Searching for Binary Supermassive Black Holes via Variable Broad Emission Line Shifts: Low Binary Fraction

TL;DR

This study searches for supermassive black hole binaries through variable broad emission line shifts in quasars, finding that less than 1% of active SMBHs are in close binaries at ~0.1 pc separation, suggesting rapid evolution or stalling.

Contribution

It provides the first large-scale statistical constraint on the fraction of SMBHs in close binaries using long-term spectroscopic data.

Findings

Less than 1% of SMBHs are in close binaries at ~0.1 pc separation.

Velocity shifts over 10 years are consistent with single SMBH models.

Most SMBH binaries either merge quickly or stall at larger radii.

Abstract

Supermassive black hole binaries (SMBHs) are expected to result from galaxy mergers, and thus are natural byproducts (and probes) of hierarchical structure formation in the Universe. They are also the primary expected source of low-frequency gravitational wave emission. We search for binary BHs using time-variable velocity shifts in broad Mg II emission lines of quasars with multi-epoch observations. First, we inspect velocity shifts of the binary SMBH candidates identified in Ju et al. (2013), using SDSS spectra with an additional epoch of data that lengthens the typical baseline to ~10 yr. We find variations in the line-of-sight velocity shifts over 10 years that are comparable to the shifts observed over 1-2 years, ruling out the binary model for the bulk of our candidates. We then analyze 1438 objects with 8 yr median time baselines, from which we would expect to see velocity shifts >1000 km/s from sub-pc binaries. We find only one object with an outlying velocity of 448 km/s, indicating, based on our modeling, that ~< 1 per cent (the value varies with different assumptions) of SMBHs that are active as quasars reside in binaries with ~0.1 pc separations. Binaries either sweep through these small separations rapidly or stall at larger radii.