Spatially Resolving the Kinematics of the <100 {\mu}as Quasar Broad Line Region using Spectroastrometry

TL;DR

This paper proposes using spectroastrometry to spatially resolve the kinematics of quasar broad line regions, enabling black hole mass measurements at high redshifts and luminosities with current and future telescopes.

Contribution

It introduces a novel spectroastrometric method to measure BLR size and kinematics, extending black hole mass estimation to high-redshift, high-luminosity quasars.

Findings

Spectroastrometry can constrain BLR size with existing telescopes for z<2.5 quasars.

Next-generation telescopes can routinely detect BLR signals up to z~6.

The method enables kinematic black hole mass measurements across cosmic time.

Abstract

The broad line region (BLR) of luminous active galactic nuclei (AGN) is a prominent observational signature of the accretion flow around supermassive black holes, which can be used to measure their masses (M_BH) over cosmic history. Due to the <100 {\mu}as angular size of the BLR, current direct constraints on BLR kinematics are limited to those provided by reverberation mapping studies, which are most efficiently carried out on low-luminosity L and low-redshift z AGN. We analyze the possibility to measure the BLR size and study its kinematic structure using spectroastrometry, whereby one measures the spatial position centroid of emission line photons as a function of velocity. We calculate the expected spectroastrometric signal of a rotation-dominated BLR for various assumptions about the ratio of random to rotational motions, and the radial distribution of the BLR gas. We show that for hyper-luminous quasars at z < 2.5, the size of the low-ionization BLR can already be constrained with existing telescopes and adaptive optics systems, thus providing a novel method to spatially resolve the kinematics of the accretion flow at 10^3 -- 10^4 gravitational radii, and measure M_BH at the high-L end of the AGN family. With a 30m-class telescope, BLR spectroastrometry should be routinely detectable for much fainter quasars out to z ~ 6, and for various emission lines. This will enable kinematic M_BH measurements as a function of luminosity and redshift, providing a compelling science case for next generation telescopes.