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

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

First listed 2026-03-18 | Last updated 2026-03-18

Abstract

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.

Short digest

JWST identifies two compact dwarfs, Pelias (z≈0.71; MACS J0416 field) and Neleus (z≈0.75; GOODS-N), whose very blue UV–optical SEDs contrast with a steep NIR–MIR rise. NIRISS/NIRSpec spectra show [O III] and Hα with extreme EWs (≥1000 Å), low metallicity (≈0.1–0.4 Z⊙), Av≈0.2 mag, and M⋆≈10^7 M⊙, while MIRI photometry requires a hot-dust component best explained by a buried AGN. SED fits give Lbol≈10^43.7–10^44.0 erg s⁻¹ and MBH≈10^5.7–10^6.7 M⊙ (if Eddington-limited), implying BH/M⋆≈6–60%—well above local scaling extrapolations. Their X-ray non-detections and V-shaped continua (with a redder turnover) link them to Blue-Excess DOGs and LRD-like phases, pushing overmassive BH searches deep into the dwarf regime.

Key figures to inspect

• SED decomposition for Pelias and Neleus (UV–MIR): verify the MIR excess over stellar templates, the location of the V-shaped turnover, and the necessity of a hot-dust/AGN component.
• NIRISS/NIRSpec rest-optical spectra: inspect [O III] λλ4959,5007 and Hα line regions for ≥1000 Å EWs, line ratios used for Z≈0.1–0.4 Z⊙, and evidence for burst-dominated populations.
• NIRCam vs MIRI cutouts: compare compact morphologies and check for a centrally concentrated MIR component indicative of embedded accretion relative to the blue stellar light.
• BH mass versus host mass plane: show Pelias and Neleus at BH/M⋆≈6–60% contrasted with local MBH–M⋆ relations to emphasize the overmassive nature.
• X-ray limits versus MIR/bolometric luminosity: assess whether upper limits require heavy obscuration or intrinsic X-ray weakness relative to standard AGN correlations.