---
DOAJ_listed: '1'
OA_place: publisher
OA_type: gold
PlanS_conform: '1'
_id: '21882'
abstract:
- lang: eng
text: 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.
acknowledgement: "B.W. thanks Michael Eracleous for valuable discussions. B.W. and
J.L. acknowledge support from JWST-GO-04233.009. B.W. also acknowledges support
provided by NASA through Hubble Fellowship grant HST-HF2-51592.001 awarded by the
Space Telescope Science Institute, which is operated by the Association of Universities
for Research in Astronomy, Inc., for NASA, under the contract NAS 5-26555. K.I.
acknowledges support from the National Natural Science Foundation of China (12573015,
W2532003), the Beijing Natural Science Foundation (IS25003), and the China Manned
Space Program (CMS-CSST-2025-A09). R.E.H. acknowledges support by the German Aerospace
Center (DLR) and the Federal Ministry for Economic Affairs and Energy (BMWi) through
program 50OR2403 “RUBIES.”\r\n\r\nThis 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 program
# 1433, 2561, 4106, 4233, 5224, 6585. The specific observations analyzed can be
accessed via DOI: 10.17909/9hpc-nc45. Computations for this research were performed
on the Pennsylvania State University’s Institute for Computational and Data Sciences’
Roar supercomputer; and on computational resources managed and supported by Princeton
Research Computing, a consortium of groups including the Princeton Institute for
Computational Science and Engineering (PICSciE) and Research Computing at Princeton
University. Some of the stellar spectra are retrieved from the POLLUX database (pollux.oreme.org)
operated at LUPM (Université de Montpellier—CNRS, France) with the support of the
PNPS and INSU. This publication made use of the NASA Astrophysical Data System for
bibliographic information."
article_number: '10'
article_processing_charge: Yes
article_type: original
arxiv: 1
author:
- first_name: Bingjie
full_name: Wang, Bingjie
last_name: Wang
- first_name: Joel
full_name: Leja, Joel
last_name: Leja
- first_name: Harley
full_name: Katz, Harley
last_name: Katz
- first_name: Kohei
full_name: Inayoshi, Kohei
last_name: Inayoshi
- first_name: Nikko J.
full_name: Cleri, Nikko J.
last_name: Cleri
- first_name: Anna
full_name: De Graaff, Anna
last_name: De Graaff
- first_name: Raphael E.
full_name: Hviding, Raphael E.
last_name: Hviding
- first_name: Pieter
full_name: Van Dokkum, Pieter
last_name: Van Dokkum
- first_name: Jenny E.
full_name: Greene, Jenny E.
last_name: Greene
- first_name: Ivo
full_name: Labbé, Ivo
last_name: Labbé
- first_name: Jorryt J
full_name: Matthee, Jorryt J
id: 7439a258-f3c0-11ec-9501-9df22fe06720
last_name: Matthee
orcid: 0000-0003-2871-127X
- first_name: Ian
full_name: Mcconachie, Ian
last_name: Mcconachie
- first_name: Rohan P.
full_name: Naidu, Rohan P.
last_name: Naidu
- first_name: Erica J.
full_name: Nelson, Erica J.
last_name: Nelson
citation:
ama: '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:10.3847/1538-4357/ae5bab'
apa: '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. https://doi.org/10.3847/1538-4357/ae5bab'
chicago: '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. https://doi.org/10.3847/1538-4357/ae5bab.'
ieee: '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.'
ista: '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.'
mla: '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:10.3847/1538-4357/ae5bab.'
short: 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).
date_created: 2026-05-17T22:02:10Z
date_published: 2026-05-01T00:00:00Z
date_updated: 2026-05-18T08:18:39Z
day: '01'
ddc:
- '520'
department:
- _id: JoMa
doi: 10.3847/1538-4357/ae5bab
external_id:
arxiv:
- '2508.18358'
file:
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has_accepted_license: '1'
intvolume: ' 1003'
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language:
- iso: eng
month: '05'
oa: 1
oa_version: Published Version
publication: The Astrophysical Journal
publication_identifier:
eissn:
- 1538-4357
issn:
- 0004-637X
publication_status: published
publisher: IOP Publishing
quality_controlled: '1'
scopus_import: '1'
status: public
title: 'The missing hard photons of Little Red Dots: Their incident ionizing spectra
resemble massive stars'
tmp:
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legal_code_url: https://creativecommons.org/licenses/by/4.0/legalcode
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