Unveiling the hosts of parsec-scale massive black hole binaries: morphology and electromagnetic signatures

Parsec-scale massive black hole binaries (MBHBs) are expected to form in hierarchical models of structure formation. Even though different observational strategies have been designed to detect these systems, a theoretical study is a further guide for their search and identification. In this work, we investigate the hosts properties and the electromagnetic signatures of massive black holes gravitationally bound on parsec-scales with primary mass > 107 M☉. For that, we construct a full-sky light-cone by the use of the semi-analytical model L-Galaxies in which physically motivated prescriptions for the formation and evolution of MBHBs have been included. Our predictions show that the large majority of the MBHBs are placed either in spiral galaxies with a classical bulge structure or in elliptical galaxies. Besides, the scaling relations followed by MBHBs are indistinguishable from the ones of single massive black holes. We find that the occupation fraction of parsec-scale MBHBs reaches up to ∼ 50 per cent in galaxies with Mstellar > 1011 M☉ and drops below 10 per cent for Mstellar < 1011 M☉. Our model anticipates that the majority of parsec-scale MBHBs are unequal mass systems and lie at z ∼ 0.5, with ∼ 20 objects per deg2 in the sky. However, most of these systems are inactive, and only 1–0.1 objects per deg2 have an electromagnetic counterpart with a bolometric luminosity in excess of 1043 erg s−1. Very luminous phases of parsec-scale MBHBs are more common at z > 1, but the number of binaries per deg2 is ≤ 0.01 at Lbol > 1045 erg s−1.