of ne,FIR ≲ 500 cm⁻³ from the [O iii] 52 μm/[O iii] 88 μm ratio, systematically lower than the optical [O ii]-based measurements. These low FIR-based densities are comparable to those at both z ∼ 0 and z > 6 in the literature, including JADES-GS-z14-0 at z = 14.18, suggesting little evolution up to z ∼ 14, in contrast to the increasing trend of optical-based densities with redshift. By conducting a JWST and ALMA joint analysis using emission lines detected with both telescopes, we find that the observed FIR [O iii] 52 and 88 μm luminosities are too high to be explained by the optical-based densities at which they would be significantly collisionally de-excited. Instead, a two-zone model with distinct high- and low-density regions is required to reproduce all observed lines, indicating that FIR [O iii] emission arises predominantly from low-density gas, while the optical [O iii] and [O ii] lines trace both regions. We further demonstrate that the direct-Te method can sometimes significantly underestimate metallicities up to 0.8 dex due to the presence of the low-density gas not fully traced by optical lines alone, highlighting the importance of combining optical and FIR lines to accurately determine gas-phase metallicities in the early Universe.