HI Observations of the Supermassive Binary Black Hole System in 0402+379

TL;DR

This study reports the discovery of the most compact supermassive binary black hole system in galaxy 0402+379, using VLBI observations to analyze HI absorption lines and propose a rotating disk model around the black holes.

Contribution

It provides the first detailed HI absorption analysis of a supermassive binary black hole system and introduces a model of a thick disk with rotating clumps around one black hole.

Findings

Detected two HI absorption lines indicating rotating gas clumps.

Estimated a lower limit of 7 x 10^8 solar masses for the central mass.

Proposed a geometrically thick disk model explaining the observed velocities.

Abstract

We have recently discovered a supermassive binary black hole system with a projected separation between the two black holes of 7.3 parsecs in the radio galaxy 0402+379. This is the most compact supermassive binary black hole pair yet imaged by more than two orders of magnitude. We present Global VLBI observations at 1.3464 GHz of this radio galaxy, taken to improve the quality of the HI data. Two absorption lines are found toward the southern jet of the source, one redshifted by 370 +/- 10 km/s and the other blueshifted by 700 +/- 10 km/s with respect to the systemic velocity of the source, which, along with the results obtained for the opacity distribution over the source, suggests the presence of two mass clumps rotating around the central region of the source. We propose a model consisting of a geometrically thick disk, of which we only see a couple of clumps, that reproduces the velocities measured from the HI absorption profiles. These clumps rotate in circular Keplerian orbits around an axis that crosses one of the supermassive black holes of the binary system in 0402+379. We find an upper limit for the inclination angle of the twin jets of the source to the line of sight of 66 degrees, which, according to the proposed model, implies a lower limit on the central mass of ~7 x 10^8 Msun and a lower limit for the scale height of the thick disk of ~12 pc .