Catálogo de publicaciones - libros

We discuss the AGN intermittent activity. Accretion onto a supermassive black hole (SMBH) is generally accepted as the basis of AGN activity, however there are still many questions about properties of the accretion flow as well as the fuel supply to the innermost regions of the host galaxy. There is growing evidence that the AGN activity could be intermittent, however the origin of this intermittency has not been identified. Several large scale processes have been discussed including mergers and feedback. However, processes related directly to the accretion disk can influence AGN activity as well. Here we link evolution of the accretion disk to AGN activity and describe different stages of AGN activity and corresponding timescales. This evolution scenario is decoupled from any external fuel supply, although it requires a continuous fuel supply into an accretion region. The amount of radiative energy and the duration of outbursts depend on the model parameters.

A realistic model of the AGN central engine probably should include the effects of the magnetic field. The black hole magnetosphere is studied first by Blandford and Znajek 1977 [1] in the context of winds and jets from radio-loud AGNs. The work has been extended to magnetospheric physics of accreting AGNs by various authors [2–6]. In this case, plasma particles will be to the magnetic field lines, and hence the accreting fluid will fall onto the black hole from regions above the equatorial plane along the field lines (see, e.g., Figure 1 of Ref. [6] and Figure 2 of Ref. [5]). Then, magnetohydrodynamics (MHD) will become important to describe the motion of the particles associated with the background field. Recently the resulting relativistic MHD shocks were explored by our group [5]. In general, the MHD shocks can be hydro-dominated or magneto-dominated [4]. Obviously the MHD case is, however, very complicated. Owing to these complexities, it was not straightforward for us to study the exact global shock-included accreting MHD flow solutions further in detail in a wider parameter/solution space [5]. Therefore, in our most recent paper [7], as a starting point, we investigated the hydrodynamic limit, which should be valid in the case of small magnetization. Our model should apply to the hydro-dominated shocks where the magnetic field does not make a significant contribution to the properties of the shocks under a weak field limit. Here we briefly report on our findings. Further details are found in Fukumura and Tsuruta [7].

The merger of two black holes is one of the most extraordinary events in the natural world. Made of pure gravity, the holes combine to form a single hole, emitting a strong burst of gravitational radiation. Ground-based detectors are currently searching for such bursts from holes formed in the evolution of binary stars, and indeed the very first gravitational wave event detected may well be a black-hole merger. The space-based LISA detector is being designed to search for such bursts from merging massive black holes in the centers of galaxies, events that would emit many thousands of solar masses of pure gravitational wave energy over a period of only a few minutes. To assist gravitational wave astronomers in their searches, and to be in a position to understand the details of what they see, numerical relativists are performing supercomputer simulations of these events. I review here the state of the art of these simulations, what we have learned from them so far, and what challenges remain before we have a full prediction of the waveforms to be expected from these events.

Gravitational waves from the coalescence of binary black holes carry linear momentum, causing center of mass recoil. This “radiation rocket” has important implications for systems with escape speeds of order the recoil velocity. We describe new recoil calculations using high precision black hole perturbation theory to estimate the magnitude of the recoil for the slow “inspiral” coalescence phase; coupled with a cruder calculation for the final “plunge”, we estimate the total recoil imparted to a merged black hole. We find that velocities of many tens to a few hundred km/sec can be achieved fairly easily. The recoil probably never exceeds about 500 km/sec.

Here we report on simulations which, for the first time, simultaneously track star formation and black hole growth, and associated feedback processes, during collisions of galaxies. We show that encounters and mergers of galaxies lead to strong nuclear gas inflows, fueling both powerful starbursts and rapid growth of central black holes. Energetic feedback from accretion onto black holes eventually expels sufficient amount of gas from the merger remnant to quench star formation as well as further black hole growth. At the completion of such an event, the stellar velocity dispersion of the remnant spheroid correlates with the final black hole mass in the manner indicated by observations of the – relation.

We analyze the effect of dissipation on the orbital evolution of supermassive black holes (SMBHs) using high-resolution gasdynamical simulations of binary equal- and unequal-mass mergers of disk dominated galaxies that include the effects of radiative cooling and star formation. We find that equal-mass mergers lead to the formation of a SMBH pair at the center of the remnant with separations limited solely by the adopted force resolution of ~ 100pc. Instead, the final separation of the SMBHs in unequal-mass mergers depends sensitively on how the central structure of the merging galaxies is modified by dissipation. In the absence of dissipation, the companion galaxy is entirely disrupted before the merger is completed leaving its SMBH wandering at a distance too far from the center of the remnant for the formation of a close pair. In contrast, gas cooling facilitates the pairing process by increasing the resilience of the companion galaxy to tidal disruption. Our results suggest that semi-analytic models of hierarchical SMBH growth that neglect the effect of dissipation likely overestimate the number of wandering SMBHs in massive galaxies. The large gas inflows associated with the strong tidal torques during the merger lead to the formation of massive, rotationally supported nuclear disks with sizes, masses and rotational velocities similar to the ones observed spectroscopically for few AGNs and ULRIGs. These disks are gravitationally unstable and likely provide the necessary fuel for feeding the SMBHs and bridging the gap between the large scale flows and the viscous accretion taking place once the gas has reached the AGN accretion disk at subparsec scales.

Using high-resolution SPH numerical simulations, we investigate the effects of gas on the inspiral and merger of a massive black hole binary. This study is motivated by the very massive nuclear gas disks observed in the central regions of merging galaxies. Here we present results that expand on the treatment in a previous work [1], by studying more realistic models in which the gas is in a disk with significant clumpiness. In the variety of simulations that we perform, we find that gravitational drag is able to reduce the separation to distances where gravitational radiation is efficient in a timescale that varies between 5× 10 yr and 2.5× 10 yr.

The dynamic evolution of binary systems of supermassive black holes (SMBH) may be a key factor affecting a large fraction of the observed properties of active galactic nuclei (AGN) and galaxy evolution. Different classes of AGN can be related in general to four evolutionary stages in a binary SMBH: 1) early merger stage; 2) wide pair stage; 3) close pair stage; and 4) pre-coalescence stage.

We compute the expected gravitational wave (GW) signal from coalescing massive black hole (MBH) binaries in a hierarchical structure formation scenario based on the standard CDM cosmology. The integrated emission from the MBH binaries results in a gravitational wave background (GWB). We discuss observability of such GWB by planned Laser Interferometer Space Antenna () [1,2].

Many clusters have surface brightness distributions that are regular with strong peaks on a bright, central, often cD, galaxy. In the Einstein X-ray survey of 215 clusters, 64% of clusters contained a bright X-ray peak, centered on an optically bright galaxy [27]. These systems, described as -ray ominant (XD) clusters, are those that have high central gas densities and hence short cooling times. In the absence of energy input, the most remarkable XD systems are calculated to be depositing mass at rates as high as 1000 M yr  (e.g., [14]). For many years, these centrally peaked, XD clusters have been described with a “cooling flow” model, first developed by Fabian & Nulsen [16] and Cowie & Binney [11]. However, observations with XMM-Newton have shown that mass deposition rates in cooling flow clusters, although still significant, are at least five times smaller than expected in the standard model ([37] and references therein). This requires considerable energy input to compensate for radiative losses.