Academic literature on the topic 'Super-massive Black Hole (SMBH)'

Black hole, one of the most inscrutable objects in the universe have been confirmed that nearly every galaxy contain a super massive black hole (SMBH) at its centre. SMBH’s mass is astounding high which contains millions of solar masses, such huge black hole will generate huge gravitational effect on the region nearby. However, studies indeed found the correlation between black hole and its host galaxy. This paper investigates the impact of black holes on galaxies’ structure and stars based on different black hole mass relation simulations. The exact effects on black holes on galaxies remain unclear, but different simulations certainly found the correlations of black hole’s mass with galaxy’s variables and stars. According to the analysis, correlations of MBH, σe, stellar mass, star formation rate and kinetic energy shows the energy emitted by the ANG/black hole affect the inner structure of the galaxy and the properties of stars which disturbing the gas reservoir in the galaxy. The impact on stars can be reflected by the impact on galaxies. Study galaxies is a good way to find more properties and details about the growth and the formation of SMBHs at the centre of the galaxies. The general structure of the galaxy also provides an indication of the structure of the universe and stars’ organizations.

We investigated whether the two recently discovered nuclei in NGC 7727 both host a super-massive black hole (SMBH). We used the high spatial resolution mode of the integral-field spectrograph MUSE on the VLT in adaptive optics mode to resolve the stellar kinematics within the sphere of influence of both putative black holes. We combined the kinematic data with an HST-based mass model and used Jeans models to measure their SMBH mass. We report the discovery of a dual SMBH system in NGC 7727. We detect a SMBH in the photometric center of the galaxy in Nucleus 1, with a mass of MSMBH = 1.54−0.15+0.18 × 108 M⊙. In the second nucleus, which is 500 pc offset from the main nucleus, we also find a clear signal for a SMBH with a mass of MBH = 6.33−1.40+3.32 × 106 M⊙. Both SMBHs are detected at high significance. The off-axis nature of Nucleus 2 makes modeling the system challenging; however, a number of robustness tests suggest that a black hole is required to explain the observed kinematics. The SMBH in the offset Nucleus 2 makes up 3.0% of its total mass, which means its SMBH is over-massive compared to the MBH − MBulge scaling relation. This confirms it as the surviving nuclear star cluster of a galaxy that has merged with NGC 7727. This discovery is the first dynamically confirmed dual SMBH system with a projected separation of less than a kiloparsec and the nearest dynamically confirmed dual SMBH at a distance of 27.4 Mpc. The second Nucleus is in an advanced state of inspiral, and it will eventually result in a 1:24 mass ratio SMBH merger. Optical emission lines suggest Nucleus 2 is a Seyfert galaxy, making it a low-luminosity Active Galactic Nuclei. There are likely many more quiescent SMBHs as well as dual SMBH pairs in the local Universe that have been missed by surveys that focus on bright accretion signatures.

AbstractTriple supermassive black holes (SMBH) can form during the hierarchical mergers of massive galaxies with an existing binary. Perturbations by a third black hole may accelerate the merging process of an inner binary, for example through the Kozai mechanism. We analyze the evolution of simulated hierarchical triple SMBHs in galactic centers, and find resonances in the evolution of the semi-major axis, the eccentricity and the inclination, for both the inner and the outer orbits of the triple system, which are not only Kozai like. Through resonant oscillations, SMBH can trigger a significant increase of the inner SMBH binary eccentricity shortening the merger timescale expected from gravitational wave (GW) emission. As hierarchical triple SMBHs may be frequent in massive galaxies, the influence of orbital resonances is of great importance to our understanding of black hole coalescence and gravitational wave detection. Although Kozai mechanism is believed to play an important role in this process, detailed studies on the pattern of these resonances is necessary.

AbstractI present examples of radio AGN with binary nuclei which provide the direct radio evidence for binary Super Massive Black Holes (SMBH) driving the AGN activity. There is also other evidence for distorted radio morphology and periodic variability which may indicate the presence of a second (inactive) SMBH. Finally I enumerate a number of possible radio tracers for the binary SMBH merger events.

ABSTRACT Accretion on to black holes at rates above the Eddington limit has long been discussed in the context of supermassive black hole (SMBH) formation and evolution, providing a possible explanation for the presence of massive quasars at high redshifts (z ≳ 7), as well as having implications for SMBH growth at later epochs. However, it is currently unclear whether such ‘super-Eddington’ accretion occurs in SMBHs at all, how common it is, or whether every SMBH may experience it. In this work, we investigate the observational consequences of a simplistic model for super-Eddington accretion flows – an optically thick, geometrically thin accretion disc where the inner-most parts experience severe photon-trapping, which is enhanced with increased accretion rate. The resulting spectral energy distributions (SEDs) show a dramatic lack of rest-frame UV, or even optical, photons. Using a grid of model SEDs spanning a wide range in parameter space (including SMBH mass and accretion rate), we find that large optical quasar surveys (such as SDSS) may be missing most of these luminous systems. We then propose a set of colour selection criteria across optical and infrared colour spaces designed to select super-Eddington SEDs in both wide-field surveys (e.g. using SDSS, 2MASS, and WISE) and deep and narrow-field surveys (e.g. COSMOS). The proposed selection criteria are a necessary first step in establishing the relevance of advection-affected super-Eddington accretion on to SMBHs at early cosmic epochs.

AbstractSmall-JASMINE (hearafter SJ), infrared astrometric satellite, will measure the positions and the proper motions which are located around the Galactic center, by operating at near infrared wave-lengths. SJ will clarify the formation process of the super massive black hole (hearafter SMBH) at the Galactic center. In particular, SJ will determine whether the SMBH was formed by a sequential merging of multiple black holes. The clarification of this formation process of the SMBH will contribute to a better understanding of merging process of satellite galaxies into the Galaxy, which is suggested by the standard galaxy formation scenario. A numerical simulation (Tanikawa and Umemura, 2014) suggests that if the SMBH was formed by the merging process, then the dynamical friction caused by the black holes have influenced the phase space distribution of stars. The phase space distribution measured by SJ will make it possible to determine the occurrences of the merging process.

