Theoretical astrophysics and plasma physics at RPC

Abstract:

Supermassive black hole binaries (SMBHBs) are expected by the hierarchical galaxy formation model in ACDM 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 super-massive 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. © 2008. The American Astronomical Society. All rights reserved.

Abstract:

We analyze the distribution of G and K type stars towards the Galactic poles using RAVE and ELODIE radial velocities, 2MASS photometric star counts, and UCAC2 proper motions. The combination of photometric and 3D kinematic data allows us to disentangle and describe the vertical distribution of dwarfs, sub-giants and giants and their kinematics. We identify discontinuities within the kinematics and magnitude counts that separate the thin disk, thick disk and a hotter component. The respective scale heights of the thin disk and thick disk are 225 ± 10 pc and 1048 ± 36 pc. We also constrain the luminosity function and the kinematic distribution function. The existence of a kinematic gap between the thin and thick disks is incompatible with the thick disk having formed from the thin disk by a continuous process, such as scattering of stars by spiral arms or molecular clouds. Other mechanisms of formation of the thick disk such as "created on the spot" or smoothly "accreted" remain compatible with our findings. © 2008 ESO.