The missing hard photons of Little Red Dots: Their incident ionizing spectra resemble massive stars

article The missing hard photons of Little Red Dots: Their incident ionizing spectra resemble massive stars published yes Bingjie Wang author Joel Leja author Harley Katz author Kohei Inayoshi author Nikko J. Cleri author Anna De Graaff author Raphael E. Hviding author Pieter Van Dokkum author Jenny E. Greene author Ivo Labbé author Jorryt J Matthee author 7439a258-f3c0-11ec-9501-9df22fe067200000-0003-2871-127X Ian Mcconachie author Rohan P. Naidu author Erica J. Nelson author JoMa department The nature of little red dots (LRDs) has largely been investigated through their continuum emission, with lines assumed to arise from a broad-line region. In this paper, we instead use recombination lines to infer the intrinsic properties of the central engine. Our analysis first reveals a tension between the ionizing properties implied from Hα and He ii λ4686. The high Hα EWs require copious H-ionizing photons, more than the bluest active galactic nucleus (AGN) ionizing spectra can provide. In contrast, He ii emission is marginally detected, and its low EW is, at most, consistent with the softest AGN spectra. The low He ii/Hβ (∼10−2, <20× local AGN median) further points to an unusually soft ionizing spectrum. We extend our analysis to dense gas envelopes (quasi-star/black-hole star) and find that hydrogen recombination lines become optically thick and lose diagnostic power, but He ii remains optically thin and a robust tracer. Photoionization modeling with Cloudy rules out standard AGN accretion disk spectra. Alternative explanations include exotic AGN with red rest-optical emission, high average optical depth (>10) from gas/dust, and soft ionizing spectra with abundant H-ionizing photons, consistent with, e.g., a cold accretion disk or a composite of AGN and stars. The latter is an intriguing scenario since high hydrogen densities are highly conducive for star formation, and nuclear star clusters are found in the vicinity of local massive black holes. While previous studies have mostly focused on features dominated by the absorbing hydrogen cloud, the He ii-based diagnostic proposed here represents a crucial step toward understanding the central engine of LRDs. https://research-explorer.ista.ac.at/download/21882/21891/2026_AstrophysicalJourn_Wang.pdf application/pdfno IOP Publishing2026 eng The Astrophysical Journal 0004-637X 1538-4357 2508.1835810.3847/1538-4357/ae5bab 10031 B. Wang et al., “The missing hard photons of Little Red Dots: Their incident ionizing spectra resemble massive stars,” The Astrophysical Journal, vol. 1003, no. 1. IOP Publishing, 2026. Wang, Bingjie, Joel Leja, Harley Katz, Kohei Inayoshi, Nikko J. Cleri, Anna De Graaff, Raphael E. Hviding, et al. “The Missing Hard Photons of Little Red Dots: Their Incident Ionizing Spectra Resemble Massive Stars.” The Astrophysical Journal. IOP Publishing, 2026. <a href="https://doi.org/10.3847/1538-4357/ae5bab">https://doi.org/10.3847/1538-4357/ae5bab</a>. Wang, Bingjie, et al. “The Missing Hard Photons of Little Red Dots: Their Incident Ionizing Spectra Resemble Massive Stars.” The Astrophysical Journal, vol. 1003, no. 1, 10, IOP Publishing, 2026, doi:<a href="https://doi.org/10.3847/1538-4357/ae5bab">10.3847/1538-4357/ae5bab</a>. Wang B, Leja J, Katz H, Inayoshi K, Cleri NJ, De Graaff A, Hviding RE, Van Dokkum P, Greene JE, Labbé I, Matthee JJ, Mcconachie I, Naidu RP, Nelson EJ. 2026. The missing hard photons of Little Red Dots: Their incident ionizing spectra resemble massive stars. The Astrophysical Journal. 1003(1), 10. Wang, B., Leja, J., Katz, H., Inayoshi, K., Cleri, N. J., De Graaff, A., … Nelson, E. J. (2026). The missing hard photons of Little Red Dots: Their incident ionizing spectra resemble massive stars. The Astrophysical Journal. IOP Publishing. <a href="https://doi.org/10.3847/1538-4357/ae5bab">https://doi.org/10.3847/1538-4357/ae5bab</a> Wang B, Leja J, Katz H, et al. The missing hard photons of Little Red Dots: Their incident ionizing spectra resemble massive stars. The Astrophysical Journal. 2026;1003(1). doi:<a href="https://doi.org/10.3847/1538-4357/ae5bab">10.3847/1538-4357/ae5bab</a> B. Wang, J. Leja, H. Katz, K. Inayoshi, N.J. Cleri, A. De Graaff, R.E. Hviding, P. Van Dokkum, J.E. Greene, I. Labbé, J.J. Matthee, I. Mcconachie, R.P. Naidu, E.J. Nelson, The Astrophysical Journal 1003 (2026). 218822026-05-17T22:02:10Z2026-05-18T08:18:39Z