AbstractWe have developed a new method for post-Newtonian, high-precision integration of stellar systems containing a super-massive black hole (SMBH), splitting the forces on a particle between a dominant central force and perturbations. We used this method to perform fully collisional N-body simulations of inspiralling intermediate-mass black holes (IMBHs) in the centre of the Milky Way.

ABSTRACT Accretion discs around active galactic nuclei (AGNs) are potentially unstable to star formation at large radii. We note that when the compact objects formed from some of these stars spiral into the central supermassive black hole (SMBH), there is no radiative feedback and therefore the accretion rate is not limited by radiation forces. Using a set of accretion disc models, we calculate the accretion rate on to the central SMBH in both gas and compact objects. We find that the time-scale for an SMBH to double in mass can decrease by factors ranging from ∼0.7 to as low as ∼0.1 in extreme cases, compared to gas accretion alone. Our results suggest that the formation of extremely massive black holes at high redshift may occur without prolonged super-Eddington gas accretion or very massive seed black holes. We comment on potential observational signatures as well as implications for other observations of AGNs.

AbstractTidal Disruption Events (TDE) allow to probe the super massive black holes (SMBH) in the cores of galaxies and could be a source of black hole mass growth. We present the search for candidates for TDEs conducted within OGLE and Gaia surveys. Our preliminary results indicate that TDEs can occur in cores of galaxies exhibiting different levels of activity, from quiescent, through weak-AGNs to highly active QSOs. We also present how Gaia can help study the mass distribution of Milky Way single black holes via microlensing.

Abstract Active galactic nuclei (AGNs) probably control the growth of their host galaxies via feedback in the form of wide-angle wind-driven outflows. These establish the observed correlations between supermassive black hole (SMBH) masses and host galaxy properties, e.g. the spheroid velocity dispersion σ. In this paper we consider the growth of the SMBH once it starts driving a large-scale outflow through the galaxy. To clear the gas and ultimately terminate further growth of both the SMBH and the host galaxy, the black hole must continue to grow its mass significantly, by up to a factor of a few, after reaching this point. The mass increment ΔMBH depends sensitively on both galaxy size and SMBH spin. The galaxy size dependence leads to ΔMBH ∝ σ5 and a steepening of the M–σ relation beyond the analytically calculated M ∝ σ4, in agreement with observation. Slowly spinning black holes are much less efficient in producing feedback, so at any given σ the slowest spinning black holes should be the most massive. Current observational constraints are consistent with this picture, but insufficient to test it properly; however, this should change with upcoming surveys.

Active Galactic Nuclei (AGN) are among the most powerful sources of energy in the Universe. The “central engine” is likely a super massive (M >~ 10^6 M_sun) black hole accreting matter from the nuclei of host galaxies. In order to study the AGN demography, formation, evolution, accretion physics and galaxy feedback processes we need a reliable method to estimate the black hole mass. The most reliable ones (direct methods) can be applied only to a few tens of nearby AGNs, strongly limiting the possibility to perform statistical studies on large samples and high redshift sources. The issue of black hole mass estimation on large samples of Type 1 AGN is addressed using an indirect (hence less reliable) procedure: the Single Epoch Virial (SEV) method. In this thesis I discuss the assumptions, biases and possible systematic errors affecting the SEV estimates, and propose a completely independent method to estimate the Type 1 AGN black hole mass. The method is based on the assumption that accretion occur through a standard Shakura & Sunyaev (1973) accretion disk. The calibration is performed by studying the statistical relationships between the broad–band spectral features of Type 1 AGN and the optical emission line luminosities. I apply the method to a sample of 23 radio–loud narrow–line Seyfert 1 galaxies, for which the SEV masses were suspected to be strongly biased. The resulting black hole mass estimates are significantly greater than SEV ones. I discuss the reliability of these estimates, and the consequence on the physical interpretation of the class of narrow–line Seyfert 1 galaxies in the framework of the AGN unified model.

Ces dernières années les principaux relevés cosmologiques ont collectés plusieurs dizaines de milliers de spectres de noyaux actifs de galaxies (connus aussi sous l'abréviation anglaise AGN), illustrant ainsi la grande éfficacité des techniques de pré-sélections optiques de candidat AGN. Ils ont ainsi permis une étude statistique détaillée de la population d'AGN.
Le revers de ces techniques de pré-sélection est que certaines catégories de la population globale des AGN optiques sont peut-être sous-représentées dans ces échantillons, biaisant notre compréhension actuelle de l'évolution de ces objets.

Cette thèse de doctorat est dédiée à l'étude des propriétés des AGN de type 1 de faible luminosité. Dans ce but nous utilisons un échantillon spectroscopique de 130 AGN à raies
d'émission larges, extrait d'un grand relevé de galaxie: le VIRMOS VLT Deep Survey (VVDS).
Cet échantillon présente un intérêt unique, de part la simplicité des critères de pré-sélection de ces objets (une simple limite en magnitude) ainsi que par la profondeur en magnitude atteinte: il fournit d'une part un aperçu des propriétés des AGN de type 1 à des magnitudes encore peu explorées spectroscopiquement (les AGN étudiés ici sont jusqu'à cent fois plus faibles que ceux du SDSS) et il permet d'autre part de quantifier les biais qui seraient introduits par des critères classiques de sélection.

Nous mesurons une densité sur le ciel d'environ 470 AGN par degré carré à notre magnitude limite (Iab=24). Une fraction importante des AGN que nous observons ne serait pas sélectionée par les techniques de sélections morphologiques et d'excès d'UV classiquement appliquées. Nous attribuons cet effet à la contamination de nos AGN par leur galaxie hôte, vue leur faible luminosité. La fonction de luminosité des AGN montre qu'il y a relativement plus d'AGN de faible luminosité à bas redshift qu'á plus grand redshift. Cette observation corrobore le scénario de croissance anti-hierarchique de trous-noirs galactiques suggéré par les echantillons d'AGN sélectionnés en rayons-X.
Finalement nous nous sommes intéressés à la nature des trous noirs qui sont à l'origine de nos AGN. S'agit-il de petits trous noirs galactiques ou bien, au contraire, de trous noirs de grandes masses accrétant faiblement ?
Nos résultats, encore préliminaires, suggèrent que nos AGN correspondent en moyenne à des trous noirs galactique de masses intermediaires (~100 millions de masses solaires) accrétant à des taux d'Eddington modeste (~< 10%).

