Properties and merger signatures of galaxies hosting LISA coalescing massive black hole binaries

¹ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
e-mail: david.izquierdovillalba@unimib.it
² INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
³ Institut d’Astrophysique de Paris, Sorbonne Université, CNRS, UMR 7095, 98 bis bd Arago, 75014 Paris, France
⁴ Department of Astronomy, MongManWai Building, Tsinghua University, Beijing 100084, PR China
⁵ Donostia International Physics Centre (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastian, Spain
⁶ IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain

Received: 25 May 2023
Accepted: 24 June 2023

Abstract

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 (f_gas > 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.