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Register a new userDynamics of a Superdense Cluster of Black Holes and the Formation of the Galactic SMBH
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Added by
Roberto Capuzzo-Dolcetta
Publication date
2021
fields
Physics
and research's language is
English
Authors
P. Chassonnery Dep. ofn Physics
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The center of our Galaxy is known to host a massive compact object, Sgr A$^*$, which is commonly considered as a super-massive black hole of $sim 4times 10^6$ M$_odot$. It is surrounded by a dense and massive nuclear star cluster, with a half mass radius about $5$~pc and a mass larger than $10^{7}$ M$_odot$. In this paper we studied the evolutionary fate of a very dense cluster of intermediate mass black holes, possible remnants of the dissipative orbital evolution of massive globular cluster hosts. We performed a set of high precision $N$-body simulations taking into account deviations from pure Newtonian gravitational interaction via a Post Newtonian development up to $2.5$ order, which is the one accounting for energy release by gravitational wave emission. The violent dynamics of the system leads to various successive merger events such to grow a single object containing $sim 25$ per cent of the total cluster mass before partial dispersal of the cluster, and such to generate, in different bursts, a significant quantity of gravitational waves emission. If generalized, the present results suggest a mechanism of mass growth up to the scale of a super massive black hole.
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The spin angular momentum S of a supermassive black hole (SBH) precesses due to torques from orbiting stars, and the stellar orbits precess due to dragging of inertial frames by the spinning hole. We solve the coupled post-Newtonian equations describing the joint evolution of S and the stellar angular momenta Lj, j = 1...N in spherical, rotating nuclear star clusters. In the absence of gravitational interactions between the stars, two evolutionary modes are found: (1) nearly uniform precession of S about the total angular momentum vector of the system; (2) damped precession, leading, in less than one precessional period, to alignment of S with the angular momentum of the rotating cluster. Beyond a certain distance from the SBH, the time scale for angular momentum changes due to gravitational encounters between the stars is shorter than spin-orbit precession times. We present a model, based on the Ornstein-Uhlenbeck equation, for the stochastic evolution of star clusters due to gravitational encounters and use it to evaluate the evolution of S in nuclei where changes in the Lj are due to frame dragging close to the SBH and to encounters farther out. Long-term evolution in this case is well described as uniform precession of the SBH about the clusters rotational axis, with an increasingly important stochastic contribution when SBH masses are small. Spin precessional periods are predicted to be strongly dependent on nuclear properties, but typical values are 10-100 Myr for low-mass SBHs in dense nuclei, 100 Myr - 10 Gyr for intermediate mass SBHs, and > 10 Gyr for the most massive SBHs. We compare the evolution of SBH spins in stellar nuclei to the case of torquing by an inclined, gaseous accretion disk.
Chandrasekhars most important contribution to stellar dynamics was the concept of dynamical friction. I briefly review that work, then discuss some implications of Chandrasekhars theory of gravitational encounters for motion in galactic nuclei.
We calculate the exact formation probability of primordial black holes generated during the collapse at horizon re-entry of large fluctuations produced during inflation, such as those ascribed to a period of ultra-slow-roll. We show that it interpolates between a Gaussian at small values of the average density contrast and a Cauchy probability distribution at large values. The corresponding abundance of primordial black holes may be larger than the Gaussian one by several orders of magnitude. The mass function is also shifted towards larger masses.
Primordial black hole (PBH) mergers have been proposed as an explanation for the gravitational wave events detected by the LIGO collaboration. Such PBHs may be formed in the early Universe as a result of the collapse of extremely rare high-sigma peaks of primordial fluctuations on small scales, as long as the amplitude of primordial perturbations on small scales is enhanced significantly relative to the amplitude of perturbations observed on large scales. One consequence of these small-scale perturbations is generation of stochastic gravitational waves that arise at second order in scalar perturbations, mostly before the formation of the PBHs. These induced gravitational waves have been shown, assuming gaussian initial conditions, to be comparable to the current limits from the European Pulsar Timing Array, severely restricting this scenario. We show, however, that models with enhanced fluctuation amplitudes typically involve non-gaussian initial conditions. With such initial conditions, the current limits from pulsar timing can be evaded. The amplitude of the induced gravitational-wave background can be larger or smaller than the stochastic gravitational-wave background from supermassive black hole binaries.
We derive the first constraints on the time delay distribution of binary black hole (BBH) mergers using the LIGO-Virgo Gravitational-Wave Transient Catalog GWTC-2. Assuming that the progenitor formation rate follows the star formation rate (SFR), the data favor that $43$--$100%$ of mergers have delay times $<4.5$ Gyr (90% credibility). Adopting a model for the metallicity evolution, we derive joint constraints for the metallicity-dependence of the BBH formation efficiency and the distribution of time delays between formation and merger. Short time delays are favored regardless of the assumed metallicity dependence, although the preference for short delays weakens as we consider stricter low-metallicity thresholds for BBH formation. For a $p(tau) propto tau^{-1}$ time delay distribution and a progenitor formation rate that follows the SFR without metallicity dependence, we find that $tau_mathrm{min}<2.2$ Gyr, whereas considering only the low-metallicity $Z < 0.3,Z_odot$ SFR, $tau_mathrm{min} < 3.0$ Gyr (90% credibility). Alternatively, if we assume long time delays, the progenitor formation rate must peak at higher redshifts than the SFR. For example, for a $p(tau) propto tau^{-1}$ time delay distribution with $tau_mathrm{min} = 4$ Gyr, the inferred progenitor rate peaks at $z > 3.9$ (90% credibility). Finally, we explore whether the inferred formation rate and time delay distribution vary with BBH mass.
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