Probing high redshift massive black hole coalescence through gravitational waves and electromagnetic signals

The existence of supermassive black holes (SMBHs) with masses exceeding billions of solar masses at early cosmic epochs (less than one Gyr after the Big Bang) poses an intriguing challenge in astrophysics. Despite ongoing investigations, the origin and evolution of such SMBHs are still riddled with many uncertainties related to their formation and growth mechanisms and remain intriguing and actively debated topics of research in astrophysics. In this thesis we study the formation and evolution of SMBHs with the use of gravitational waves (GWs) arising from the coalescence of the progenitor massive black holes (MBHs). We investigate the coalescence of massive black hole (MBH ≳ 106 M⊙) binaries (MBHBs) at 6 < z < 10 by adopting a suite of cosmological hydrodynamical simulations of galaxy formation, zoomed-in on biased (> 3σ) overdense regions (Mh ∼ 1012 M⊙ dark matter halos at z = 6) of the Universe. We first analyse the impact of different resolutions and active galactic nucleus (AGN) feedback prescriptions on the merger rate of MBHs, assuming instantaneous mergers. Then, we compute the halo bias correction factor due to the overdense simulated region. We then select our fiducial model, for which we further study the effect of delay in the MBHB coalescence due to dynamical friction. Next, we compute the expected properties of the GW signals and find the fraction of MBHB mergers to be detected by the Laser Interferometer Space Antenna (LISA) and calculate the angular resolution of LISA detectable events (LDEs) to estimate the sky-localization limits of the detectable MBHB mergers. For the LDEs with known GW properties, we further investigate the intrinsic and observational properties of z≳6 galaxies that host the coalescing MBHBs. We associate the LDEs to viii Abstract their host galaxies and select those that, based on their intrinsic properties (˙M, SFR, gas metallicity, and dust mass), are expected to be bright in one or more electromagnetic (EM) bands, e.g. rest-frame X-ray, ultra-violet (UV) and far-infrared (FIR). We further restrict our selection to those LDEs that, after considering the effect of delay due to dynamical friction in the MBH coalescence, are still occurring at z≳6. We find that ∼20-30% of the LDEs and their host galaxies are also detectable with EM telescopes. We post-process these events with dust radiative transfer calculations to make accurate predictions about their spectral energy distributions (SEDs) and continuum maps in the JWST to ALMA wavelength range. We compare the spectra arising from galaxies hosting the merging MBHs with those arising from AGN powered by single accreting BHs. We find that it will be impossible to identify an LDE from the continuum SEDs because of the absence of specific imprints from the merging MBHs. Finally, we compute the profile of the Hαline arising from LDEs, taking into account both the contribution from their star-forming regions and the accreting MBHs. We conclude that the combined detection of GW and EM signals from z≳6 MBHs is challenging (if not impossible) not only because of the poor sky-localization (∼10 deg2 ) provided by LISA, but also because the loudest GW emitters (MBH ∼ 105−6 M⊙) are not massive enough to leave significant signatures (e.g. extended wings) in the emission lines arising from the broad line region. This thesis is composed of four parts: theoretical background, numerical background, analysis and results and finally discussions and summary. Each part is composed of several chapters, the overview of which we highlight below: • We begin with Chapter 1 to give a general introduction to the readers about the cosmological framework this work is based on, timeline of the various epochs of our Universe, theoretical modelling of AGN and different possible pathways of the origin and evolution of SMBHs. • We then describe in Chapter 2 the theory of gravitational waves and its relevance to the formation and evolution of massive black hole binaries (MBHBs). We begin with a brief overview of the general theory of relativity, then describe the different timescales related to the coalescence of MBHs. We then describe some key aspects of GWs from binary systems and a basic concept of GW detectors. We end this chapter with a synopsis of electomagnetic (EM) signals from merging MBHs and the expected detections till date. • Next, in Chapter 3 we outline the key components of a cosmological simulation and provide an overview of modern simulations. Thereafter, we describe the hydrodynamical simulations (HDS) used in this work and highlight the comparison of the two simulations we used as well as the differences within each for varying feedback effects. • Chapter 4 gives a rundown on the principle of radiative transfer (RT), different approachesto solve the RT problem and finally the numerical aspect of the code skirt used in this work to model dust surrounding the AGNs in our model. • In Chapter 5 we derive the merger rate of MBHBs in different simulation runs and also compute the effect of considering overdense halos as MBHB hosting sites and the bias produced from such assumptions. Correcting for this bias, we then compare out results with contemporary merger rates derived from various semi-analytical models (SAMs) and HDS. • We begin Chapter 6 with the technique we used to associate MBHBs in our simulations to the galaxy hosting them. We then compute the time delays that can be induced in MBHB coalescence due to dynamical friction against the stars surrounding the MBHB in post-processing. • In Chapter 7 we study the GW properties arising from the coalescence of MBHBs. We compute the signal-to-noise ratio of the binaries for LISA and the angular resolution of the mergers which leads to GW emission. We also discuss about the mass ratio and different time, frequqency and spatial scales of interest for the GW events. • Finally, in Chapter 8 we delve into the possible EM signals from GW events discussed preciously. We first select the LDEs associated with a host galaxy and compute various intrinsic and observable properties of the hosts. We then produce synthetic spectra and simulated maps of the events in different EM bands and discuss the possibility of detecting EM signals from LDEs before, during or after the coalescence. We also analyze the Hαline profile from LDEs that we calculate considering the contributions from both star-forming regions and accreting MBHs. • We conclude this thesis with Chapter 9 where we summarise our work and results anddiscuss the caveats involved in this work and future prospects.