The Most Massive Active Galactic Nuclei at $1\lesssim z \lesssim 2$

TL;DR

This study critically examines the methods used to estimate supermassive black hole masses in high-redshift quasars, revealing systematic uncertainties and suggesting that the most massive black holes are likely intrinsically very massive, possibly overmassive compared to their host galaxies.

Contribution

It provides a detailed analysis of the systematic effects in black hole mass estimations for high-redshift quasars, highlighting issues with emission line widths and proposing more reliable mass estimates.

Findings

AGNs over $10^{10}M_{ m ext{sun}}$ often show double-peaked H$ ext{ extalpha}$ emission.

Emission lines with widths over 8000 km/s should be used cautiously for mass estimation.

High-mass black holes at $1 extless z extless 2$ are likely intrinsically very massive, possibly overmassive relative to their host galaxies.

Abstract

We obtained near-infrared spectra of 26 SDSS quasars at $0.7<z<2.5$ with reported rest-frame ultraviolet $M_{\rm BH} \sim 10^{10}M_{\odot}$ to critically examine the systematic effects involved with their mass estimations. We find that AGNs heavier than $10^{10}M_{\odot}$ often display double-peaked H$\alpha$ emission, extremely broad FeII complex emission around MgII, and highly blueshifted and broadened CIV emission. The weight of this evidence, combined with previous studies, cautions against the use of $M_{\rm BH}$ values based on any emission line with a width over 8000 km/s. Also, the $M_{\rm BH}$ estimations are not positively biased along the presence of ionized narrow line outflows, anisotropic radiation, or the use of line FWHM instead of $\sigma$ for our sample, and unbiased with variability, scatter in broad line equivalent width, or obscuration for general type-1 quasars. Removing the systematically uncertain $M_{\rm BH}$ values, $\sim10^{10}M_{\odot}$ BHs in $1\lesssim z \lesssim 2$ AGNs can still be explained by anisotropic motion of the broad line region from $\sim10^{9.5}M_{\odot}$ BHs, although current observations support they are intrinsically most massive, and overmassive to the host's bulge mass.