Synopsis According to the hierarchical structure formation model, massive structures like galaxy clusters are formed due to the gravitational collapse of initial density perturbations and their subsequent mergers. As the formation of galaxy clusters is driven by gravity, they are expected to follow self-similar profiles for density, temperature, entropy etc (Kaiser 1986). However, observations show that self-similarity assumption is not followed in clusters due to the presence of cooling and other such non-gravitational processes (Markevitch et al. 1998; Ponman et al. 1999). More than a third of galaxy clusters have cooling time of the hot diffuse gas in the intra-cluster medium (ICM) in their core smaller than their lifetime (Cavagnolo et al. 2009). As a result, the hot gas in cluster core is expected to cool down catastrophically with total cold gas mass deposition in the core greater than 1012 M⊙ during their lifetime and a star formation rate of several 100 M⊙yr−1. However, lack of observational support of these cooling flow signatures (Peterson et al. 2003) in clusters with short cooling time (cool core clusters) point to the presence of some heating mechanism to compensate the cooling loses and prevent the runway cooling. Among many possible candidates, AGN jets associated with the supermassive black hole present in member central galaxy of the cluster has emerged as the principle heating source (McNamara and Nulsen 2007). Observations show that the energy required to form the structures in the ICM as a result of AGN outbursts, are sufficient to overcome the radiative losses of the ICM. However, the details of AGN feedback to control the cooling flow remains sketchy. My thesis is based on numerical study of AGN feedback in galaxy clusters and trying to answer some important questions related to it. In chapter 1, we discuss the process of galaxy cluster formation and how self-similarity arises naturally in such systems. We then discuss the observational evidences of the breaking of self-similarity in galaxy clusters. We look at the early history of X-ray observations of galaxy clusters and the quest to find signatures of cooling flow as predicted by theoretical models. The non-detection of cooling flow signatures like absence of line emissions below 0.5 keV in several cool core clusters gave rise to the possibility of presence of some heating mechanism to control the cooling flow. We discuss the observational evidences pointing to AGN jets being the possible heating source to compensate for the cooling losses of the ICM. We discuss the different modes of AGN feedback in galaxy clusters and their role in the evolution of these systems. We finally give a brief history of the numerical work done in the area of AGN feedback in galaxy clusters. This chapter ends with the big questions in AGN feedback model that needed investigation. In chapter 2, using high-resolution 3-D and 2-D (axisymmetric) hydrodynamic simulations in spherical geometry, we study the evolution of cool cluster cores heated by feedback-driven bipolar active galactic nuclei (AGN) jets. Condensation of cold gas, and the consequent en-hanced accretion, is required for AGN feedback to balance radiative cooling with reasonable efficiencies, and to match the observed cool core properties. A feedback efficiency (mechanical luminosity ≈ ǫM˙accc2; where M˙acc is the mass accretion rate at 1 kpc) as small as 6 × 10−5 is sufficient to reduce the cooling/accretion rate by ∼ 10 compared to a pure cooling flow in clusters (with M200 � 7 × 1014 M⊙). This value is much smaller compared to the ones consid-ered earlier, and is consistent with the jet efficiency and the fact that only a small fraction of gas at 1 kpc is accreted on to the super-massive black hole (SMBH). The feedback efficiency in earlier works was so high that the cluster core reached equilibrium in a hot state without much precipitation, unlike what is observed in cool-core clusters. We find hysteresis cycles in all our simulations with cold mode feedback: condensation of cold gas when the ratio of the cooling-time to the free-fall time (tcool/tff ) is � 10 leads to a sudden enhancement in the accretion rate; a large accretion rate causes strong jets and overheating of the hot ICM such that tcool/tff > 10; further condensation of cold gas is suppressed and the accretion rate falls, leading to slow cooling of the core and condensation of cold gas, restarting the cycle. Therefore, there is a spread in core properties, such as the jet power, accretion rate, for the same value of core entropy or tcool/tff . A fewer number of cycles are observed for higher efficiencies and for lower mass halos because the core is overheated to a longer cooling time. The 3-D simulations show the formation of a few-kpc scale, rotationally-supported, massive (∼ 1011M⊙) cold gas torus. Since the torus gas is not accreted on to the SMBH, it is largely decoupled from the feedback cycle. The radially dominant cold gas (T < 5 × 104 K; |vr| > |vφ|) consists of fast cold gas uplifted by AGN jets and freely-infalling cold gas condensing out of the core. The radially dominant cold gas extends out to 25 kpc for the fiducial run (halo mass 7 × 1014M⊙ and feedback efficiency 6 × 10−5), with the average mass inflow rate dominating the outflow rate by a factor of ≈ 2. We compare our simulation results with recent observations. In chapter 3, we investigate the stochastic condensation of cold gas and its accretion onto the central super-massive black hole (SMBH) which is essential for active galactic nuclei (AGN) feedback to work in the most massive galaxies that lie at the centres of galaxy clusters. Our 3-D hydrodynamic AGN jet-ICM (intracluster medium) simulations, looking at the detailed angular momentum distribution of cold gas and its time variability for the first time, show that the angular momentum of the cold gas crossing � 1 kpc is essentially isotropic. With almost equal mass in clockwise and counter-clockwise orientations, we expect a cancellation of angular momentum on roughly the dynamical time. This means that a compact accretion flow with a short viscous time ought to form, through which enough accretion power can be channeled into jet mechanical energy sufficiently quickly to prevent a cooling flow. The inherent stochasticity, expected in feedback cycles driven by cold gas condensation, gives rise to a large variation in the cold gas mass at the centres of galaxy clusters, for similar cluster and SMBH masses, in agreement with the observations. Such correlations are expected to be much tighter for the smoother hot/Bondi accretion. The weak correlation between cavity power and Bondi power obtained from our simulations also match observations. Recent analysis shows that it is important to explicitly include the gravitational potential of the central brightest central galaxy (BCG) to infer the acceleration due to gravity (g) and the free-fall time (tff ≡ [2r/g]1/2 ) in cool cluster cores. Accurately measuring tff is crucial because according to numerical simulations cold gas condensation and strong feedback occur in cluster cores with min(tcool/tff ) below a threshold value close to 10. Recent observations which include the BCG gravity show that the observed threshold in min(tcool/tff ) lies at a somewhat higher value, close to 10-30; there are only a few clusters in which this ratio falls much below 10. In chapter 4, we compare numerical simulations of feedback AGN (Active Galactic Nuclei) jets interacting with the intracluster medium (ICM), with and without a BCG potential. We find that, for a fixed feedback efficiency, the presence of a BCG does not significantly affect the temperature but increases (decreases) the core density (entropy) on average. Most importantly, min(tcool/tff ) is only affected slightly by the inclusion of the BCG gravity. Also notable is that the lowest value of min(tcool/tff ) in the NFW+BCG runs are about twice larger than in the NFW runs because of a shorter time for feedback heating (which scales with the free-fall time) in the former. We also look at the role of depletion of cold gas due to star formation and show that it only affects the rotationally dominant component (torus), while the radially dominant component (which regulates the feedback cycle) remains largely unaffected. Stellar gas depletion also increases the duty cycle of AGN jets. The distribution of metals due to AGN jets in our simulations is predominantly along the jet direction and the radial spread of metals is less compared to the observations. We also show that the turbulence in cool core clusters is weak, consistent with recent Hitomi results on Perseus cluster.

