{"type":"journal_article","oa_version":"Published Version","file":[{"relation":"main_file","date_updated":"2025-09-29T06:59:14Z","checksum":"cf93d635121dbf4865fd080c517927d0","content_type":"application/pdf","file_id":"20409","file_name":"2025_AstronomyAstrophysics_deGraaff2.pdf","access_level":"open_access","creator":"dernst","file_size":1218479,"success":1,"date_created":"2025-09-29T06:59:14Z"}],"language":[{"iso":"eng"}],"article_processing_charge":"No","article_number":"A168","scopus_import":"1","title":"A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5","OA_place":"publisher","date_updated":"2025-09-30T14:43:31Z","oa":1,"quality_controlled":"1","tmp":{"short":"CC BY (4.0)","image":"/images/cc_by.png","legal_code_url":"https://creativecommons.org/licenses/by/4.0/legalcode","name":"Creative Commons Attribution 4.0 International Public License (CC-BY 4.0)"},"month":"09","doi":"10.1051/0004-6361/202554681","OA_type":"diamond","abstract":[{"lang":"eng","text":"The origin of the rest-optical emission of compact, red, high-redshift sources known as little red dots (LRDs) poses a major puzzle. If interpreted as starlight, it would imply that LRDs constitute the densest stellar systems in the Universe. However, alternative models suggest active galactic nuclei (AGN) may instead power the rest-optical continuum. Here, we present JWST/NIRSpec, NIRCam, and MIRI observations from the RUBIES and PRIMER programs of The Cliff: a bright LRD at z = 3.55 with an exceptional Balmer break, twice as strong as that of any high-redshift source previously observed. The spectra also reveal broad hydrogen (Hα FWHM ∼ 1500 km s−1) and He I emission, but no significant metal lines. We demonstrate that massive evolved stellar populations cannot explain the observed spectrum, even when considering unusually steep and strong dust attenuation or reasonable variations in the initial mass function. Moreover, the formally best-fit stellar mass and compact size (M* ∼ 1010.5 M⊙,  re ∼ 40 pc) would imply densities at which near-monthly stellar collisions might lead to significant X-ray emission. We argue that the Balmer break, emission lines, and Hα absorption line are instead most plausibly explained by a black hole star (BH*) scenario, in which dense gas surrounds a powerful ionising source. In contrast to recently proposed BH* models of dust-reddened AGN, we show that spectral fits in the rest UV to near-infrared favour an intrinsically redder continuum over strong dust reddening. This may point to a super-Eddington accreting massive black hole or, possibly, the presence of (super)massive stars in a nuclear star cluster. The Cliff is the clearest evidence to date that at least some LRDs are not ultra-dense massive galaxies, and are instead powered by a central ionising source embedded in dense, absorbing gas."}],"date_created":"2025-09-28T22:01:27Z","author":[{"first_name":"Anna","full_name":"De Graaff, Anna","last_name":"De Graaff"},{"first_name":"Hans Walter","full_name":"Rix, Hans Walter","last_name":"Rix"},{"first_name":"Rohan P.","last_name":"Naidu","full_name":"Naidu, Rohan P."},{"last_name":"Labbé","full_name":"Labbé, Ivo","first_name":"Ivo"},{"full_name":"Wang, Bingjie","last_name":"Wang","first_name":"Bingjie"},{"last_name":"Leja","full_name":"Leja, Joel","first_name":"Joel"},{"first_name":"Jorryt J","id":"7439a258-f3c0-11ec-9501-9df22fe06720","full_name":"Matthee, Jorryt J","last_name":"Matthee","orcid":"0000-0003-2871-127X"},{"last_name":"Katz","full_name":"Katz, Harley","first_name":"Harley"},{"first_name":"Jenny E.","last_name":"Greene","full_name":"Greene, Jenny E."},{"full_name":"Hviding, Raphael E.","last_name":"Hviding","first_name":"Raphael E."},{"full_name":"Baggen, Josephine","last_name":"Baggen","first_name":"Josephine"},{"last_name":"Bezanson","full_name":"Bezanson, Rachel","first_name":"Rachel"},{"last_name":"Boogaard","full_name":"Boogaard, Leindert A.","first_name":"Leindert A."},{"last_name":"Brammer","full_name":"Brammer, Gabriel","first_name":"Gabriel"},{"last_name":"Dayal","full_name":"Dayal, Pratika","first_name":"Pratika"},{"first_name":"Pieter","last_name":"Van Dokkum","full_name":"Van Dokkum, Pieter"},{"full_name":"Goulding, Andy D.","last_name":"Goulding","first_name":"Andy D."},{"first_name":"Michaela","last_name":"Hirschmann","full_name":"Hirschmann, Michaela"},{"last_name":"Maseda","full_name":"Maseda, Michael V.","first_name":"Michael V."},{"full_name":"Mcconachie, Ian","last_name":"Mcconachie","first_name":"Ian"},{"last_name":"Miller","full_name":"Miller, Tim B.","first_name":"Tim B."},{"last_name":"Nelson","full_name":"Nelson, Erica","first_name":"Erica"},{"full_name":"Oesch, Pascal A.","last_name":"Oesch","first_name":"Pascal A."