Galaxy formation and evolution

Abstract:

We present interferometru; observations resolving the CO emission in the four gas-rich lenticular galaxies NGC 3 032, NGC 4150, NGC 4459, and NGC 4526, and we compare the CO distribution and kinematics to those of the stars and ionized gas. Counterrotation documents an external origin for the gas in at least one case (NGC 3032), and the comparisons to stellar and ionized gas substructures in all four galaxies offer insights into their formation histories. The molecular gas is found in kpc-scale disks with mostly regular kinematics and average surface densities of 100-200 M⊙ pc -2. The disks are well aligned with the stellar photometric and kinematic axes. In the two more luminous Virgo Cluster members NGC 4459 and NGC 4526 the molecular gas shows excellent agreement with circular velocities derived independently from detailed modeling of stellar kinematic data. There are also two puzzling instances of disagreements between stellar kinematics and gas kinematics on subkiloparsec scales. In the inner arcseconds of NGC 3032 the CO velocities are significantly lower than the inferred circular velocities, and the reasons may possibly be related to the external origin of the gas but are not well understood. In addition, the very young population of stars in the core of NGC 4150 appears to have the opposite sense of rotation from the molecular gas. © 2008. The American Astronomical Society. All rights reserved.

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.