Binary systems comprising of compact objects like neutron stars (NS) and/or black holes (BH) lose their energy and angular momentum via gravitational waves (GW). Radiation reaction due to the emission of GW results in a gradual shrinking of the binary orbit and an accompanying gradual increase in the orbital frequency. The preliminary phase of the binary evolution when the radiation-reaction time-scale is much larger than the orbital time-scale is called the inspiral phase. GW emitted during the final stages of the inspiral phase constitute one of the most important sources for the ground-based laser interferometric GW detectors like LIGO, VIRGO and the proposed space-based detector LISA. For the ground-based detectors, NS and/or stellar mass BH binaries are primary sources, while for LISA super-massive BH (SMBH) binaries are potential targets. Inspiralling compact binaries (ICB) are among the prime targets for interferometric detectors because using approximation schemes in general relativity (GR) like the post-Minkowskian (PM) and the post-Newtonian (PN) approximations one can compute the GW emitted by them with sufficient accuracy both for their detection and parameter estimation leading to GW astronomy. The extreme weakness of gravitational interactions implies that if a GW signal from an ICB is incident on a detector, it will be buried in the noisy detector output. Therefore, sophisticated data analysis techniques are required for detecting the signal in presence of the dominant noise and also estimating the parameters of the signal. From the pre-calculated theoretical waveforms called templates, one already knows the structure of the waveform from an ICB. The technique for detecting signals which are of known form in a noisy detector is matched filtering. This technique consists of cross-correlating the output of a noisy detector assumed to contain the signal of known form with a set of templates. It then finds an ‘optimal’ template that would produce, on average, the highest signal-to-noise ratio (SNR). The efficient performance of matched filtering as a data-analysis strategy for GW signals from ICB presupposes very accurate theoretical templates. Slight mismatches between the signal and the template will result in a loss of signal to noise ratio. Computing very accurate theoretical templates and including effects such as eccentricity are challenging tasks for the theoreticians. This thesis addresses some of the issues related to the waveform modelling of the ICB and their implications for GW data analysis. It is known theoretically that compact binaries reduce their eccentricity through the emission of GW. When GW signals from prototype ICB reach the GW detector bandwidth, their orbits are almost circular. Hence one usually models the binary orbit to be circular for computation of the search templates. The waveform from an ICB in a circular orbit is, at any given PN order of approximation, a linear combination of a finite number of harmonics of the orbital frequency. At the lowest order of approximation, called the Newtonian order, the waveform comprises a single harmonic at twice the orbital frequency. Inclusion of higher order PN corrections lead to the appearance of higher harmonics of the orbital frequency. Since the amplitudes of the higher harmonics contain higher powers of the PN expansion parameter, relative to the Newtonian order, they are referred to as amplitude corrections. The phase of each harmonic, determined by the orbital phase, is known upto 3.5PN order (nPN is the order of approximation equivalent to terms ~(v/c)2n beyond the Newtonian order, where v denotes the binary’s orbital velocity and c is the speed of light). Matched filtering is more sensitive to the phase of the signal rather than its amplitude, since the correlation builds up as long as the signal and the template remain in phase. Motivated by this fact, search templates so far have been a waveform model involving only the dominant harmonic (at twice the orbital frequency), although the phase evolution itself is included upto the maximum available PN order. Such waveforms, in which all amplitude corrections are neglected, but the phase is treated to the maximum available order, are called restricted waveforms (RWF) and these are generally used in the data-analysis of ground-based detectors and also simulated searches for the planned LISA. However, recent studies, in the case of ground-based interferometers, showed that going beyond the RWF approximation could improve the efficiency of detection as well as parameter estimation of the inspiral signal. After a brief overview of the properties of GW and their detection strategies in chapter 1, in chapters 2 and 3, we investigate the implications of going beyond the RWF, in the context of the planned space-based Laser Interferometric Space Antenna (LISA). The sensitivity of ground-based detectors is limited by seismic noise below 20Hz. On the other hand, the space-based LISA will be designed to be sensitive to GWs of frequency (10−4 _1)Hz. The most important source in this frequency band are supermassive BH (SMBH) binaries. There is strong observational evidence for the existence of SMBH with masses in the range of in most galactic nuclei. Mergers of such galaxies result in SMBH binaries whose evolution is governed by the emission of GW. Observation of the GW from SMBH binaries at high redshifts is one of the major science goals of LISA. These observations will allow us to probe the evolution of SMBHs and structure formation and provide an unique opportunity to test General Relativity (and its alternatives) in the strong field regime of the theory. Observing SMBH coalescences with high (100-1000) SNR is crucial for performing all the aforementioned tests. The LISA bandwidth (10−4_ 1)Hz determines the range of masses accessible to LISA because the inspiral signal would end when the system’s orbital frequency reaches the mass-dependent last stable orbit (LSO). In the test-mass approximation, the angular velocity ι at LSO is given by where M is the total mass of the binary. Search templates using the RWF, which contains only the dominant harmonic at twice the orbital frequency, cannot extract power in the signal beyond This further implies that the frequency range [0.1, 100] mHz corresponds to the range for the total mass of BH binaries that would be accessible to LISA. In chapter 2, we show that inclusion of higher harmonics will enhance the mass-range of LISA (for the same frequency range) and allow for the detection of SMBH binaries with total masses higher than The template employed in chapter 2 includes amplitude corrections upto 2.5PN order, while keeping the phase upto 3.5PN order. We call this template the full waveform (FWF). The FWF defined above contains higher harmonics of the orbital frequency, the highest of them being 7 times the orbital frequency. For a SMBH binary with total mass the dominant harmonic at LSO is less than the lower cut-off of the LISA bandwidth. Therefore, if one uses the RWF as a search template, this system is ‘invisible’ to LISA. However, the seventh harmonic can still enter the LISA bandwidth and produce a significant SNR and thus allow its detection. With the FWF, LISA can observe sources which are favoured by astronomical observations, but not observable with the RWF. More specifically, with the inclusion of all known harmonics LISA will be able to observe SMBH coalescences with total mass (and mass-ratio 0.1) for a low frequency cut-off of 10−4Hz (10−5Hz) with an SNR up to ~ 60 (~30) at a distance of 3 Gpc. The orbital motion of LISA around the Sun induces frequency, phase and amplitude modulations in the observed GW signal. These modulations carry information about both the source’s location and orientation. Determination of the angular coordinates of the source also allows determination of the luminosity distance of SMBH binaries. Therefore, SMBH binaries are often referred to as GW “standard sirens” (analogous to the electromagnetic “standard candles”). LISA would also be able to measure the “redshifted” masses of the component black holes with good accuracy for sources up to redshifts of a few. However, GW observations alone cannot provide any information about