Recognition: unknown
The Missing Hard Photons of Little Red Dots: Their Incident Ionizing Spectra Resemble Massive Stars
read the original abstract
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 H$\alpha$ and HeII$\,\lambda$4686. The high H$\alpha$ EWs require copious H-ionizing photons, more than the bluest AGN ionizing spectra can provide. In contrast, HeII emission is marginally detected, and its low EW is, at most, consistent with the softest AGN spectra. The low HeII/H$\beta$ ($\sim10^{-2}$, $<20\times$ 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 HeII 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/or 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 HeII-based diagnostic proposed here represents a crucial step toward understanding the central engine of LRDs.
This paper has not been read by Pith yet.
Forward citations
Cited by 3 Pith papers
-
The GlimmIr: Spectroscopic Variability in a z~7 LRD Indicates Rapid Changes in Both the Narrow and Broad Line Regions
First spectroscopic variability in a z~7 LRD shows rapid changes in both narrow and broad line regions, implying direct ionization from the central source to surrounding nebular gas.
-
A new sample of Little Red Dots at $z<0.45$ in DESI DR1: Broad Balmer lines, low ionization spectrum and no variability
Eight low-redshift Little Red Dots identified in DESI DR1 exhibit broad Balmer lines, steep decrements, compact shapes, and negligible variability, with a number density roughly 10,000 times lower than at z>4.
-
GLIMPSED: Direct evidence for a fast AGN-driven outflow from a z=6.64 Little Red Dot host galaxy
A z=6.64 LRD host galaxy exhibits a fast AGN-driven outflow with 5500 km/s velocities, dusty gas, and low metallicity, confirming AGN presence in these systems.
Reference graph
Works this paper leans on
-
[1]
Akins, H. B., Casey, C. M., Lambrides, E., et al. 2025a, ApJ, 991, 37, doi: 10.3847/1538-4357/ade984
-
[2]
Akins, H. B., Casey, C. M., Berg, D. A., et al. 2025b, ApJL, 980, L29, doi: 10.3847/2041-8213/adab76
-
[3]
Akins, H. B., Casey, C. M., Chisholm, J., et al. 2026, ApJ, 997, 218, doi: 10.3847/1538-4357/ae2c77
-
[4]
X-Ray View of Little Red Dots: Do They Host Supermassive Black Holes?
Ananna, T. T., Bogd´ an,´A., Kov´ acs, O. E., Natarajan, P., & Hickox, R. C. 2024, ApJL, 969, L18, doi: 10.3847/2041-8213/ad5669
-
[5]
2016, MNRAS, 456, 1424, doi: 10.1093/mnras/stv2758
Aret, A., Kraus, M., & ˇSlechta, M. 2016, MNRAS, 456, 1424, doi: 10.1093/mnras/stv2758
-
[6]
Asada, Y., Inayoshi, K., Fei, Q., Fujimoto, S., & Willott, C. 2026, arXiv e-prints, arXiv:2601.10573, doi: 10.48550/arXiv.2601.10573 Astropy Collaboration, Robitaille, T. P., Tollerud, E. J., et al. 2013, A&A, 558, A33, doi: 10.1051/0004-6361/201322068 Missing Ionizing Photons of LRDs25 T able A.1.Notable LRDs IDz spec JWST PID Reference for spectral mode...
