{"abstract":[{"lang":"eng","text":"We study 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~mag (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 \\beta<-2.5 to \\beta\\sim-1, 3) observational measurements of \\xi_{\\rm ion} are 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."}],"article_processing_charge":"No","department":[{"_id":"JoMa"}],"external_id":{"arxiv":["2408.03189"]},"oa_version":"Published Version","publication":"The Open Journal of Astrophysics","year":"2025","publication_identifier":{"eissn":["2565-6120"]},"_id":"20192","file":[{"creator":"dernst","file_name":"2025_OpenJourAstrophysics_Katz.pdf","content_type":"application/pdf","relation":"main_file","checksum":"ba469d132907147f9e86d87f9124dd14","date_updated":"2025-09-30T14:28:25Z","file_id":"20412","success":1,"file_size":1836432,"date_created":"2025-09-30T14:28:25Z","access_level":"open_access"}],"citation":{"apa":"Katz, H., Cameron, A. J., Saxena, A., Barrufet, L., Choustikov, N., Cleri, N. J., … Oesch, P. A. (2025). 21 Balmer Jump Street: The nebular continuum at high redshift and implications for the bright galaxy problem, UV continuum slopes, and early stellar populations. <i>The Open Journal of Astrophysics</i>. Maynooth Academic Publishing. <a href=\"https://doi.org/10.33232/001c.142570\">https://doi.org/10.33232/001c.142570</a>","ieee":"H. Katz <i>et al.</i>, “21 Balmer Jump Street: The nebular continuum at high redshift and implications for the bright galaxy problem, UV continuum slopes, and early stellar populations,” <i>The Open Journal of Astrophysics</i>, vol. 8. Maynooth Academic Publishing, 2025.","ista":"Katz H, Cameron AJ, Saxena A, Barrufet L, Choustikov N, Cleri NJ, De Graaff A, Ellis RS, Fosbury RAE, Heintz KE, Maseda M, Matthee JJ, Mcconachie I, Oesch PA. 2025. 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. 8.","short":"H. Katz, A.J. Cameron, A. Saxena, L. Barrufet, N. Choustikov, N.J. Cleri, A. De Graaff, R.S. Ellis, R.A.E. Fosbury, K.E. Heintz, M. Maseda, J.J. Matthee, I. Mcconachie, P.A. Oesch, The Open Journal of Astrophysics 8 (2025).","ama":"Katz H, Cameron AJ, Saxena A, et al. 21 Balmer Jump Street: The nebular continuum at high redshift and implications for the bright galaxy problem, UV continuum slopes, and early stellar populations. <i>The Open Journal of Astrophysics</i>. 2025;8. doi:<a href=\"https://doi.org/10.33232/001c.142570\">10.33232/001c.142570</a>","mla":"Katz, Harley, et al. “21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations.” <i>The Open Journal of Astrophysics</i>, vol. 8, Maynooth Academic Publishing, 2025, doi:<a href=\"https://doi.org/10.33232/001c.142570\">10.33232/001c.142570</a>.","chicago":"Katz, Harley, Alex J. Cameron, Aayush Saxena, Laia Barrufet, Nicholas Choustikov, Nikko J. Cleri, Anna De Graaff, et al. “21 Balmer Jump Street: The Nebular Continuum at High Redshift and Implications for the Bright Galaxy Problem, UV Continuum Slopes, and Early Stellar Populations.” <i>The Open Journal of Astrophysics</i>. Maynooth Academic Publishing, 2025. <a href=\"https://doi.org/10.33232/001c.142570\">https://doi.org/10.33232/001c.142570</a>."},"language":[{"iso":"eng"}],"PlanS_conform":"1","intvolume":"         8","month":"07","ddc":["520"],"has_accepted_license":"1","OA_type":"diamond","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","type":"journal_article","arxiv":1,"file_date_updated":"2025-09-30T14:28:25Z","status":"public","tmp":{"legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","short":"CC BY (4.0)","image":"/images/cc_by.png","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"oa":1,"day":"25","date_updated":"2025-09-30T14:29:33Z","author":[{"full_name":"Katz, Harley","last_name":"Katz","first_name":"Harley"},{"last_name":"Cameron","first_name":"Alex J.","full_name":"Cameron, Alex J."},{"first_name":"Aayush","last_name":"Saxena","full_name":"Saxena, Aayush"},{"full_name":"Barrufet, Laia","last_name":"Barrufet","first_name":"Laia"},{"first_name":"Nicholas","last_name":"Choustikov","full_name":"Choustikov, Nicholas"},{"last_name":"Cleri","first_name":"Nikko J.","full_name":"Cleri, Nikko J."},{"last_name":"De Graaff","first_name":"Anna","full_name":"De Graaff, Anna"},{"last_name":"Ellis","first_name":"Richard S.","full_name":"Ellis, Richard S."},{"full_name":"Fosbury, Robert A.E.","first_name":"Robert A.E.","last_name":"Fosbury"},{"full_name":"Heintz, Kasper E.","first_name":"Kasper E.","last_name":"Heintz"},{"first_name":"Michael","last_name":"Maseda","full_name":"Maseda, Michael"},{"full_name":"Matthee, Jorryt J","id":"7439a258-f3c0-11ec-9501-9df22fe06720","orcid":"0000-0003-2871-127X","last_name":"Matthee","first_name":"Jorryt J"},{"full_name":"Mcconachie, Ian","first_name":"Ian","last_name":"Mcconachie"},{"full_name":"Oesch, Pascal A.","last_name":"Oesch","first_name":"Pascal A."}],"volume":8,"OA_place":"publisher","date_created":"2025-08-17T22:01:37Z","acknowledgement":"HK thanks Andrey Kravtsov for insightful comments and thoughtful discussions. We sincerely thank the PIs and Co-Is of the JWST programs where spectral data was made publicly available on the DJA. We refer interested readers to the following papers for survey descriptions regarding the spectral data: Bunker et al. (2023a); D’Eugenio et al. (2024); Bezanson et al. (2022); Barrufet et al. (2024); de Graaff et al. (2024); Finkelstein et al. (2024); Glazebrook et al. (2024); Pierel et al. (2024); Siebert et al. (2024); Maseda et al. (2024). This work is based in part on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with programs listed in Table 1. AJC and AS acknowledge funding from the “FirstGalaxies” Advanced Grant from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No. 789056). ","quality_controlled":"1","article_type":"original","date_published":"2025-07-25T00:00:00Z","scopus_import":"1","user_id":"2DF688A6-F248-11E8-B48F-1D18A9856A87","publication_status":"published","publisher":"Maynooth Academic Publishing","doi":"10.33232/001c.142570"}