Parallelization, Special Hardware and Post-Newtonian Dynamics in Direct N - Body Simulations

The formation and evolution of supermassive black hole (SMBH) binaries during and after galaxy mergers is an important ingredient for our understanding of galaxy formation and evolution in a cosmological context, e.g. for predictions of cosmic star formation histories or of SMBH demographics (to predict events that emit gravitational waves). If galaxies merge in the course of their evolution, there should be either many binary or even multiple black holes, or we have to find out what happens to black hole multiples in galactic nuclei, e.g. whether they come sufficiently close to merge resulting from emission of gravitational waves, or whether they eject each other in gravitational slingshot interactions

Gravitational radiation reaction in compact binary systems: Contribution of the magnetic dipole-magnetic dipole interaction

We study the gravitational radiation reaction in compact binary systems composed of neutron stars with spin and huge magnetic dipole moments (magnetars). The magnetic dipole moments undergo a precessional motion about the respective spins. At sufficiently high values of the magnetic dipole moments, their interaction generates second post-Newtonian order contributions both to the equations of motion and to the gravitational radiation escaping the system. We parametrize the radial motion and average over a radial period in order to find the secular contributions to the energy and magnitude of the orbital angular momentum losses, in the generic case of \textit{eccentric} orbits. Similarly as for the spin-orbit, spin-spin, quadrupole-monopole interactions, here too we deduce the secular evolution of the relative orientations of the orbital angular momentum and spins. These equations, supplemented by the evolution equations for the angles characterizing the orientation of the dipole moments form a first order differential system, which is closed. The circular orbit limit of the energy loss agrees with Ioka and Taniguchi's earlier result

Gravitational waves from eccentric compact binaries: Reduction in signal-to-noise ratio due to nonoptimal signal processing

Inspiraling compact binaries have been identified as one of the most promising sources of gravitational waves for interferometric detectors. Most of these binaries are expected to have circularized by the time their gravitational waves enter the instrument's frequency band. However, the possibility that some of the binaries might still possess a significant eccentricity is not excluded. We imagine a situation in which eccentric signals are received by the detector but not explicitly searched for in the data analysis, which uses exclusively circular waveforms as matched filters. We ascertain the likelihood that these filters, though not optimal, will nevertheless be successful at capturing the eccentric signals. We do this by computing the loss in signal-to-noise ratio incurred when searching for eccentric signals with those nonoptimal filters. We show that for a binary system of a given total mass, this loss increases with increasing eccentricity. We show also that for a given eccentricity, the loss decreases as the total mass is increased.Comment: 14 pages, 4 figures, ReVTeX; minor changes made after referee's comment

Pulsar timing arrays and the challenge of massive black hole binary astrophysics

Pulsar timing arrays (PTAs) are designed to detect gravitational waves (GWs) at nHz frequencies. The expected dominant signal is given by the superposition of all waves emitted by the cosmological population of supermassive black hole (SMBH) binaries. Such superposition creates an incoherent stochastic background, on top of which particularly bright or nearby sources might be individually resolved. In this contribution I describe the properties of the expected GW signal, highlighting its dependence on the overall binary population, the relation between SMBHs and their hosts, and their coupling with the stellar and gaseous environment. I describe the status of current PTA efforts, and prospect of future detection and SMBH binary astrophysics.Comment: 18 pages, 4 figures. To appear in the Proceedings of the 2014 Sant Cugat Forum on Astrophysics. Astrophysics and Space Science Proceedings, ed. C.Sopuerta (Berlin: Springer-Verlag

Evolution of supermassive black holes

Supermassive black holes (SMBHs) are nowadays believed to reside in most local galaxies, and the available data show an empirical correlation between bulge luminosity - or stellar velocity dispersion - and black hole mass, suggesting a single mechanism for assembling black holes and forming spheroids in galaxy halos. The evidence is therefore in favour of a co-evolution between galaxies, black holes and quasars. In cold dark matter cosmogonies, small-mass subgalactic systems form first to merge later into larger and larger structures. In this paradigm galaxy halos experience multiple mergers during their lifetime. If every galaxy with a bulge hosts a SMBH in its center, and a local galaxy has been made up by multiple mergers, then a black hole binary is a natural evolutionary stage. The evolution of the supermassive black hole population clearly has to be investigated taking into account both the cosmological framework and the dynamical evolution of SMBHs and their hosts. The seeds of SMBHs have to be looked for in the early Universe, as very luminous quasars are detected up to redshift higher than z=6. These black holes evolve then in a hierarchical fashion, following the merger hierarchy of their host halos. Accretion of gas, traced by quasar activity, plays a fundamental role in determining the two parameters defining a black hole: mass and spin. A particularly intriguing epoch is the initial phase of SMBH growth. It is very challenging to meet the observational constraints at z=6 if BHs are not fed at very high rates in their infancy.Comment: Extended version of the invited paper to appear in the Proceedings of the Conference "Relativistic Astrophysics and Cosmology - Einstein's Legacy

Gravitational Waves from Mergin Compact Binaries: How Accurately Can One Extract the Binary's Parameters from the Inspiral Waveform?

