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

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.

Figures (11)

• Figure 1: Top panel: $5" \times 4"$ cutouts of the 24 HST/ACS and JWST/NIRCam and MIRI available filters for this study centred at the coordinates of Pelias (red dot marker, $RA = 64.05654$ deg, $DEC = -24.08365$ deg). The RGB cutout was made combining all filters with effective wavelength below $1.2~\mu$m (observed) for the B channel, between $1.2~\mu$m and $3~\mu$m for the G channel, and $> 3~\mu$m for the R channel. In the bottom right corner of each cutout we show the size of the corresponding PSF. Bottom panel: 2D (top) and 1D (bottom) spectra of Pelias as obtained from NIRISS WFSS observations in the F090W, F115W, F150W and F200W filters Willott+22Sarrouh+25. In the 1D spectrum, we highlight the wavelength range covered by the different NIRISS filters with different background colors. We also point out the position of the main rest-frame optical features found in the spectrum.
• Figure 2: Similar to Figure but for Neleus (red dot marker, $\rm RA = 189.23767$ deg, $\rm DEC = +62.13801$ deg). The cutouts size is $5'\times 5'$ while the 2D and 1D spectra shown are from the publicly available NIRSpec PRISM observations Maseda+24.
• Figure 3: Comparison between our targets photometry and galaxy empirical templates: IRAS 08572+3915 (orange) from Brown+14, a QSO with little dust attenuation ($\tau = 0.5$, light blue) by Lyu+17, a heavily obscured QSO ($\tau = 20$, red) from Lyu+18, the maximal SED template of LRDs/"V"-shaped objects (purple) by Akins+25, a young SSP (5 Myr, green) from the bpass models by Eldridge+17 and the the Blue Hot DOG W0204-0506 by Assef+20 (magenta). Pelias and Neleus data points are reported with open circles and open squares, respectively. Upperlimits are shown with black downward triangles. In grey, we present the best-fit model obtained by cigale (see § ).
• Figure 4: The OHNO diagram. The solid, dashed and dotted black demarcation lines are based on the work of Backhaus+22, Feuillet+24 and Arevalo-Gonzalez+25, respectively. In the background, the 2D density distribution is representative of galaxies from the Sloan Digital Sky Survey York+00Kauffmann+03. The brown data-points show the location of recent literature at $z \gtrsim 4$ from Killi+24Kocevski+23Kokorev+23Larson+23Trump+23Ubler+24bScholtz+25Calabro+24aRinaldi+24 while the red pentagon highlight the position of our target Neleus. We also present a grid of SFG and AGN models by Calabro+23, showing with different colors and sizes the representative points of models of different metallicity ($Z = 0.04, 0.1, 0.2, 0.4, 0.8~Z_\odot$) and ionisation parameter ($\log_{10}(U) = -3.0, -2.5, -2.0, -1.5$).
• Figure 5: Top panel: DGs templates from Lyu+24 normalised to the MIRI F770W photometry. Pelias and Neleus photometry is reported with open circles and open squares, respectively. Upperlimits are reported with black downward triangles. In grey, we show the best-fit model obtained by cigale (see § ). The inset displays a zoom-in of the 6 - 30 $\mu$m spectral region (observed frame). Bottom panel: as above, DGs templates (without smoothing) fixing the total dust luminosity to the one predicted by cigale based on the available rest-frame UV-to-optical photometry.