Dr Harley Katz

Abstract:

Reliable nebular emission line diagnostics are essential for accurately inferring the physical properties (e.g. electron temperature, density, pressure, and metallicity) of H II regions from spectra. When interpreting spectra, it is typical to adopt a single zone model, e.g. at fixed density, pressure, or temperature, to infer H II region properties. However, such an assumption may not fully capture the complexities of a turbulent interstellar medium. To understand how a complex density field driven by supersonic turbulence impacts nebular emission lines, we simulate 3D H II regions surrounding a single O star, both with and without supersonic turbulence. We find that turbulence directly impacts the values of common strong line ratios. For example turbulent H II regions exhibit systematically higher [N II]/Hα, lower [O III]/Hβ, and lower O32, compared to homogeneous H II regions with the same mean density and ionizing source. These biases can impact inferences of metallicity, ionization parameter, excitation, and ionization source. For our choice of turbulence, direct Te method metallicity inferences are biased low, by up to 0.1 dex, which is important for metallicity studies, but not enough to explain the abundance discrepancy problem. Finally, we show that large differences between measured electron densities emerge between infrared, optical, and UV density indicators. Our results motivate the need for large grids of turbulent H II regions models that span the range of conditions seen at both high and low redshift to better interpret observed spectra.

Abstract:

We study the stellar mass-iron metallicity relation of dwarf galaxies in the new high-resolution megatron cosmological radiation-hydrodynamics simulations. These simulations model galaxy formation up to z ≈ 8 in a region that will collapse into a Milky-Way-like galaxy at z = 0, while self-consistently tracking Population III and II (Pop. III, Pop. II) star formation, feedback and chemical enrichment. megatron dwarf galaxies are in excellent agreement with the observed stellar mass-metallicity relation at z = 0, including an over-abundance of dwarfs along a flat plateau in metallicity (⟨[Fe/H]⟩ ≈ −2.5) at low stellar masses (M⋆ ≤ 10⁵ M⊙). We tie this feature to the chemical enrichment of dwarf galaxies by Pop. III pair-instability supernova (PISN) explosions. The strong Lyman-Werner background (LW) from the protogalaxy ensures that PISNe occur in haloes massive enough (≈ 10⁷ M⊙) to retain their ejecta. We also predict a tail of ≈ 20% of iron-deficient (⟨[Fe/H]⟩ ≤ −3) dwarf galaxies. We show that both plateau and tail (i) are robust to large variations in Pop. II feedback assumptions, and (ii) survive in bound satellites surrounding the central galaxy at z = 0.

Abstract:

We present the MEGATRON suite of cosmological radiation hydrodynamics simulations following the formation of Milky Way-mass galaxies from the earliest cosmic epochs when Population III stars form to Cosmic Noon. The suite represents the first set of cosmological simulations that couples a vast nonequilibrium thermochemistry network of primordial species, metals, and molecules to multifrequency, on-the-fly radiation transport, allowing us to directly predict the spectral properties of early galaxies. By initializing the simulations at zero metallicity, resolving haloes well below the atomic cooling threshold, reaching parsec-scale resolution, and modeling a Milky Way-mass environment, we aim to address four key science themes: 1) Star formation at cosmic dawn, 2) Galaxy formation and the interstellar medium in the epoch of reionization, 3) The circumgalactic medium towards cosmic noon, and 4) Reionization in a local volume environment and near-field cosmology. In this introductory work, we present an overview of the physical characteristics of high-redshift MEGATRON galaxies and their environment at z > 8. We present a library of > 175, 000 simulated galaxy spectra and demonstrate how much of the diversity of galaxy spectra seen by JWST is naturally reproduced in the context of a ΛCDM cosmology. Caveats are discussed, such as the lack of AGN in our simulations and the limitations of our adopted stellar population and chemical yield models. This project represents a step towards making more direct comparisons between simulations and observations and is particularly applicable for optimizing methods to infer galaxy properties from existing high-redshift JWST spectra and imaging data.

Abstract:

The ultraviolet (UV) spectral slope, β, serves as a fundamental probe of stellar population properties and interstellar medium conditions in galaxies. At high redshift, a small sample of galaxies have been confirmed to have extremely blue spectral slopes of β < −2.8, as measured directly from spectroscopy. One explanation for such galaxies is that they are leaking copious amounts of ionizing photons such that these photons are never processed into the nebular continuum and we are observing the bluer intrinsic slopes of the stellar populations. Here, we use a detailed set of radiation hydrodynamics simulations with parsec-scale resolution and non-equilibrium chemistry to elucidate the physical origin of extremely blue UV slopes at high redshift. We show that, while some extremely blue galaxies (EBGs) do indeed have high escape fractions, a second population of EBGs has fesc < 1%. In this second population, the ISM gas densities tend to be much lower, and the timescale for ionization fronts to reach their Strömgren radii can be longer than the main-sequence lifetime of massive stars, leading to a lag in nebular emission. Our results demonstrate that extremely blue UV slopes do not uniquely imply high escape fractions, which highlights the importance of time-dependent (non-equilibrium) nebular emission for interpreting the spectra of galaxies in the early Universe.

Abstract:

Identifying Lyman continuum (LyC) leaking galaxies is an important step towards understanding the reionization history of the Universe. However, measuring the escape fraction of LyC photons (f LyC) in high-z galaxies is a very challenging task. We examine whether the O32 ratio, defined as the ratio between [OIII] λ5007Å and [OII] λ3727Å lines, can be used as a proxy for f LyC. To this end, we construct synthetic spectra by post-processing high-resolution, radiation-hydrodynamic simulations of giant molecular clouds (GMCs) with the photo-ionization code, CLOUDY. We then combine the results with a random forest machine learning algorithm to calculate emission line strengths at different evolutionary phases of the GMC.