The Stellar Mass-Black Hole Mass Relation at z ∼ 2 down to M_BH ∼ 10⁷ M_⊙ Determined by HETDEX

We investigate the stellar mass-black hole mass ( M * - M BH ) relation with type 1 active galactic nuclei (AGNs) down to M BH = 10 7 M ⊙ , corresponding to a ≃ −21 absolute magnitude in rest-frame ultraviolet, at z = 2-2.5. Exploiting the deep and large-area spectroscopic survey of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), we identify 66 type 1 AGNs with M BH ranging from 10⁷-10¹⁰ M _⊙ that are measured with single-epoch virial method using C iv emission lines detected in the HETDEX spectra. M * of the host galaxies are estimated from optical to near-infrared photometric data taken with Spitzer, the Wide-field Infrared Survey Explorer, and ground-based 4-8 m class telescopes by CIGALE spectral energy distribution (SED) fitting. We further assess the validity of SED fitting in two cases by host-nuclear decomposition performed through surface brightness profile fitting on spatially resolved host galaxies with the James Webb Space Telescope/NIRCam CEERS data. We obtain the M * - M BH relation covering the unexplored low-mass ranges of M BH ∼ 10 7 - 10 8 M ⊙ , and conduct forward modeling to fully account for the selection biases and observational uncertainties. The intrinsic M * - M BH relation at z ∼ 2 has a moderate positive offset of 0.52 ± 0.14 dex from the local relation, suggestive of more efficient black hole growth at higher redshift even in the low-mass regime of M BH ∼ 10 7 - 10 8 M ⊙ . Our M * - M BH relation is inconsistent with the M BH suppression at the low- M * regime predicted by recent hydrodynamic simulations at a 98% confidence level, suggesting that feedback in the low-mass systems may be weaker than those produced in hydrodynamic simulations.