Theoretical astrophysics and plasma physics at RPC

Abstract:

We show that the usual relation between redshift and angular-diameter distance can be obtained by considering light from a source to be gravitationally lensed by material that lies in the telescope beam as it passes from source to observer through an otherwise empty universe. This derivation yields an equation for the dependence of angular diameter on redshift in an inhomogeneous universe. We use this equation to model the distribution of angular-diameter distance for redshift z=3 in a realistically clustered cosmology. The distribution is such that attempts to determine q_0 from angular-diameter distances will systematically underestimate q_0 by ~0.15, and large samples would be required to beat down the intrinsic dispersion in measured values of q_0.

Abstract:

We study the persistence of warps in galactic discs in the presence of massive halos. A disc is approximated by a set of massive rings, while a halo is represented by a conventional n-body simulation. We confirm the conclusion of Nelson & Tremaine (1995) that a halo responds strongly to an embedded precessing disc. This response invalidates the approximations made by in the derivation of classical `modified tilt' modes. We show that the response of the halo causes the line of nodes of a disc that starts from a modified tilt mode to wind up within a few dynamical times. We explain this finding in terms of the probable spectrum of true normal modes of a combined disc-halo system.

Abstract:

T The stellar kinematics of the low-luminosity elliptical galaxy NGC 4486B have been measured in seeing s∗5 0-22 with the Canada-France-Hawaii Telescope and Subarcsecond Imaging Spectrograph. Lauer and collaborators have shown that NGC 4486B is similar to M31 in having a double nucleus. Here we show that it also resembles M31 in its kinematics. Like M31, NGC 4486B rotates fairly rapidly near the center (V 5 76 H 7 kms21 at 0-6) but more slowly farther out (V 3 20 H 6 km s21 at r 3 40). Also, the velocity dispersion gradient is very steep: s increases from 116 H 6 km s21 at r 5 20-60 to s 5 281 H 11 km s21 at the center. This is much higher than expected for an elliptical galaxy of absolute magnitude MB 3 216.8: even more than M31, NGC 4486B is far above the scatter in the Faber-Jackson correlation between s and bulge luminosity. Therefore, the King core mass-to-light ratio, M/LV 3 20, is unusually high compared with normal values for old stellar populations (M/LV 5 4 H 1 at MB 3 217). We construct simple dynamical models with isotropic velocity dispersions and show that they reproduce black hole (BH) masses derived by more detailed methods. We also fit axisymmetric, three-integral models. Isotropic models imply that NGC 4486B contains a central dark object, probably a BH, of mass MF 5 622 13 3 108 MJ. However, anisotropic models fit the data without a BH if the ratio of radial to azimuthal dispersions is 12 at r 3 10. Therefore, this is a less strong BH detection than the ones in M31, M32,and NGC 3115. A dark mass of 6 3 108 MJ is 19% of the mass Mbulge in stars; even if MF is somewhat smaller than the isotropic value, MF/Mbulge is likely to be unusually large. Double nuclei are a puzzle because the dynamical friction timescales for self-gravitating star clusters in close orbit around each other are short. Since both M31 and NGC 4486B contain central dark objects, our results support models in which the survival of a double nucleus is connected with the presence of a BH. For example, they support the Keplerian eccentric disk model due to Tremaine.