work page internal anchor Pith review doi:10.48550/arxiv.2601.10573 2026
-
[7]
Baggen, J. F. W., van Dokkum, P., Brammer, G., et al. 2024, ApJL, 977, L13, doi: 10.3847/2041-8213/ad90b8
-
[8]
Baldwin, J. A., Phillips, M. M., & Terlevich, R. 1981, PASP, 93, 5, doi: 10.1086/130766
-
[9]
L., et al., 1999, @doi [ ] 10.1086/308056 , 527, 54
Balogh, M. L., Morris, S. L., Yee, H. K. C., Carlberg, R. G., & Ellingson, E. 1999, ApJ, 527, 54, doi: 10.1086/308056 B¨ ar, R. E., Weigel, A. K., Sartori, L. F., et al. 2017, MNRAS, 466, 2879, doi: 10.1093/mnras/stw3283
-
[10]
2010, ApJ, 708, 834, doi: 10.1088/0004-637X/708/1/834
Bartko, H., Martins, F., Trippe, S., et al. 2010, ApJ, 708, 834, doi: 10.1088/0004-637X/708/1/834
-
[11]
arXiv , author =:2507.09085 , journal =
Begelman, M. C., & Dexter, J. 2026, ApJ, 996, 48, doi: 10.3847/1538-4357/ae274a 28Wang et al
-
[12]
Begelman, M. C., Rossi, E. M., & Armitage, P. J. 2008, MNRAS, 387, 1649, doi: 10.1111/j.1365-2966.2008.13344.x
-
[13]
2025, ApJL, 984, L55, doi: 10.3847/2041-8213/adce6c
Bellovary, J. 2025, ApJL, 984, L55, doi: 10.3847/2041-8213/adce6c
-
[14]
Boylan-Kolchin, M. 2023, Nature Astronomy, 7, 731, doi: 10.1038/s41550-023-01937-7
-
[15]
2023, msaexp: NIRSpec analyis tools , 0.6.17, Zenodo, 10.5281/zenodo.7299500
Brammer, G. 2023,, 0.6.17 Zenodo, doi: 10.5281/zenodo.7299500
-
[16]
Brazzini, M., D’Eugenio, F., Maiolino, R., et al. 2025, MNRAS, 544, L167, doi: 10.1093/mnrasl/slaf116
-
[17]
2003, ApJ, 596, 34, doi: 10.1086/377529
Bromm, V., & Loeb, A. 2003, ApJ, 596, 34, doi: 10.1086/377529
-
[18]
2003, MNRAS, 339, 937, doi: 10.1046/j.1365-8711.2003.06241.x G¨ otberg, Y., de Mink, S
Bruzual, G., & Charlot, S. 2003, MNRAS, 344, 1000, doi: 10.1046/j.1365-8711.2003.06897.x Bruzual A., G. 1983, ApJ, 273, 105, doi: 10.1086/161352
-
[19]
Busche, J. R., & Hillier, D. J. 2005, AJ, 129, 454, doi: 10.1086/426362
-
[20]
Cameron, A. J., Saxena, A., Bunker, A. J., et al. 2023, A&A, 677, A115, doi: 10.1051/0004-6361/202346107
-
[21]
Casey, C. M., Akins, H. B., Finkelstein, S. L., et al. 2025, ApJL, 990, L61, doi: 10.3847/2041-8213/adfa91
-
[22]
2025, MNRAS, 542, 2597, doi: 10.1093/mnras/staf1362
Cenci, E., & Habouzit, M. 2025, MNRAS, 542, 2597, doi: 10.1093/mnras/staf1362
-
[23]
Chabrier, G. 2003, PASP, 115, 763, doi: 10.1086/376392
-
[24]
Chang, S.-J., Gronke, M., Matthee, J., & Mason, C. 2026, MNRAS, 545, staf2131, doi: 10.1093/mnras/staf2131
-
[25]
2023, RMxAA, 59, 327, doi: 10.22201/ia.01851101p.2023.59.02.12 9
Chatzikos, M., Bianchi, S., Camilloni, F., et al. 2023, RMxAA, 59, 327, doi: 10.22201/ia.01851101p.2023.59.02.12
-
[26]
