Dr Harley Katz

Abstract:

The Hα emission line in galaxies is a powerful tracer of their recent star formation activity. With the advent of JWST, we are now able to routinely observe Hα in galaxies at high redshift (z ≳ 3) and thus measure their star formation rates (SFRs). However, using classical SFR(Hα) calibrations to derive the SFRs leads to biased results because high-redshift galaxies are commonly characterized by low metallicities and bursty star formation histories, affecting the conversion factor between the Hα luminosity (LHα ) and the SFR. We developed a set of new SFR(Hα) calibrations that allowed us to predict the SFRs of Hα-emitters at z ≳ 3 with very little error. We used the SPHINX cosmological simulations to select a sample of star-forming galaxies representative of the Hα-emitter population observed with JWST. We then derived linear corrections to the classical SFR(Hα) calibrations that took variations in the physical properties (e.g., stellar metallicities) among individual galaxies into account. We obtained two new SFR(Hα) calibrations that compared to the classical calibrations reduce the root mean squared error (RMSE) in the predicted SFRs by ΔRMSE ≈ 0.04 dex and ΔRMSE ≈ 0.06 dex, respectively. Using the recent JWST NIRCam/grism observations of Hα-emitters at z ∼ 6, we show that the new calibrations affect the high-redshift galaxy population statistics: (i) the estimated cosmic SFR density decreases by ΔρSFR ≈ 12%, and (ii) the observed slope of the star formation main sequence increases by Δ∂ log SFR/∂ log M★ = 0.08 ± 0.02.

Abstract:

Numerous high-z galaxies have recently been observed with the James Webb Space Telescope (JWST), providing new insights into early galaxy evolution. Their physical properties are typically derived through spectral energy distribution (SED) fitting, but the reliability of this approach remains uncertain owing to limited constraints on star formation histories (SFHs) and on the contribution from emission for such early systems. Applying BAGPIPES on simulated SEDs with SFR10 > 0.3 M ⊙ yr⁻¹ at z = 6 from the SPHINX ²⁰ cosmological simulation, we examine the uncertainties related to the recovery of stellar masses, star formation rates (SFR10), and stellar metallicities from mock JWST/Near-Infrared Camera photometry, spanning F115W–F444W. Even without dust or emission lines, fitting the intrinsic stellar continuum overestimates the stellar mass by about 60%, on average (and by up to a factor of five for low-mass galaxies with recent starbursts). It also underestimates the SFR10 by a factor of 2, due to inaccurate SFHs and age–metallicity degeneracies. In full SED-fitting models that include dust attenuation and nebular emission, stellar mass estimates are primarily affected by age–metallicity degeneracy and emission lines. Short-term SFRs are most sensitive to dust attenuation and nebular emission, while long-term SFRs additionally depend on the assumed SFHs. Incorporating bands that are free of strong emission lines, such as F410M, helps mitigate stellar mass overestimation by disentangling line emission from older stellar populations. We also find that best fit or likelihood-weighted estimates are generally more accurate than median posterior values. Although stellar mass functions are reproduced reasonably well (particularly when the minimum-χ ² estimates are used), the slope of the main sequence of star formation acutely depends on the adopted fitting model. Overall, these results underscore the importance of careful modelling when interpreting high-z photometry, particularly for galaxies with recent star formation burst and/or strong emission lines, to minimise systematic biases in derived physical properties.

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:

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.