},{"full_name":"Setton, David J.","last_name":"Setton","first_name":"David J."},{"first_name":"Irene","full_name":"Shivaei, Irene","last_name":"Shivaei"},{"last_name":"Weibel","full_name":"Weibel, Andrea","first_name":"Andrea"},{"first_name":"Katherine E.","full_name":"Whitaker, Katherine E.","last_name":"Whitaker"},{"first_name":"Christina C.","full_name":"Williams, Christina C.","last_name":"Williams"}],"publisher":"EDP Sciences","user_id":"317138e5-6ab7-11ef-aa6d-ffef3953e345","PlanS_conform":"1","has_accepted_license":"1","external_id":{"arxiv":["2503.16600"],"isi":["001570450900004"]},"year":"2025","status":"public","ddc":["520"],"date_published":"2025-09-01T00:00:00Z","arxiv":1,"department":[{"_id":"JoMa"}],"article_type":"original","_id":"20406","file_date_updated":"2025-09-29T06:59:14Z","publication":"Astronomy and Astrophysics","volume":701,"publication_status":"published","day":"01","isi":1,"publication_identifier":{"eissn":["1432-0746"],"issn":["0004-6361"]},"intvolume":"       701","citation":{"ieee":"A. De Graaff <i>et al.</i>, “A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5,” <i>Astronomy and Astrophysics</i>, vol. 701. EDP Sciences, 2025.","mla":"De Graaff, Anna, et al. “A Remarkable Ruby: Absorption in Dense Gas, Rather than Evolved Stars, Drives the Extreme Balmer Break of a Little Red Dot at z = 3.5.” <i>Astronomy and Astrophysics</i>, vol. 701, A168, EDP Sciences, 2025, doi:<a href=\"https://doi.org/10.1051/0004-6361/202554681\">10.1051/0004-6361/202554681</a>.","apa":"De Graaff, A., Rix, H. W., Naidu, R. P., Labbé, I., Wang, B., Leja, J., … Williams, C. C. (2025). A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5. <i>Astronomy and Astrophysics</i>. EDP Sciences. <a href=\"https://doi.org/10.1051/0004-6361/202554681\">https://doi.org/10.1051/0004-6361/202554681</a>","chicago":"De Graaff, Anna, Hans Walter Rix, Rohan P. Naidu, Ivo Labbé, Bingjie Wang, Joel Leja, Jorryt J Matthee, et al. “A Remarkable Ruby: Absorption in Dense Gas, Rather than Evolved Stars, Drives the Extreme Balmer Break of a Little Red Dot at z = 3.5.” <i>Astronomy and Astrophysics</i>. EDP Sciences, 2025. <a href=\"https://doi.org/10.1051/0004-6361/202554681\">https://doi.org/10.1051/0004-6361/202554681</a>.","short":"A. De Graaff, H.W. Rix, R.P. Naidu, I. Labbé, B. Wang, J. Leja, J.J. Matthee, H. Katz, J.E. Greene, R.E. Hviding, J. Baggen, R. Bezanson, L.A. Boogaard, G. Brammer, P. Dayal, P. Van Dokkum, A.D. Goulding, M. Hirschmann, M.V. Maseda, I. Mcconachie, T.B. Miller, E. Nelson, P.A. Oesch, D.J. Setton, I. Shivaei, A. Weibel, K.E. Whitaker, C.C. Williams, Astronomy and Astrophysics 701 (2025).","ista":"De Graaff A, Rix HW, Naidu RP, Labbé I, Wang B, Leja J, Matthee JJ, Katz H, Greene JE, Hviding RE, Baggen J, Bezanson R, Boogaard LA, Brammer G, Dayal P, Van Dokkum P, Goulding AD, Hirschmann M, Maseda MV, Mcconachie I, Miller TB, Nelson E, Oesch PA, Setton DJ, Shivaei I, Weibel A, Whitaker KE, Williams CC. 2025. A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5. Astronomy and Astrophysics. 701, A168.","ama":"De Graaff A, Rix HW, Naidu RP, et al. A remarkable ruby: Absorption in dense gas, rather than evolved stars, drives the extreme Balmer break of a little red dot at z = 3.5. <i>Astronomy and Astrophysics</i>. 2025;701. doi:<a href=\"https://doi.org/10.1051/0004-6361/202554681\">10.1051/0004-6361/202554681</a>"},"acknowledgement":"We thank the PRIMER team for making their imaging data publicly available immediately. We thank Jaime Villaseñor and Friedrich Röpke for helpful discussions. This work is based 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 #1837 and #4233. Support for program #4233 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127. REH acknowledges support by the German Aerospace Center (DLR) and the Federal Ministry for Economic Affairs and Energy (BMWi) through program 50OR2403 ‘RUBIES’. This research was supported by the International Space Science Institute (ISSI) in Bern, through ISSI International Team project #562. The Cosmic Dawn Center is funded by the Danish National Research Foundation (DNRF) under grant #140. This work has received funding from the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract number MB22.00072, as well as from the Swiss National Science Foundation (SNSF) through project grant 200020_207349. Support for this work for RPN was provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51515.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. TBM was supported by a CIERA fellowship. Open Access funding provided by Max Planck Society."}