A Candidate of a Least-Massive Black Hole at the First 1.1 Billion Years of the Universe

Abstract: We report a candidate of a low-luminosity active galactic nucleus (AGN) at z=5 that was selected from the first near-infrared images of the JWST CEERS project. This source, named CEERS-AGN-z5-1 at absolute 1450 A magnitude M1450=-19.5 +/- 0.3, was found via a visual selection of compact sources from a catalog of Lyman break galaxies at z>4, taking advantage of the superb spatial resolution of the JWST/NIRCam images. The 20 photometric data available from CFHT, HST, Spitzer, and JWST suggest that the continuum shape of this source is reminiscent of that for an unobscured AGN, and there is a clear color excess in the filters where the redshifted Hbeta+[OIII] and Halpha are covered. The estimated line luminosity is L_Hbeta+[OIII] =10^43.0 erg s-1 and L_Halpha =10^42.9 erg s-1 with the corresponding rest-frame equivalent width EW_{Hbeta+[OIII]} =1100 A and EW_Halpha =1600 A, respectively. Our SED fitting analysis favors the scenario that this object is either a strong broad-line emitter or even a super-Eddington accreting black hole (BH), although a possibility of an extremely young galaxy with moderate dust attenuation is not completely ruled out. The bolometric luminosity, L_bol=2.5 +/- 0.3 \times 10^44 erg s-1, is consistent with those of z<0.35 broad-line AGNs with M_BH = 10^6 M_sun accreting at the Eddington limit. This new AGN population at the first 1.1 billion years of the universe may close the gap between the observed BH mass range at high redshift and that of BH seeds. Spectroscopic confirmation is awaited to secure the redshift and its AGN nature.