Spatially Resolving the Kinematics of the ≲ 100 μas Quasar Broad-line Region Using Spectroastrometry
Abstract
The broad-line region (BLR) of luminous active galactic nuclei (AGNs) 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 μ 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) AGNs. We analyze the possibility of measuring 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\lt 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 103-104 gravitational radii and measure {{M}BH} at the high-L end of the AGN family. With a 30 m 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 2015
- DOI:
- 10.1088/0004-637X/804/1/57
- arXiv:
- arXiv:1502.07767
- Bibcode:
- 2015ApJ...804...57S
- Keywords:
-
- quasars: emission lines;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 14 pages, 7 figures, accepted for publication in ApJ