the redshift of the source. If the host galaxy or galaxy cluster is known one can disentangle the redshift from the masses by optical measurement of the redshift. This would not only allow one to extract the “physical” masses, but also provide an exciting possibility to study the luminosity distance-redshift relation providing a totally independent confirmation of the cosmological parameters. Further, this combined observation can be used to map the distribution of black hole masses as a function of redshift. Another outstanding issue in present day cosmology in which LISA can play a role is the dark energy and its physical origin. Probing the equation-of-state-ratio (w(z)) provides an important clue to the question of whether dark energy is truly a cosmological constant (i.e., w = -1). Assuming the Universe to be spatially flat, a combination of WMAP and Supernova Legacy Survey (SNLS) data yields significant constraints on Without including the spatial flatness as a prior, WMAP, large-scale structure and supernova data place a stringent constraint on the dark energy equation of state, For this to be possible, LISA should (a) measure the luminosity distance to the source with a good accuracy and (b) localize the coalescence event on the sky with good angular resolution so that the host galaxy/galaxy cluster can be uniquely identified. Based on analysis with the RWF, it is found that LISA’s angular resolution is not good enough to identify the source galaxy or galaxy cluster, and that other forms of identification would be needed. Secondly, weak lensing effects would corrupt the distance estimation to the same level as LISA’s systematic error. In chapter 3, we study the problem of parameter estimation in the context of LISA, but using the FWF. We investigate systematically the variation in parameter estimation with PN orders by critically examining the role of higher harmonics in the fast GW phasing and their interplay with the slow modulations induced due to LISA’s motion. More importantly, we explore the improvement in the estimation of the luminosity distance and the angular parameters due to the inclusion of higher harmonics in the waveform. We translate the error in the angular resolution to obtain the number of galaxies (or galaxy clusters) within the error box on the sky. We find that independent of the angular position of the source on the sky, higher harmonics improve LISA’s performance on both counts raised in earlier works based on the RWF. We show that the angular resolution enhances typically by a factor of ~2-500 (greater at higher masses) and the error on the estimation of the luminosity distance goes down by a factor of ~ 2-100 (again, larger at higher masses). For many possible sky positions and orientations of the source, the inaccuracy in our measurement of the dark energy would be at the level of a few percent, so that it would only be limited by weak lensing. We conclude that LISA could provide interesting constraints on cosmological parameters, especially the dark energy equation-of-state, and yet circumvent all the lower rungs of the cosmic distance ladder. Having emphasized the need to consider the FWF as a more powerful template, in chapter 4 we calculate a higher order term in the amplitude corrections of the waveform. In chapters 2 & 3, the FWF incorporated amplitude corrections upto 2.5PN order. In chapter 4 the waveform is calculated upto 3PN order. Recent progress in Numerical Relativity (NR) has resulted in computation of the late inspiral and subsequent merger and ringdown phases of the binary evolution (where PN theory does not hold good) by a full-fledged numerical integration of the Einstein field equations. A new field has emerged recently consisting of high-accuracy comparisons between the PN predictions and the numerically-generated waveforms. Such comparisons and matching to the PN results have proved currently to be very successful. They clearly show the need to include high PN corrections not only for the evolution of the binary’s orbital phase but also for the modulation of the gravitational amplitude. This leads to one more motivation for the work in this chapter: providing the associated spin-weighted spherical harmonic decomposition to facilitate comparison and match of the high PN prediction for the inspiral waveform to the numerically-generated waveforms for the merger and ringdown. For the computation of waveforms from the inspiralling compact binaries one needs to solve the two-body problem in general relativity. The nonlinear structure of general relativity prevents one from obtaining a general solution to this problem. The two-body problem is tackled using the multipolar post-Minkowskian (MPM) wave generation formalism. The MPM formalism describes the radiation field of any isolated post-Newtonian source. The radiation field is first of all parametrized by means of two sets of radiative multipole moments. These moments are then related (by means of an algorithm for solving the non-linearities of the field equations) to the so-called canonical moments which constitute some useful intermediaries for describing the external field of the source. The canonical moments are then expressed in terms of the operational source moments obtained by matching to a PN source and are given by explicit integrals extending over the matter source and gravitational field. The extension of the waveform by half a PN order requires as inputs the relations between the radiative, canonical and source multipole moments for general sources at 3PN order. We also require the 3PN extension of the source multipole moments in the case of compact binaries. The waveform in the far-zone consists of two types of terms, instantaneous and hereditary. The instantaneous terms are determined by the dynamical state of the binary at the retarded time. The hereditary terms, on the other hand, depend on the entire past history of the source. These terms originate from the nonlinear interactions between the various multipole moments and also from backscattering off the curved spacetime generated by the waves themselves. In this chapter, we compute the contributions of all the instantaneous and hereditary terms (which include tails, tails-of-tails and memory integrals) up to 3PN order. The end results of this chapter are given in terms of both the 3PN plus and cross polarizations and the separate spin-weighted spherical harmonic modes. Though most of the sources will be in circular orbits by the time the GWs emitted by the system enter the sensitivity band of the laser interferometers, astrophysical scenarios such as Kozai mechanism could produce binaries which have nonzero eccentricity. Studies have shown that filtering the signal from an eccentric binary with circular orbit templates could significantly degrade the SNR. For constructing a phasing formula for eccentric binaries one has to compute the energy and angular momentum fluxes carried away by the GWs and then compute how the orbital elements evolve with time under gravitational radiation reaction. The far-zone energy and angular momentum fluxes, like the waveform, contain both instantaneous and hereditary contributions. The complete 3PN energy flux and instantaneous terms in the 3PN angular momentum flux are already known. In chapter 5, the hereditary terms in the 3PN angular momentum flux from an ICB moving in quasi-elliptical orbits are computed. A semi-analytic method in the frequency domain is used to compute the hereditary contributions. At 3PN order, the quasi-Keplerian representation of elliptical orbits at 1PN order is required. To calculate the tail contributions we exploit the doubly periodic nature of the motion to average the 3PN fluxes over the binary’s orbit. The hereditary part of the angular momentum flux provided here has to be supplemented with the instantaneous part to obtain the final input needed for the construction of templates for binaries moving in elliptical orbits, a class of sources for both the space based detectors and the ground based ones. Using the hereditary contributions in the 3PN energy flux, we also compute the 3PN accurate hereditary contributions to the secular evolution of the orbital elements of the quasi-Keplerian orbit description.