The Host Galaxy (If Any) of the Little Red Dots
Chen, C.-H., Ho, L. C., Li, R., & Zhuang, M.-Y. 2025, ApJ, 983, 60, doi: 10.3847/1538-4357/ada93a
-
[27]
2017, ApJ, 838, 159, doi: 10.3847/1538-4357/aa679f
Choi, J., Conroy, C., & Byler, N. 2017, ApJ, 838, 159, doi: 10.3847/1538-4357/aa679f
-
[28]
2016, ApJ, 823, 102, doi: 10.3847/0004-637X/823/2/102
Choi, J., Dotter, A., Conroy, C., et al. 2016, ApJ, 823, 102, doi: 10.3847/0004-637X/823/2/102
work page internal anchor Pith review doi:10.3847/0004-637x/823/2/102 2016
-
[29]
Cleri, N. J., Olivier, G. M., Backhaus, B. E., et al. 2025, ApJ, 994, 146, doi: 10.3847/1538-4357/ae0f17
-
[30]
Conroy, C., & Gunn, J. E. 2010, ApJ, 712, 833, doi: 10.1088/0004-637X/712/2/833
-
[31]
Davis, S. W., & Laor, A. 2011, ApJ, 728, 98, doi: 10.1088/0004-637X/728/2/98 de Graaff, A., Rix, H.-W., Carniani, S., et al. 2024, A&A, 684, A87, doi: 10.1051/0004-6361/202347755 de Graaff, A., Rix, H.-W., Naidu, R. P., et al. 2025a, A&A, 701, A168, doi: 10.1051/0004-6361/202554681 de Graaff, A., Brammer, G., Weibel, A., et al. 2025b, A&A, 697, A189, doi:...
-
[32]
2016, ApJS, 222, 8, doi: 10.3847/0067-0049/222/1/8 Ekstr¨ om, S., Georgy, C., Eggenberger, P., et al
Dotter, A. 2016, ApJS, 222, 8, doi: 10.3847/0067-0049/222/1/8
-
[33]
Eldridge, J. J., Stanway, E. R., Xiao, L., et al. 2017, PASA, 34, e058, doi: 10.1017/pasa.2017.51
-
[34]
2016, MNRAS, 456, 3354, doi: 10.1093/mnras/stv2794
Feltre, A., Charlot, S., & Gutkin, J. 2016, MNRAS, 456, 3354, doi: 10.1093/mnras/stv2794
-
[35]
Ferland, G. J. 2006, Hazy, A Brief Introduction to Cloudy 06.02
work page 2006
-
[36]
J., Done, C., Jin, C., Landt, H., & Ward, M
Ferland, G. J., Done, C., Jin, C., Landt, H., & Ward, M. J. 2020, MNRAS, 494, 5917, doi: 10.1093/mnras/staa1207
-
[37]
Ferland, G. J., & Netzer, H. 1983, ApJ, 264, 105, doi: 10.1086/160577
-
[38]
Ferland, G. J., Chatzikos, M., Guzm´ an, F., et al. 2017, RMxAA, 53, 385, doi: 10.48550/arXiv.1705.10877
-
[39]
Ferrara, A., Salvadori, S., Yue, B., & Schleicher, D. 2014, MNRAS, 443, 2410, doi: 10.1093/mnras/stu1280
-
[40]
Furtak, L. J., Labb´ e, I., Zitrin, A., et al. 2024, Nature, 628, 57, doi: 10.1038/s41586-024-07184-8
-
[41]
Furtak, L. J., Secunda, A. R., Greene, J. E., et al. 2025, A&A, 698, A227, doi: 10.1051/0004-6361/202554110
-
[42]
Galavis, M. E., Mendoza, C., & Zeippen, C. J. 1997, A&AS, 123, 159, doi: 10.1051/aas:1997344
-
[43]
2010, Reviews of Modern Physics, 82, 3121, doi: 10.1103/RevModPhys.82.3121
Genzel, R., Eisenhauer, F., & Gillessen, S. 2010, Reviews of Modern Physics, 82, 3121, doi: 10.1103/RevModPhys.82.3121
-
[44]
2019, MNRAS, 482, 321, doi: 10.1093/mnras/sty2514
