Hitting the slopes: A spectroscopic view of UV continuum slopes of galaxies reveals a reddening at z > 9.5

Abstract The UV continuum slope of galaxies, β, is a powerful diagnostic of the metallicity and ages of stars, nebular gas properties, dust content, and the escape of Lyman continuum photons. In this study, we present β measurements for 395 spectroscopically confirmed galaxies at 5 < z < 14.3 selected primarily from JADES, using high quality JWST NIRSpec/PRISM spectra. We find a median β = −2.15, finding a mild increase in blueness of β with increasing redshift and fainter UV magnitudes. Interestingly, we find evidence for reddening of the average β at z > 9.5, deviating from the trend observed at z < 9.5. Using stacked spectra in bins of redshift and β, we derive trends between β and dust attenuation, metallicity, ionization parameter, and stellar age indicators, finding a lack of dust attenuation to be the dominant driver of bluer β values. We further report five galaxies with β ≤ −2.9, which show a range of spectroscopic properties and signs of significant LyC photon leakage. Finally, we show that the redder β values at z > 9.5 may require rapid build-up of dust reservoirs in the very early Universe or a significant contribution from the nebular continuum emission to the observed UV spectra, with the nebular continuum fraction depending on the gas temperatures and densities. We show that in the absence of dust, nebular emission at ne > 10, 000 cm−3 can reproduce the range of red β that we see in our sample. Higher gas densities can also redden the nebular continuum emission, potentially explaining the observed β values.

Authors

Institutions

Publication Details

Journal
Monthly Notices of the Royal Astronomical Society
Published
2026-04-30
DOI
https://doi.org/10.1093/mnras/stag808
Primary Topic
Galaxies: Formation, Evolution, Phenomena
Type
article
Field-Weighted Citation Impact
0.00
Controls
|||
ALL TIME
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
article

Hitting the slopes: A spectroscopic view of UV continuum slopes of galaxies reveals a reddening at z > 9.5

Joris Witstok, Phillip A. Cargile, Santiago Arribas, Harley Katz et al.
Monthly Notices of the Royal Astronomical Society
Galaxies: Formation, Evolution, Phenomena
article

Hitting the slopes: A spectroscopic view of UV continuum slopes of galaxies reveals a reddening at z > 9.5

Joris Witstok, Phillip A. Cargile, Santiago Arribas, Harley Katz, Isaac Laseter, A Saxena, Stefano Carniani, Yongda Zhu, Brant Robertson, Charlotte Simmonds, Francesco D’Eugenio, Emma Curtis-Lake, Christina C Williams, Kristan Boyett, Benjamin D Johnson, Andrew J Bunker, Rachana Bhatawdekar, Gareth C Jones, Hannah Übler, Michael V Maseda, Chris Willott, Alex J Cameron, Zhiyuan Ji, Stéphane Charlot, Sandro Tacchella, Nimisha Kumari, Kevin Hainline, Jacopo Chevallard, Mirko Curti
article en

Abstract

Abstract The UV continuum slope of galaxies, β, is a powerful diagnostic of the metallicity and ages of stars, nebular gas properties, dust content, and the escape of Lyman continuum photons. In this study, we present β measurements for 395 spectroscopically confirmed galaxies at 5 < z < 14.3 selected primarily from JADES, using high quality JWST NIRSpec/PRISM spectra. We find a median β = −2.15, finding a mild increase in blueness of β with increasing redshift and fainter UV magnitudes. Interestingly, we find evidence for reddening of the average β at z > 9.5, deviating from the trend observed at z < 9.5. Using stacked spectra in bins of redshift and β, we derive trends between β and dust attenuation, metallicity, ionization parameter, and stellar age indicators, finding a lack of dust attenuation to be the dominant driver of bluer β values. We further report five galaxies with β ≤ −2.9, which show a range of spectroscopic properties and signs of significant LyC photon leakage. Finally, we show that the redder β values at z > 9.5 may require rapid build-up of dust reservoirs in the very early Universe or a significant contribution from the nebular continuum emission to the observed UV spectra, with the nebular continuum fraction depending on the gas temperatures and densities. We show that in the absence of dust, nebular emission at ne > 10, 000 cm−3 can reproduce the range of red β that we see in our sample. Higher gas densities can also redden the nebular continuum emission, potentially explaining the observed β values.

Monthly Notices of the Royal Astronomical Society
Herzberg Institute of Astrophysics (CA), University of Copenhagen (DK), Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), University of Wisconsin System (US), European Southern Observatory (DE), University of Wisconsin–Madison (US), University of Arizona (US), University of Hertfordshire (GB), Scuola Normale Superiore (IT), University of California, Santa Cruz (US), University of Cambridge (GB), Institut d'Astrophysique de Paris (FR), European Space Agency (FR), Sorbonne Université (FR), University of Oxford (GB), University of Chicago (US), Max Planck Institute for Extraterrestrial Physics (DE), European Space Astronomy Centre (ES), DAWN Center for Independent Living (US), Center for Astrophysics Harvard & Smithsonian (US), Centro de Astrobiología (ES), NSF NOIRLab (US), University College London (GB)
Openalex Percentile: Top 7%
Galaxies: Formation, Evolution, Phenomena
DOIOpen Access PDFOpenAlex
AI Navigator

Ask Laika to Summarize, Analyze, and Connect papers live on the map.

Summarize Papers & Methodologies

Extract key findings, datasets, and comparative methods across publications.

Benchmark Rankings & Visual Analytics

Rank top research institutions, authors, funders, topics, and journals by Field-Weighted Citation Impact (FWCI) and paper volume with instant charts.

Connect Distant Disciplines

Bridge topological clusters on the map to find hidden collaborative intersections.