Binary systems comprising of compact objects like neutron stars (NS) and/or black holes (BH) lose their energy and angular momentum via gravitational waves (GW). Radiation reaction due to the emission of GW results in a gradual shrinking of the binary orbit and an accompanying gradual increase in the orbital frequency. The preliminary phase of the binary evolution when the radiation-reaction time-scale is much larger than the orbital time-scale is called the inspiral phase. GW emitted during the final stages of the inspiral phase constitute one of the most important sources for the ground-based laser interferometric GW detectors like LIGO, VIRGO and the proposed space-based detector LISA. For the ground-based detectors, NS and/or stellar mass BH binaries are primary sources, while for LISA super-massive BH (SMBH) binaries are potential targets. Inspiralling compact binaries (ICB) are among the prime targets for interferometric detectors because using approximation schemes in general relativity (GR) like the post-Minkowskian (PM) and the post-Newtonian (PN) approximations one can compute the GW emitted by them with sufficient accuracy both for their detection and parameter estimation leading to GW astronomy. The extreme weakness of gravitational interactions implies that if a GW signal from an ICB is incident on a detector, it will be buried in the noisy detector output. Therefore, sophisticated data analysis techniques are required for detecting the signal in presence of the dominant noise and also estimating the parameters of the signal. From the pre-calculated theoretical waveforms called templates, one already knows the structure of the waveform from an ICB. The technique for detecting signals which are of known form in a noisy detector is matched filtering. This technique consists of cross-correlating the output of a noisy detector assumed to contain the signal of known form with a set of templates. It then finds an ‘optimal’ template that would produce, on average, the highest signal-to-noise ratio (SNR). The efficient performance of matched filtering as a data-analysis strategy for GW signals from ICB presupposes very accurate theoretical templates. Slight mismatches between the signal and the template will result in a loss of signal to noise ratio. Computing very accurate theoretical templates and including effects such as eccentricity are challenging tasks for the theoreticians. This thesis addresses some of the issues related to the waveform modelling of the ICB and their implications for GW data analysis. It is known theoretically that compact binaries reduce their eccentricity through the emission of GW. When GW signals from prototype ICB reach the GW detector bandwidth, their orbits are almost circular. Hence one usually models the binary orbit to be circular for computation of the search templates. The waveform from an ICB in a circular orbit is, at any given PN order of approximation, a linear combination of a finite number of harmonics of the orbital frequency. At the lowest order of approximation, called the Newtonian order, the waveform comprises a single harmonic at twice the orbital frequency. Inclusion of higher order PN corrections lead to the appearance of higher harmonics of the orbital frequency. Since the amplitudes of the higher harmonics contain higher powers of the PN expansion parameter, relative to the Newtonian order, they are referred to as amplitude corrections. The phase of each harmonic, determined by the orbital phase, is known upto 3.5PN order (nPN is the order of approximation equivalent to terms ~(v/c)2n beyond the Newtonian order, where v denotes the binary’s orbital velocity and c is the speed of light). Matched filtering is more sensitive to the phase of the signal rather than its amplitude, since the correlation builds up as long as the signal and the template remain in phase. Motivated by this fact, search templates so far have been a waveform model involving only the dominant harmonic (at twice the orbital frequency), although the phase evolution itself is included upto the maximum available PN order. Such waveforms, in which all amplitude corrections are neglected, but the phase is treated to the maximum available order, are called restricted waveforms (RWF) and these are generally used in the data-analysis of ground-based detectors and also simulated searches for the planned LISA. However, recent studies, in the case of ground-based interferometers, showed that going beyond the RWF approximation could improve the efficiency of detection as well as parameter estimation of the inspiral signal. After a brief overview of the properties of GW and their detection strategies in chapter 1, in chapters 2 and 3, we investigate the implications of going beyond the RWF, in the context of the planned space-based Laser Interferometric Space Antenna (LISA). The sensitivity of ground-based detectors is limited by seismic noise below 20Hz. On the other hand, the space-based LISA will be designed to be sensitive to GWs of frequency (10−4 _1)Hz. The most important source in this frequency band are supermassive BH (SMBH) binaries. There is strong observational evidence for the existence of SMBH with masses in the range of in most galactic nuclei. Mergers of such galaxies result in SMBH binaries whose evolution is governed by the emission of GW. Observation of the GW from SMBH binaries at high redshifts is one of the major science goals of LISA. These observations will allow us to probe the evolution of SMBHs and structure formation and provide an unique opportunity to test General Relativity (and its alternatives) in the strong field regime of the theory. Observing SMBH coalescences with high (100-1000) SNR is crucial for performing all the aforementioned tests. The LISA bandwidth (10−4_ 1)Hz determines the range of masses accessible to LISA because the inspiral signal would end when the system’s orbital frequency reaches the mass-dependent last stable orbit (LSO). In the test-mass approximation, the angular velocity ι at LSO is given by where M is the total mass of the binary. Search templates using the RWF, which contains only the dominant harmonic at twice the orbital frequency, cannot extract power in the signal beyond This further implies that the frequency range [0.1, 100] mHz corresponds to the range for the total mass of BH binaries that would be accessible to LISA. In chapter 2, we show that inclusion of higher harmonics will enhance the mass-range of LISA (for the same frequency range) and allow for the detection of SMBH binaries with total masses higher than The template employed in chapter 2 includes amplitude corrections upto 2.5PN order, while keeping the phase upto 3.5PN order. We call this template the full waveform (FWF). The FWF defined above contains higher harmonics of the orbital frequency, the highest of them being 7 times the orbital frequency. For a SMBH binary with total mass the dominant harmonic at LSO is less than the lower cut-off of the LISA bandwidth. Therefore, if one uses the RWF as a search template, this system is ‘invisible’ to LISA. However, the seventh harmonic can still enter the LISA bandwidth and produce a significant SNR and thus allow its detection. With the FWF, LISA can observe sources which are favoured by astronomical observations, but not observable with the RWF. More specifically, with the inclusion of all known harmonics LISA will be able to observe SMBH coalescences with total mass (and mass-ratio 0.1) for a low frequency cut-off of 10−4Hz (10−5Hz) with an SNR up to ~ 60 (~30) at a distance of 3 Gpc. The orbital motion of LISA around the Sun induces frequency, phase and amplitude modulations in the observed GW signal. These modulations carry information about both the source’s location and orientation. Determination of the angular coordinates of the source also allows determination of the luminosity distance of SMBH binaries. Therefore, SMBH binaries are often referred to as GW “standard sirens” (analogous to the electromagnetic “standard candles”). LISA would also be able to measure the “redshifted” masses of the component black holes with good accuracy for sources up to redshifts of a few. However, GW observations alone cannot provide any information about