Glatzle, M., Ciardi, B., & Graziani, L. 2019, MNRAS, 482, 321, doi: 10.1093/mnras/sty2514
-
[45]
Glikman, E., Helfand, D. J., & White, R. L. 2006, ApJ, 640, 579, doi: 10.1086/500098 Gonz´ alez-D´ ıaz, R., V´ ılchez, J. M., Kehrig, C., del
-
[46]
2025, A&A, 702, A3, doi: 10.1051/0004-6361/202555935
Moral-Castro, I., & Iglesias-P´ aramo, J. 2025, A&A, 702, A3, doi: 10.1051/0004-6361/202555935
-
[47]
2003, MNRAS, 339, 937, doi: 10.1046/j.1365-8711.2003.06241.x G¨ otberg, Y., de Mink, S
Goodman, J. 2003, MNRAS, 339, 937, doi: 10.1046/j.1365-8711.2003.06241.x G¨ otberg, Y., de Mink, S. E., Groh, J. H., et al. 2018, A&A, 615, A78, doi: 10.1051/0004-6361/201732274
-
[48]
Goulding, A. D., Greene, J. E., Setton, D. J., et al. 2023, ApJL, 955, L24, doi: 10.3847/2041-8213/acf7c5
-
[49]
Estimating Black Hole Masses in Active Galaxies Using the Halpha Emission Line
Greene, J. E., & Ho, L. C. 2005, ApJ, 630, 122, doi: 10.1086/431897
-
[50]
Greene, J. E., Labbe, I., Goulding, A. D., et al. 2024, ApJ, 964, 39, doi: 10.3847/1538-4357/ad1e5f
-
[51]
Groves, B. A., Heckman, T. M., & Kauffmann, G. 2006, MNRAS, 371, 1559, doi: 10.1111/j.1365-2966.2006.10812.x
-
[52]
Hamann, W. R., & Gr¨ afener, G. 2003, A&A, 410, 993, doi: 10.1051/0004-6361:20031308
-
[53]
1985, ApJ, 297, 371, doi: 10.1086/163537 Missing Ionizing Photons of LRDs29
Hamilton, D. 1985, ApJ, 297, 371, doi: 10.1086/163537 Missing Ionizing Photons of LRDs29
-
[54]
Harris, C. R., Millman, K. J., van der Walt, S. J., et al. 2020, Nature, 585, 357, doi: 10.1038/s41586-020-2649-2
-
[55]
Hillier, D. J., & Miller, D. L. 1998, ApJ, 496, 407, doi: 10.1086/305350
-
[56]
Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results
Hinshaw, G., Larson, D., Komatsu, E., et al. 2013, ApJS, 208, 19, doi: 10.1088/0067-0049/208/2/19
-
[57]
1995, ApJ, 439, 875, doi: 10.1086/175226
Hubeny, I., & Lanz, T. 1995, ApJ, 439, 875, doi: 10.1086/175226
-
[58]
Hunter, J. D. 2007, Computing in Science and Engineering, 9, 90, doi: 10.1109/MCSE.2007.55
-
[59]
Hviding, R. E., de Graaff, A., Miller, T. B., et al. 2025, A&A, 702, A57, doi: 10.1051/0004-6361/202555816
-
[60]
Inayoshi, K., Kimura, S. S., & Noda, H. 2025a, PASJ, 77, 811, doi: 10.1093/pasj/psaf050
-
[61]
Inayoshi, K., & Maiolino, R. 2025, ApJL, 980, L27, doi: 10.3847/2041-8213/adaebd
-
[62]
The Emergence of Little Red Dots from Binary Massive Black Holes
Haiman, Z. 2025b, arXiv e-prints, arXiv:2505.05322, doi: 10.48550/arXiv.2505.05322
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.2505.05322
-
[63]
2020, ARA&A, 58, 27, doi: 10.1146/annurev-astro-120419-014455
