Paper

JWST Reveals Two Overmassive Black Hole Candidates in Dwarf Galaxies at z \\(\\) 0.7: Pushing Black Hole Searches into the Dwarf-Galaxy Regime

Noirot, G, Kramarenko, I, Cooper, R, Rinaldi, P, Hayes, M J, Colina, L, Langeroodi, D, Rieke, G H, Rodighiero, G, Torralba, A, Matthee, J, Iani, E, Alonso-Herrero, A, Willott, C, Yang, G, Zhu, Y, Catone, M, Muzzin, A, Lyu, J, Navarro-Carrera, R, Sun, F, Sawicki, M, Jermann, I, Annunziatella, M, Costantin, L, Melinder, J, Östlin, G, Z Wu, Pacucci, F, Caputi, K, Navarrete, B, C Di Cesare, Bergamini, P, Kotiwale, G, A Crespo Gómez, Desprez, G, Mascia, S, Sun, Y

2026

Overview

We report the discovery and characterization of two compact galaxies, Pelias and Neleus, at z ~ 0.71 and z ~ 0.75, identified in MACS J0416.1-2403 and GOODS-North. Both exhibit unusual spectral energy distributions (SEDs), with very blue rest-frame UV-optical emission and a steep rise toward near- and mid-infrared wavelengths. JWST/NIRISS and JWST/NIRSpec spectroscopy show strong rest-frame optical lines ([O III] 4959,5007 and Halpha) with extreme equivalent widths (>= 1000 Angstrom), indicating young burst-dominated populations with low metallicities (Z ~ 0.1-0.4 Zsun), low dust attenuation (Av ~ 0.2 mag), and stellar masses of Mstar ~ 10^7 Msun. Nonetheless, JWST/MIRI photometry reveals a strong mid-infrared excess that cannot be explained by stellar populations or star-formation-heated dust alone, requiring a hot-dust component most naturally associated with a deeply embedded active galactic nucleus (AGN). SED modelling yields log10(Lbol [erg/s]) ~ 43.7-44.0, implying black hole masses of log10(MBH [Msun]) ~ 5.7-6.7 under the assumption of Eddington-limited accretion. Given the very low stellar masses of the hosts, this corresponds to black-hole-to-stellar mass ratios of about 6-60%, well above the extrapolation of local scaling relations. The lack of X-ray detections suggests that the accretion may be either heavily obscured or intrinsically X-ray weak. Their SEDs also resemble those of Blue Excess Hot Dust Obscured Galaxies and show the characteristic V-shaped continuum seen in Little Red Dots, although with the inflection occurring at redder wavelengths.

• Publisher: Cornell University Library, arXiv.org
• Subject: Active galactic nuclei / Black holes / Compact galaxies / Cosmic dust / Dwarf galaxies / Infrared astronomy / Infrared photometry / James Webb Space Telescope / Mass ratios / Stellar mass / Stellar populations / Wavelengths / X-ray astronomy