A systematic search for massive black hole binaries in SDSS spectroscopic sample

TL;DR

This study systematically searches for massive black hole binaries in SDSS spectra, identifying nine promising candidates and revealing a new class of extreme double-peaked emitters, thereby doubling known candidates and advancing understanding of black hole mergers.

Contribution

It introduces a data-driven spectral modeling method to identify black hole binary candidates in SDSS data, significantly increasing the number of known candidates.

Findings

Identified 9 black hole binary candidates in SDSS data.

Discovered a new class of extreme double-peaked emitters.

Doubled the number of known black hole binary candidates.

Abstract

We present the results of a systematic search for massive black hole binaries in the Sloan Digital Sky Survey spectroscopic database. We focus on bound binaries, under the assumption that one of the black holes is active. In this framework, the broad lines associated to the accreting black hole are expected to show systematic velocity shifts with respect to the narrow lines, which trace the rest-frame of the galaxy. For a sample of 54586 quasars and 3929 galaxies at redshifts 0.1<z<1.5 we brute-force model each spectrum as a mixture of two quasars at two different redshifts. The spectral model is a data-driven dimensionality reduction of the SDSS quasar spectra based on a matrix factorization. We identified 32 objects with peculiar spectra. Nine of them can be interpreted as black hole binaries. This doubles the number of known black hole binary candidates. We also report on the discovery of a new class of extreme double-peaked emitters with exceptionally broad and faint Balmer lines. For all the interesting sources, we present detailed analysis of the spectra, and discuss possible interpretations.