Inayoshi, K., Visbal, E., & Haiman, Z. 2020, ARA&A, 58, 27, doi: 10.1146/annurev-astro-120419-014455
-
[64]
Jeon, J., Liu, B., Bromm, V., et al. 2026, ApJ, 998, 148, doi: 10.3847/1538-4357/ae3725
-
[65]
Ji, X., Maiolino, R., ¨Ubler, H., et al. 2025, MNRAS, 544, 3900, doi: 10.1093/mnras/staf1867
-
[66]
2026, MNRAS, 545, staf2235, doi: 10.1093/mnras/staf2235
Ji, X., D’Eugenio, F., Juodˇ zbalis, I., et al. 2026, MNRAS, 545, staf2235, doi: 10.1093/mnras/staf2235
-
[67]
2012, MNRAS, 420, 1825, doi: 10.1111/j.1365-2966.2011.19805.x
Jin, C., Ward, M., Done, C., & Gelbord, J. 2012, MNRAS, 420, 1825, doi: 10.1111/j.1365-2966.2011.19805.x
-
[68]
Johnson, J. L., Whalen, D. J., Fryer, C. L., & Li, H. 2012, ApJ, 750, 66, doi: 10.1088/0004-637X/750/1/66 Juodˇ zbalis, I., Maiolino, R., Baker, W. M., et al. 2026, MNRAS, 546, stag086, doi: 10.1093/mnras/stag086
-
[69]
Kehrig, C., V´ ılchez, J. M., Guerrero, M. A., et al. 2018, MNRAS, 480, 1081, doi: 10.1093/mnras/sty1920
-
[70]
Kewley, L. J., Nicholls, D. C., & Sutherland, R. S. 2019, ARA&A, 57, 511, doi: 10.1146/annurev-astro-081817-051832
-
[71]
Kido, D., Ioka, K., Hotokezaka, K., Inayoshi, K., & Irwin, C. M. 2025, MNRAS, 544, 3407, doi: 10.1093/mnras/staf1898
-
[72]
Killi, M., Watson, D., Brammer, G., et al. 2024, A&A, 691, A52, doi: 10.1051/0004-6361/202348857
-
[73]
Kocevski, D. D., Finkelstein, S. L., Barro, G., et al. 2025, ApJ, 986, 126, doi: 10.3847/1538-4357/adbc7d
-
[74]
Kokorev, V., Chisholm, J., Naidu, R. P., et al. 2025, arXiv e-prints, arXiv:2511.07515, doi: 10.48550/arXiv.2511.07515 Kovaˇ cevi´ c-Dojˇ cinovi´ c, J., Dojˇ cinovi´ c, I., Laki´ cevi´ c, M., & Popovi´ c, L.ˇC. 2025, A&A, 694, A289, doi: 10.1051/0004-6361/202450516 Labb´ e, I., van Dokkum, P., Nelson, E., et al. 2023, Nature, 616, 266, doi: 10.1038/s41586...
-
[75]
Labbe, I., Greene, J. E., Matthee, J., et al. 2024, arXiv e-prints, arXiv:2412.04557, doi: 10.48550/arXiv.2412.04557
-
[76]
The case for super-Eddington accretion in JWST broad-line AGN during the first billion years
Lambrides, E., Garofali, K., Larson, R., et al. 2024, arXiv e-prints, arXiv:2409.13047, doi: 10.48550/arXiv.2409.13047
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.2409.13047 2024
-
[77]
Lanz, T., & Hubeny, I. 2003, ApJS, 146, 417, doi: 10.1086/374373
-
[78]
Laor, A., & Davis, S. W. 2014, MNRAS, 438, 3024, doi: 10.1093/mnras/stt2408
-
[79]
Leitherer, C., Schaerer, D., Goldader, J. D., et al. 1999, ApJS, 123, 3, doi: 10.1086/313233
-
[80]
Li, Y., Leja, J., Johnson, B. D., et al. 2025, ApJ, 986, 9, doi: 10.3847/1538-4357/adcab4
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.