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