S21B0126

We propose optical spectroscopic follow-up of variability-selected intermediate-mass black hole (IMBH) AGN candidates discovered by the Subaru HSC COSMOS and SXDS UltraDeep (UD) time-domain survey. Using the five-band (grizy) light curves at a depth of i ∼ 26 mag per epoch obtained in the COSMOS and SXDS UD fields, we have identified thousands of AGNs solely based on their optical variability, many of which are likely to be missed by other AGN selection techniques. From this variability-selected AGN sample, we have found 10 AGNs at intermediate redshifts (z_phot < 1) hosted by dwarf galaxies with stellar masses of M_* < 10^9.5 M_⊙ (estimated from multi-wavelength photometry), i.e., IMBH AGN candidates with BH masses of M_BH < 10⁵⁻⁶ M_⊙. Since the variability-selected AGNs are by definition unobscured type 1 AGNs, the proposed deep spectroscopy enables us to not only determine spectroscopic redshifts but also detect broad Balmer emission line components, and then estimate BH mass and Eddington ratio of these IMBH AGN candidates. This observation will allow us to put unique observational constraints on AGN occupation fraction in dwarf galaxies, which will provide profound insights into seed BH formation mechanisms.

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