21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations
We study, from both a theoretical and observational perspective, the physical origin and spectroscopic 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 ha...
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| Language: | English |
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Maynooth Academic Publishing
2025-07-01
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| Series: | The Open Journal of Astrophysics |
| Online Access: | https://doi.org/10.33232/001c.142570 |
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| author | Harley Katz Alex J. Cameron Aayush Saxena Laia Barrufet Nichloas Choustikov Nikko J. Cleri Anna de Graff Richard S. Ellis Robert A.E. Fosbury Kasper E. Heintz Michael Maseda Jorryt Matthee Ian McConachie Pascal A. Oesch |
| author_facet | Harley Katz Alex J. Cameron Aayush Saxena Laia Barrufet Nichloas Choustikov Nikko J. Cleri Anna de Graff Richard S. Ellis Robert A.E. Fosbury Kasper E. Heintz Michael Maseda Jorryt Matthee Ian McConachie Pascal A. Oesch |
| author_sort | Harley Katz |
| collection | DOAJ |
| description | We study, from both a theoretical and observational perspective, the physical origin and spectroscopic 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 $z\gtrsim10$, 2) at high gas densities, UV slopes can redden from $\beta\lesssim-2.5$ to $\beta\sim-1$, 3) observational measurements of $\xi_{\rm ion}$ are \textcolor{red}{gross underestimates}, and 4) UV downturns from two-photon emission can masquerade as damped Ly$\alpha$ 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. |
| format | Article |
| id | doaj-art-e2d92740f05d42c9abc4fc51f5d57572 |
| institution | Kabale University |
| issn | 2565-6120 |
| language | English |
| publishDate | 2025-07-01 |
| publisher | Maynooth Academic Publishing |
| record_format | Article |
| series | The Open Journal of Astrophysics |
| spelling | doaj-art-e2d92740f05d42c9abc4fc51f5d575722025-08-20T03:55:49ZengMaynooth Academic PublishingThe Open Journal of Astrophysics2565-61202025-07-01810.33232/001c.14257021 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar PopulationsHarley KatzAlex J. CameronAayush SaxenaLaia BarrufetNichloas ChoustikovNikko J. CleriAnna de GraffRichard S. EllisRobert A.E. FosburyKasper E. HeintzMichael MasedaJorryt MattheeIan McConachiePascal A. OeschWe study, from both a theoretical and observational perspective, the physical origin and spectroscopic 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 $z\gtrsim10$, 2) at high gas densities, UV slopes can redden from $\beta\lesssim-2.5$ to $\beta\sim-1$, 3) observational measurements of $\xi_{\rm ion}$ are \textcolor{red}{gross underestimates}, and 4) UV downturns from two-photon emission can masquerade as damped Ly$\alpha$ 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.https://doi.org/10.33232/001c.142570 |
| spellingShingle | Harley Katz Alex J. Cameron Aayush Saxena Laia Barrufet Nichloas Choustikov Nikko J. Cleri Anna de Graff Richard S. Ellis Robert A.E. Fosbury Kasper E. Heintz Michael Maseda Jorryt Matthee Ian McConachie Pascal A. Oesch 21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations The Open Journal of Astrophysics |
| title | 21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations |
| title_full | 21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations |
| title_fullStr | 21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations |
| title_full_unstemmed | 21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations |
| title_short | 21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations |
| title_sort | 21 balmer jump street the nebular continuum at high redshift and implications for the bright galaxy problem uv continuum slopes and early stellar populations |
| url | https://doi.org/10.33232/001c.142570 |
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