The Stellar Mass - Black Hole Mass Relation at $z\sim 2$ Down to ${\mathcal{M}}_{\mathrm{BH}}\sim 10^7{M}_{\odot }$ Determined by HETDEX

Abstract

We investigate the stellar mass - black hole mass (${\mathcal{M}}_{\ast }-{\mathcal{M}}_{\mathrm{BH}}$) relation with type 1 AGN down to ${\mathcal{M}}_{\mathrm{BH}}=10^7{M}_{\odot }$, corresponding to a $\simeq -21$ absolute magnitude in rest-frame ultraviolet (UV), 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 AGN with ${\mathcal{M}}_{\mathrm{BH}}$ ranging from $10^7$ to $10^{10}{M}_{\odot }$ that are measured with single-epoch virial method using C{\sc iv} emission lines detected in the HETDEX spectra. ${\mathcal{M}}_{\ast }$ of the host galaxies are estimated from optical to near-infrared photometric data taken with Spitzer, WISE, and ground-based 4-8m class telescopes by CIGALE 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 JWST/NIRCam CEERS data. We obtain the ${\mathcal{M}}_{\ast }-{\mathcal{M}}_{\mathrm{BH}}$ relation covering the unexplored low-mass ranges of ${\mathcal{M}}_{\mathrm{BH}}\sim 10^7-10^8{M}_{\odot }$, and conduct forward modelling to fully account for the selection biases and observational uncertainties. The intrinsic ${\mathcal{M}}_{\ast }-{\mathcal{M}}_{\mathrm{BH}}$ relation at $z\sim 2$ has a moderate positive offset of $0.52\pm 0.14$~dex from the local relation, suggestive of more efficient black hole growth at higher redshift even in the low-mass regime of ${\mathcal{M}}_{\mathrm{BH}}\sim 10^7-10^8{M}_{\odot }$. Our ${\mathcal{M}}_{\ast }-{\mathcal{M}}_{\mathrm{BH}}$ relation is inconsistent with the ${\mathcal{M}}_{\mathrm{BH}}$ suppression at the low-${\mathcal{M}}_{\ast }$ 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.

16 pages, 8 figures, accepted for publication in ApJ