MEGATRON: The Physical Origins of Steep UV Slopes at high redshift

Year: 2026

Type: article

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 . 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.

Source: The Open Journal of Astrophysics

DOI: 10.33232/001c.169642

Authors: Harley Katz, Nicholas Choustikov, Aayush Saxena, Corentin Cadiou, Martin P. Rey, Jeremy Blaizot, Alex J. Cameron, Fergus Cullen, Taysun Kimm, Kosei Matsumoto

Institutions: University of Chicago, University of Oxford, Institut d'Astrophysique de Paris, University of Bath, Lyon 1 Université, University of Copenhagen, University of Edinburgh, Yonsei University, Ghent University

Language: English

Topic: Galaxies: Formation, Evolution, Phenomena

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: diamond

Funders: National Aeronautics and Space Administration, UK Research and Innovation, Durham University, Yonsei University, Science and Technology Facilities Council

OpenAlex ID: W7214945574