Repeated Imaging of Massive Black Hole Binary Orbits with Millimeter Interferometry: measuring black hole masses and the Hubble constant

TL;DR

This paper proposes using millimeter-wavelength VLBI to directly observe and measure the orbits of massive black hole binaries, enabling precise mass measurements and an independent determination of the Hubble constant.

Contribution

It introduces a method to identify resolvable massive black hole binaries via periodic quasar light curves and proposes VLBI observations to measure their orbits and masses.

Findings

Approximately 100 resolvable MBHBs may exist at z<0.5.

VLBI can measure binary masses with 30% precision.

Potential to determine the Hubble constant with ~10% accuracy.

Abstract

Very long baseline interferometry (VLBI) at millimeter (mm) wavelengths is being employed to resolve event-horizon scale structure of the environment surrounding the Milky-Way black hole, at an angular resolution of a few tens of micro-arcseconds. The same approach could also resolve the orbital separation of a population of massive black hole binaries (MBHBs). Modeling the inspiral of binaries due to gravitational wave emission and gas and requiring binary orbital periods of less than 10 years, we estimate that there may exist ~100 resolvable MBHBs that are bright enough to be observed by mm-wavelength VLBI instruments over the entire sky, at redshifts z<0.5. We propose to search for these resolvable MBHBs by identifying binaries with the required orbital separations from periodic quasar light curves identified in optical and near-IR surveys. These periodic-light-curve candidates can be followed up with radio observations to determine their promise for observation with VLBI at mm wavelengths. VLBI observations over the timescale of a binary orbit can allow unprecedented precision in the measurement of the binary mass, to within 30%. In combination with an independent binary mass measurement, VLBI observation would allow a novel (~10%) measurement of the Hubble constant, independent from those currently proposed and employed.