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Abstract We study the black hole mass–host galaxy stellar mass relation,M_BH–M_*, of a sample ofz< 4 optically variable active galactic nuclei (AGNs) in the COSMOS field. The parent sample of 491 COSMOS AGNs were identified by optical variability from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) program. Using publicly available catalogs and spectra, we consolidate their spectroscopic redshifts and estimate virial black hole masses using broad-line widths and luminosities. We show that variability searches with deep, high-precision photometry like the HSC-SSP can identity AGNs in low-mass galaxies up toz∼ 1. However, their black holes are more massive given their host galaxy stellar masses than predicted by the local relation for active galaxies. We report thatz∼ 0.5–4 variability-selected AGNs are meanwhile more consistent with theM_BH–M_*relation for local inactive early-type galaxies. This result is in agreement with most previous studies of theM_BH–M_*relation at similar redshifts and indicates that AGNs selected from variability are not intrinsically different from the broad-line Type 1 AGN population at similar luminosities. Our results demonstrate the need for robust black hole and stellar mass estimates for intermediate-mass black hole candidates in low-mass galaxies at similar redshifts to anchor this scaling relation. Assuming that these results do not reflect a selection bias, they appear to be consistent with self-regulated feedback models wherein the central black hole and stars in galaxies grow in tandem. 

ABSTRACT Jetted active galactic nuclei (AGNs) are the principal extragalactic γ-ray sources. Fermi-detected high-redshift (z > 3) blazars are jetted AGNs thought to be powered by massive, rapidly spinning supermassive black holes (SMBHs) in the early universe (<2 Gyr). They provide a laboratory to study early black hole (BH) growth and super-Eddington accretion – possibly responsible for the more rapid formation of jetted BHs. However, previous virial BH masses of z > 3 blazars were based on C iv λ1549 in the observed optical, but C iv λ1549 is known to be biased by strong outflows. We present new Gemini/GNIRS near-infrared spectroscopy for a sample of nine z > 3 Fermi γ-ray blazars with available multiwavelength observations that maximally sample the spectral energy distributions (SEDs). We estimate virial BH masses based on the better calibrated broad H β and/or Mg ii λ2800. We compare the new virial BH masses against independent mass estimates from SED modelling. Our work represents the first step in campaigning for more robust virial BH masses and Eddington ratios for high-redshift Fermi blazars. Our new results confirm that high-redshift Fermi blazars indeed host overly massive SMBHs as suggested by previous work, which may pose a theoretical challenge for models of the rapid early growth of jetted SMBHs.