JWST reveals two overmassive black hole candidates in dwarf galaxies at z~0.7. Pushing black hole searches into the dwarf-galaxy regime

We report the discovery and characterisation of two compact galaxies, and at z łeq 1). Given the very low M_⋆ of the hosts, this corresponds to black hole–to–stellar mass ratios of approx4 - 60%, well above the extrapolation of local scaling relations. The lack of X-ray detections suggests the accretion may either be heavily obscured or intrinsically X-ray weak. Moreover, the overall SED of and z łambdałambda4959,5007, Hα) that have extreme equivalent widths (≳ 1000 Å), indicating young, burst-dominated populations with low metallicities (Z , implying M_∙ ̊m M_⊙ (assuming Eddington-limited accretion λ_ identified in MACS J0416.1-2403 and GOODS-North. Both exhibit unusual spectral energy distributions (SEDs), with very blue rest-frame ultraviolet (UV)–optical emission and a steep rise towards near-infrared (NIR) and mid-infrared (MIR) wavelengths. Spectroscopy from the JWST Near Infrared Imager and Slitless Spectrograph (NIRISS) and Near Infrared Spectrograph (NIRSpec) shows strong rest-frame optical lines ( iii - 0.4 Z_⊙), low dust attenuation (A_V ≈ 0.2 mag), and stellar masses of M_⋆ ̊m M_⊙. Nonetheless, the JWST Mid-Infrared Instrument (MIRI) photometry revealed a strong MIR excess that cannot be explained by stellar populations or star-formation–heated dust alone. Indeed, it requires the presence of a hot-dust component that is most naturally associated with a deeply embedded active galactic nucleus (AGN). Modelling of the SED yielded L_ ̊m bol 43.7 - 44.0 erg s^ -1 5.7 - 6.7 ̊m Edd and shows similarities to those of blue excess hot dust obscured galaxies, suggesting that short-lived phases of buried X-ray–weak AGN activity, where rapid black hole growth dominates the MIR emission, can occur even in much lower mass systems. Finally, their SEDs also show the characteristic `V-shaped' continuum seen in little red dots (LRDs), though the inflection occurs at significantly redder wavelengths compared to classic LRDs, consistent with recently proposed scenarios in which the turnover shifts to longer wavelengths for systems with bluer optical slopes.