the redshift of the source. If the host galaxy or galaxy cluster is known one can disentangle the redshift from the masses by optical measurement of the redshift. This would not only allow one to extract the “physical” masses, but also provide an exciting possibility to study the luminosity distance-redshift relation providing a totally independent confirmation of the cosmological parameters. Further, this combined observation can be used to map the distribution of black hole masses as a function of redshift. Another outstanding issue in present day cosmology in which LISA can play a role is the dark energy and its physical origin. Probing the equation-of-state-ratio (w(z)) provides an important clue to the question of whether dark energy is truly a cosmological constant (i.e., w = -1). Assuming the Universe to be spatially flat, a combination of WMAP and Supernova Legacy Survey (SNLS) data yields significant constraints on Without including the spatial flatness as a prior, WMAP, large-scale structure and supernova data place a stringent constraint on the dark energy equation of state, For this to be possible, LISA should (a) measure the luminosity distance to the source with a good accuracy and (b) localize the coalescence event on the sky with good angular resolution so that the host galaxy/galaxy cluster can be uniquely identified. Based on analysis with the RWF, it is found that LISA’s angular resolution is not good enough to identify the source galaxy or galaxy cluster, and that other forms of identification would be needed. Secondly, weak lensing effects would corrupt the distance estimation to the same level as LISA’s systematic error. In chapter 3, we study the problem of parameter estimation in the context of LISA, but using the FWF. We investigate systematically the variation in parameter estimation with PN orders by critically examining the role of higher harmonics in the fast GW phasing and their interplay with the slow modulations induced due to LISA’s motion. More importantly, we explore the improvement in the estimation of the luminosity distance and the angular parameters due to the inclusion of higher harmonics in the waveform. We translate the error in the angular resolution to obtain the number of galaxies (or galaxy clusters) within the error box on the sky. We find that independent of the angular position of the source on the sky, higher harmonics improve LISA’s performance on both counts raised in earlier works based on the RWF. We show that the angular resolution enhances typically by a factor of ~2-500 (greater at higher masses) and the error on the estimation of the luminosity distance goes down by a factor of ~ 2-100 (again, larger at higher masses). For many possible sky positions and orientations of the source, the inaccuracy in our measurement of the dark energy would be at the level of a few percent, so that it would only be limited by weak lensing. We conclude that LISA could provide interesting constraints on cosmological parameters, especially the dark energy equation-of-state, and yet circumvent all the lower rungs of the cosmic distance ladder. Having emphasized the need to consider the FWF as a more powerful template, in chapter 4 we calculate a higher order term in the amplitude corrections of the waveform. In chapters 2 & 3, the FWF incorporated amplitude corrections upto 2.5PN order. In chapter 4 the waveform is calculated upto 3PN order. Recent progress in Numerical Relativity (NR) has resulted in computation of the late inspiral and subsequent merger and ringdown phases of the binary evolution (where PN theory does not hold good) by a full-fledged numerical integration of the Einstein field equations. A new field has emerged recently consisting of high-accuracy comparisons between the PN predictions and the numerically-generated waveforms. Such comparisons and matching to the PN results have proved currently to be very successful. They clearly show the need to include high PN corrections not only for the evolution of the binary’s orbital phase but also for the modulation of the gravitational amplitude. This leads to one more motivation for the work in this chapter: providing the associated spin-weighted spherical harmonic decomposition to facilitate comparison and match of the high PN prediction for the inspiral waveform to the numerically-generated waveforms for the merger and ringdown. For the computation of waveforms from the inspiralling compact binaries one needs to solve the two-body problem in general relativity. The nonlinear structure of general relativity prevents one from obtaining a general solution to this problem. The two-body problem is tackled using the multipolar post-Minkowskian (MPM) wave generation formalism. The MPM formalism describes the radiation field of any isolated post-Newtonian source. The radiation field is first of all parametrized by means of two sets of radiative multipole moments. These moments are then related (by means of an algorithm for solving the non-linearities of the field equations) to the so-called canonical moments which constitute some useful intermediaries for describing the external field of the source. The canonical moments are then expressed in terms of the operational source moments obtained by matching to a PN source and are given by explicit integrals extending over the matter source and gravitational field. The extension of the waveform by half a PN order requires as inputs the relations between the radiative, canonical and source multipole moments for general sources at 3PN order. We also require the 3PN extension of the source multipole moments in the case of compact binaries. The waveform in the far-zone consists of two types of terms, instantaneous and hereditary. The instantaneous terms are determined by the dynamical state of the binary at the retarded time. The hereditary terms, on the other hand, depend on the entire past history of the source. These terms originate from the nonlinear interactions between the various multipole moments and also from backscattering off the curved spacetime generated by the waves themselves. In this chapter, we compute the contributions of all the instantaneous and hereditary terms (which include tails, tails-of-tails and memory integrals) up to 3PN order. The end results of this chapter are given in terms of both the 3PN plus and cross polarizations and the separate spin-weighted spherical harmonic modes. Though most of the sources will be in circular orbits by the time the GWs emitted by the system enter the sensitivity band of the laser interferometers, astrophysical scenarios such as Kozai mechanism could produce binaries which have nonzero eccentricity. Studies have shown that filtering the signal from an eccentric binary with circular orbit templates could significantly degrade the SNR. For constructing a phasing formula for eccentric binaries one has to compute the energy and angular momentum fluxes carried away by the GWs and then compute how the orbital elements evolve with time under gravitational radiation reaction. The far-zone energy and angular momentum fluxes, like the waveform, contain both instantaneous and hereditary contributions. The complete 3PN energy flux and instantaneous terms in the 3PN angular momentum flux are already known. In chapter 5, the hereditary terms in the 3PN angular momentum flux from an ICB moving in quasi-elliptical orbits are computed. A semi-analytic method in the frequency domain is used to compute the hereditary contributions. At 3PN order, the quasi-Keplerian representation of elliptical orbits at 1PN order is required. To calculate the tail contributions we exploit the doubly periodic nature of the motion to average the 3PN fluxes over the binary’s orbit. The hereditary part of the angular momentum flux provided here has to be supplemented with the instantaneous part to obtain the final input needed for the construction of templates for binaries moving in elliptical orbits, a class of sources for both the space based detectors and the ground based ones. Using the hereditary contributions in the 3PN energy flux, we also compute the 3PN accurate hereditary contributions to the secular evolution of the orbital elements of the quasi-Keplerian orbit description.