The most promising source of gravitational waves for the planned detectors LIGO and VIRGO are merging compact binaries, i.e., neutron star/neutron star (NS/NS), neutron star/black hole (NS/BH), and black hole/black-hole (BH/BH) binaries. We investigate how accurately the distance to the source and the masses and spins of the two bodies will be measured from the gravitational wave signals by the three detector LIGO/VIRGO network using ``advanced detectors'' (those present a few years after initial operation). The combination ${\cal M} \equiv (M_1 M_2)^{3/5}(M_1 +M_2)^{-1/5}$ of the masses of the two bodies is measurable with an accuracy $\approx 0.1\%-1\%$. The reduced mass is measurable to $\sim 10\%-15\%$ for NS/NS and NS/BH binaries, and $\sim 50\%$ for BH/BH binaries (assuming $10M_\odot$ BH's). Measurements of the masses and spins are strongly correlated; there is a combination of $\mu$ and the spin angular momenta that is measured to within $\sim 1\%$. We also estimate that distance measurement accuracies will be $\le 15\%$ for $\sim 8\%$ of the detected signals, and $\le 30\%$ for $\sim 60\%$ of the signals, for the LIGO/VIRGO 3-detector network.Comment: 103 pages, 20 figures, submitted to Phys Rev D, uses revtex macros, Caltech preprint GRP-36

Massive binary black holes in galactic nuclei and their path to coalescence

Massive binary black holes form at the centre of galaxies that experience a merger episode. They are expected to coalesce into a larger black hole, following the emission of gravitational waves. Coalescing massive binary black holes are among the loudest sources of gravitational waves in the Universe, and the detection of these events is at the frontier of contemporary astrophysics. Understanding the black hole binary formation path and dynamics in galaxy mergers is therefore mandatory. A key question poses: during a merger, will the black holes descend over time on closer orbits, form a Keplerian binary and coalesce shortly after? Here we review progress on the fate of black holes in both major and minor mergers of galaxies, either gas-free or gas-rich, in smooth and clumpy circum-nuclear discs after a galactic merger, and in circum-binary discs present on the smallest scales inside the relic nucleus.Comment: Accepted for publication in Space Science Reviews. To appear in hard cover in the Space Sciences Series of ISSI "The Physics of Accretion onto Black Holes" (Springer Publisher

N-body simulations of gravitational dynamics

We describe the astrophysical and numerical basis of N-body simulations, both of collisional stellar systems (dense star clusters and galactic centres) and collisionless stellar dynamics (galaxies and large-scale structure). We explain and discuss the state-of-the-art algorithms used for these quite different regimes, attempt to give a fair critique, and point out possible directions of future improvement and development. We briefly touch upon the history of N-body simulations and their most important results.Comment: invited review (28 pages), to appear in European Physics Journal Plu

Formation and Evolution of Supermassive Black Holes

The correlation between the mass of supermassive black holes in galaxy nuclei and the mass of the galaxy spheroids or bulges (or more precisely their central velocity dispersion), suggests a common formation scenario for galaxies and their central black holes. The growth of bulges and black holes can commonly proceed through external gas accretion or hierarchical mergers, and are both related to starbursts. Internal dynamical processes control and regulate the rate of mass accretion. Self-regulation and feedback are the key of the correlation. It is possible that the growth of one component, either BH or bulge, takes over, breaking the correlation, as in Narrow Line Seyfert 1 objects. The formation of supermassive black holes can begin early in the universe, from the collapse of Population III, and then through gas accretion. The active black holes can then play a significant role in the re-ionization of the universe. The nuclear activity is now frequently invoked as a feedback to star formation in galaxies, and even more spectacularly in cooling flows. The growth of SMBH is certainly there self-regulated. SMBHs perturb their local environment, and the mergers of binary SMBHs help to heat and destroy central stellar cusps. The interpretation of the X-ray background yields important constraints on the history of AGN activity and obscuration, and the census of AGN at low and at high redshifts reveals the downsizing effect, already observed for star formation. History appears quite different for bright QSO and low-luminosity AGN: the first grow rapidly at high z, and their number density decreases then sharply, while the density of low-luminosity objects peaks more recently, and then decreases smoothly.Comment: 31 pages, 13 figures, review paper for Astrophysics Update