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Black Hole

Lensing of space time around a black hole. At Oxford we study black holes observationally and theoretically on all size and time scales - it is some of our core work.

Credit: ALAIN RIAZUELO, IAP/UPMC/CNRS. CLICK HERE TO VIEW MORE IMAGES.

Dr Harley Katz

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Sub department

  • Astrophysics
harley.katz@physics.ox.ac.uk
Telephone: 01865 273348
Denys Wilkinson Building, room 532D
  • About
  • Publications

High-z Stellar Masses Can Be Recovered Robustly with JWST Photometry

The Astrophysical Journal Letters American Astronomical Society 978:2 (2025) l42

Authors:

RK Cochrane, H Katz, R Begley, CC Hayward, PN Best
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21 BALMER JUMP STREET: THE NEBULAR CONTINUUM AT HIGH REDSHIFT AND IMPLICATIONS FOR THE BRIGHT GALAXY PROBLEM, UV CONTINUUM SLOPES, AND EARLY STELLAR POPULATIONS

Open Journal of Astrophysics 8 (2025)

Authors:

H Katz, AJ Cameron, A Saxena, L Barrufet, N Choustikov, NJ Cleri, A de Graaff, RS Ellis, RAE Fosbury, KE Heintz, M Maseda, J Matthee, I McConachie, PA Oesch

Abstract:

We study, from both a theoretical and observational perspective, the physical origin and spectro-scopic impact of extreme nebular emission in high-redshift galaxies. The nebular continuum, which can appear during an extreme starburst, is of particular importance as it tends to redden UV slopes and has a significant contribution to the UV luminosities of galaxies. Furthermore, its shape can be used to infer the gas density and temperature of the interstellar medium. First, we provide a theoretical background, showing how different stellar populations (SPS models, initial mass functions (IMFs), and stellar temperatures) and nebular conditions impact observed galaxy spectra. We demonstrate that, for systems with strong nebular continuum emission, 1) UV fluxes can increase by up to 0.7 magnitudes (or more in the case of hot/massive stars) above the stellar continuum, which may help reconcile the surprising abundance of bright high-redshift galaxies and the elevated UV luminosity density at z ≿ 10, 2) at high gas densities, UV slopes can redden from β ≾ −2.5 to β ∼ −1, 3) observational measurements of ξion are gross underestimates, and 4) UV downturns from two-photon emission can masquerade as damped Lyα systems. Second, we present a dataset of 58 galaxies observed with NIRSpec on JWST at 2.5 < z < 9.0 that are selected to have strong nebular continuum emission via the detection of the Balmer jump. Five of the 58 spectra are consistent with being dominated by nebular emission, exhibiting both a Balmer jump and a UV downturn consistent with two-photon emission. For some galaxies, this may imply the presence of hot massive stars and a top-heavy IMF. We conclude by exploring the properties of spectroscopically confirmed z > 10 galaxies, finding that UV slopes and UV downturns are in some cases redder or steeper than expected from SPS models, which may hint at more exotic (e.g. hotter/more massive stars or AGN) ionizing sources.
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Impact of star formation models on the growth of simulated galaxies at high redshifts

Astronomy & Astrophysics EDP Sciences (2024)

Authors:

Cheonsu Kang, Taysun Kimm, Daniel Han, Harley Katz, Julien Devriendt, Adrianne Slyz, Romain Teyssier
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Early Bright Galaxies from Helium Enhancements in High-Redshift Star Clusters

The Open Journal of Astrophysics Maynooth University 7 (2024)

Authors:

Harley Katz, Alexander P Ji, Grace Telford, Peter Senchyna
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A public grid of radiative transfer simulations for Lyα and metal lines in idealised galactic outflows

Astronomy & Astrophysics EDP Sciences 691 (2024) a213

Authors:

T Garel, L Michel-Dansac, A Verhamme, V Mauerhofer, H Katz, J Blaizot, F Leclercq, G Salvignol
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