Search

The gravitational wave (GW) antenna LISA will detect the signal from coalescing massive black hole binaries (MBHBs) of 104 − 107 M⊙, providing clues as to their formation and growth throughout cosmic history. Some of these events will be localised with a precision of several to less than a deg2, enabling the possible identification of their host galaxy. This work explores the properties of the host galaxies of LISA MBHBs below z ≲ 3. We generate a simulated lightcone using the semi-analytical model L-Galaxies applied to the merger trees of the high-resolution N-body cosmological simulation Millennium-II. The model shows that LISA MBHBs are expected to be found in optically dim (r > 20), star-forming (sSFR > 10−10 yr−1), gas-rich (fgas > 0.6), and disc-dominated (B/T < 0.7) low-mass galaxies of stellar masses 108 − 109 M⊙. However, these properties are indistinguishable from those of galaxies harbouring single massive black holes of comparable mass, making it difficult to select LISA hosts among the whole population of low-mass galaxies. Motivated by this, we explore the possibility of using merger signatures to select LISA hosts. We find that 40%−80% of the galaxies housing LISA MBHBs display merger features related to the interaction that brought the secondary MBH to the galaxy. Despite this, around 60% of dwarf galaxies placed in the surroundings of the LISA hosts will show these kinds of features as well, challenging the unequivocal detection of LISA hosts through the search for merger signatures. Consequently, the detection of an electromagnetic transient associated with the MBHB merger will be vital in order to pinpoint the star-forming dwarf galaxy where these binary systems evolve and coalesce. [ABSTRACT FROM AUTHOR]

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 |$10^4 \, {-}\, 10^7\, \, \rm {M}_{\odot }$| 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 |$z\, {\le }\, 3$| coalescing binaries with total mass of |$3\, {\times }\, 10^{5}$|⁠ , |$3\, {\times }\, 10^6$|⁠ , and |$3\, {\times }\, 10^7\, \rm {M}_{\odot }$|⁠ , and varying mass ratio. We find that, even at the time of merger, the number of galaxies around the LISA sources is too large (⁠|${\gtrsim }\, 10^2$|⁠) 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 |${\lesssim }\, 10$| active galactic nuclei (AGNs) emitting above |${\sim }\, 10^{-17} \, \rm erg\, cm^{-2}\, s^{-1}$|⁠. For sources at higher redshifts, the poorer sky-localization causes this number to increase up to |${\sim }\, 10^3$|⁠. Archival data from eRosita will allow discarding |${\sim }\, 10{{\ \rm per\ cent}}$| 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 |${\lesssim }\, 10^6\, \rm {M}_{\odot }$| at |$z\, {\le }\, 0.3$| 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, |${\lesssim }\, 10$| 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 |$z\, {\lesssim }\, 1$|⁠. [ABSTRACT FROM AUTHOR]

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 |$\rm {\gt }\, 10^7\, M_{\odot }$|⁠. 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 |${\sim }\, 50{{\ \rm per\ cent}}$| in galaxies with |$\rm M_{stellar}\, {\gt }\, 10^{11}\, M_{\odot }$| and drops below 10 per cent for |$\rm M_{stellar}\, {\lt }\, 10^{11}\, M_{\odot }$|⁠. Our model anticipates that the majority of parsec-scale MBHBs are unequal mass systems and lie at |$z\, {\sim }\, 0.5$|⁠ , with |${\sim }\, 20$| objects per |$\rm deg^2$| in the sky. However, most of these systems are inactive, and only |$\rm {1-0.1}$| objects per |$\rm deg^2$| 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 |$z\, {\gt }\, 1$|⁠ , but the number of binaries per |$\rm deg^2$| is |${\lesssim }\, 0.01$| at |$\rm L_{\rm bol}\, {\gt }\, 10^{45}\,\rm erg\,s^{-1}$|⁠. [ABSTRACT FROM AUTHOR]

Extreme mass ratio inspirals (EMRIs) are compact binary systems characterized by a mass ratio q  = m / M in the range 10−9–10−4 and represent primary gravitational wave (GW) sources for the forthcoming Laser Interferometer Space Antenna (LISA). While their standard formation channel involves relaxation processes deflecting compact objects on very low angular momentum orbits around the central massive black hole, a number of alternative formation channels has been proposed, including binary tidal break-up, migration in accretion discs and secular and chaotic dynamics around a massive black hole binary (MBHB). In this work, we take an extensive closer look at this latter scenario, investigating how EMRIs can be triggered by MBHBs, formed in the aftermath of galaxy mergers. By employing a suite of relativistic three-body simulations, we evaluate the efficiency of EMRI formation for different parameters of the MBHB, assessing the importance of both secular and chaotic dynamics. By modelling the distribution of compact objects in galaxy nuclei, we estimate the resulting EMRI formation rate, finding that EMRI are produced in a sharp burst, with peak rates that are 10–100 times higher than the standard two-body relaxation channel, lasting for 106–108 yr. By coupling our results with an estimate of the cosmic MBHB merger rate, we finally forecast that LISA could observe |${\cal O}(10)$| EMRIs per year formed by this channel. [ABSTRACT FROM AUTHOR]