Galaxy fields of LISA massive black hole mergers in a simulated universe

Laser Interferometer Space Antenna (LISA) will extend the search for gravitational waves (GWs) at 0.1 − 100 mHz where loud signals from coalescing binary black holes of 104 − 107 M are expected. Depending on their mass and luminosity distance, the uncertainty in the LISA sky-localization decreases from hundreds of deg2 during the inspiral phase to fractions of a deg2 after the merger. By using the semi-analytical model L-Galaxies applied to the Millennium-I merger trees, we generate a simulated universe to identify the hosts of z ≤ 3 coalescing binaries with total mass of 3 × 105, 3 × 106, and 3 × 107 M, and varying mass ratio. We find that, even at the time of merger, the number of galaxies around the LISA sources is too large (<~ 102) to allow direct host identification. However, if an X-ray counterpart is associated to the GW sources at z < 1, all LISA fields at merger are populated by <~ 10 active galactic nuclei (AGNs) emitting above ∼ 10−17 erg cm−2 s−1. For sources at higher redshifts, the poorer sky-localization causes this number to increase up to ∼ 103. Archival data from eRosita will allow discarding ∼ 10 per cent of these AGNs, being too shallow to detect the dim X-ray luminosity of the GW sources. Inspiralling binaries in an active phase with masses <~ 106 M at z ≤ 0.3 can be detected, as early as 10 h before the merger, by future X-ray observatories in less than a few minutes. For these systems, <~ 10 AGNs are within the LISA sky-localization area. Finally, the LISA-Taiji network would guarantee the identification of an X-ray counterpart 10 h before merger for all binaries at z <~ 1.