Towards measuring supermassive black hole masses with interferometric observations of the dust continuum

TL;DR

This paper demonstrates that dust continuum sizes measured by optical/near-infrared interferometry can be used to estimate supermassive black hole masses in active galactic nuclei efficiently, with accuracy comparable to reverberation mapping.

Contribution

It introduces a new method to measure black hole masses using dust continuum sizes calibrated against broad-line region measurements, offering a faster alternative to traditional techniques.

Findings

Dust continuum size is roughly twice the reverberation mapping size.

Continuum size correlates tightly with broad-line region size (scatter 0.25 dex).

Dust-based black hole mass estimates are as accurate as reverberation mapping.

Abstract

This work focuses on active galactic nuclei (AGNs), and the relation between the sizes of the hot dust continuum and the broad-line region (BLR). We find that the continuum size measured using optical/near-infrared interferometry (OI) is roughly twice that measured by reverberation mapping (RM). Both OI and RM continuum sizes show a tight relation with the H$\beta$ BLR size with only an intrinsic scatter of 0.25 dex. The masses of supermassive black holes (BHs) can hence be simply derived from a dust size in combination with a broad line width and virial factor. Since the primary uncertainty of these BH masses comes from the virial factor, the accuracy of the continuum-based BH masses is close to those based on the RM measurement of the broad emission line. Moreover, the necessary continuum measurements can be obtained on a much shorter timescale than those required monitoring for RM, and are also more time efficient than those needed to resolve the BLR with OI. The primary goal of this work is to demonstrate measuring the BH mass based on the dust continuum size with our first calibration of the $R_\mathrm{BLR}$-$R_\mathrm{d}$ relation. The current limitation and caveats are discussed in detail. Future GRAVITY observations are expected to improve the continuum-based method and have the potential to measure BH masses for a large sample of AGNs in the low-redshift Universe.