Single Supermassive Black Holes Research Articles

Tidal Disruption of Stellar Objects by Hard Supermassive Black Hole Binaries

Supermassive black hole binaries (SMBHBs) are expected by the hierarchical galaxy formation model in ΛCDM cosmology. There is some evidence in the literature for SMBHBs in active galactic nuclei, but there are few observational constraints on the evolution of SMBHBs in inactive galaxies and gas-poor mergers. On the theoretical front, it is unclear how long is needed for a SMBHB in a typical galaxy to coalesce. In this paper we investigate the tidal interaction between stars and binary black holes (BHs) and calculate the tidal disruption rates of stellar objects by the BH components of the binary. We derive the interaction cross sections between SMBHBs and stars from intensive numerical scattering experiments with particle number ~107 and calculate the tidal disruption rates by both single and binary BHs for a sample of realistic galaxy models, taking into account the general relativistic effects and the loss-cone refilling because of two-body interaction. We estimate the frequency of tidal flares for different types of galaxies using the BH mass function in the literature. We find that because of the three-body slingshot effect, the tidal disruption rate in the SMBHB system is more than 1 order of magnitude smaller than that in a single supermassive black hole (SMBH) system. The difference is more significant in less massive galaxies and does not depend on detailed stellar dynamical processes. Our calculations suggest that comparisons of the calculated tidal disruption rates for both single and binary BHs and the surveys of X-ray or UV flares at galactic centers could tell us whether most SMBHs in nearby galaxies are single and whether the SMBHBs formed in gas-poor galaxy mergers coalesce rapidly.

The tidal disruption rate in dense galactic cusps containing a supermassive binary black hole

We consider the problem of tidal disruption of stars in the centre of a galaxy containing a supermassive binary black hole with unequal masses. We assume that over the separation distance between the black holes, the gravitational potential is dominated by the more massive primary black hole. Also, we assume that the number density of stars is concentric with the primary black hole and has a power-law cusp. We show that the bulk of stars with a small angular-momentum component normal to the black hole binary orbit can reach a small value of total angular momentum through secular evolution in the gravitational field of the binary, and hence they can be tidally disrupted by the larger black hole. This effect is analogous to the so-called Kozai effect well known in celestial mechanics. We develop an analytical theory for the secular evolution of the stellar orbits and calculate the rate of tidal disruption. We compare our analytical theory with a simple numerical model and find very good agreement. Our results show that for a primary black hole mass of ∼ 1.0 6 -10 7 M ○. , the black hole mass-ratio q > 10 -2 , cusp size ∼1 pc, the tidal disruption rate can be as large as ∼10 -2 -1 M ○. yr -1 . This is at least 10 2 -10 4 times larger than estimated for the case of a single supermassive black hole. The duration of the phase of enhanced tidal disruption is determined by the dynamical-friction time-scale, and it is rather short: ∼10 5 yr. The dependence of the tidal disruption rate on the mass ratio, and on the size of the cusp, is also discussed.

The Galactic Center stellar cluster: The central arcsecond

With 10 years of high-resolution imaging data now available on the stellar cluster in the Galactic Center, we analyze the dynamics of the stars at projected distances ≤1.2″ from the central black hole candidate Sagittarius A* (Sgr A*). We find evidence for radial anisotropy of the cluster of stars surrounding Sgr A*. We confirm/find accelerated motion for 6 stars, with 4 stars having passed the pericenter of their orbits during the observed time span. We calculated/constrained the orbital parameters of these stars. All orbits have moderate to high eccentricities. The center of acceleration coincides with the radio position of Sgr A*. From the orbit of the star S2, currently the most tightly constrained one, we determine the mass of Sgr A* to 3.3 ± 0.7 × 106 M⊙ and its position to 2.0 ± 2.4 mas East and 2.7 ± 4.5 mas South of the nominal radio position. The data provide compelling evidence that Sgr A* is a single supermassive black hole.