This chapter focuses on supermassive black holes, which are sometimes abbreviated “SMBHs.” Stellar-mass black holes are clearly common consequences of stellar evolution, but they are not the only kinds of black holes identified by astronomers. Much more massive black holes are located in the center of many, and perhaps all, galaxies. These black holes are referred to as supermassive black holes. They are responsible for a range of phenomena originating from objects described as active galactic nuclei (AGN), which were first observed in the form of quasi-stellar objects (QSOs) or quasars. AGN are among the most luminous objects in the Universe and can be observed at great distances. The distances can be so great that the light travel time from the AGN to Earth is a large fraction of the age of the Universe. They are therefore often used to probe the evolution of the Universe.

This fictional description of the destruction of much of life on Earth comes from a novel by the astronomer Fred Hoyle, co-authored with his son Geoffrey. In the story, the formation of a quasar in the centre of our galaxy leads to the destruction of all life on Earth, except at a few fortuitously sheltered locations. Quasars—first described in 1963—are colossally energetic astronomical objects with extremely high output of radio waves. The novel built on some of Fred Hoyle’s own scientific interests because in the early 1960s, along with the astrophysicist W.A. Fowler, he had predicted that the collapse of a super-massive object could form a distinctive radio source—just before the discovery of the real thing. Although Hoyle and Fowler had the theoretical head start in explaining quasars, being busy with other work they failed to follow up on this advantage, and the current best explanation for these objects is largely due to Donald Lyndon-Bell and Martin Rees. Building on the ideas of Hoyle and Fowler, they argued that a quasar is formed by a rotating super-massive black hole, fed by a disk of in-falling matter, with jets of matter flying away from the system along its axis of rotation. Like the Hoyles’ novel, this chapter focuses on ways the biosphere could end; a fitting question for the close of a book on the ecology and evolution of Earth-based life. However, any answer to a question set in the far future can necessarily be only speculative and, of course, nobody will be around to put the theory to its ultimate test. This raises a philosophical problem namely, has such a question a place in science, or should it be left to science fiction writers? We believe that such questions count as science, not least because it would be good to know the answer (especially if something could be done to postpone the end), but also because in attempting to answer the question, we can extend our understanding of processes that are currently operating. Indeed, J.B.S. Haldane, one of the greatest scientists of the past century, wrote an early essay on much the same topic we consider here.

The origin of supermassive black holes (SMBH) in galaxy centers still remains uncertain. There are two possible ways oftheir formation — from massive (10 5 − 10 6 M ⊙ ) and low-mass (100 M ⊙ ) BH nuclei. The latter scenario should leave behinda large number of intermediate mass black holes (IMBH, 10 2 − 10 5 M ⊙ ). The largest published sample of bona-fide IMBH-powered AGN contains 10 objects confirmed in X-ray. Here we present a new sample of 15 bona-fide IMBHs, obtainedby confirming the optically selected IMBH candidates by the presence of radiation from the galactic nucleus in the X-rayrange, which increases the number of confirmed IMBHs at the centers of galaxies by 2.5 times. In the same way, 99 blackholes with masses of 2 · 10 5 − 10 6 M ⊙ were confirmed. The sources of X-ray data were publicly available catalogs, archivesof data, and our own observations on XMM-Newton, Chandra and Swift. The Eddington coefficients for 30% of the objectsfrom both samples turned out to be close to critical, from 0.5 to 1, which is an unusually high fraction. Also for the firsttime for light-weight SMBH the correlations between the luminosity in the [OIII] emission line or the broad component ofthe Hα line and the luminosity in the X-ray range were plotted.