Theoretical astrophysics and plasma physics at RPC

Abstract:

Techniques for the construction of dynamical Galaxy models should be considered essential infrastructure that should be put in place before Gaia flies. Three possible modelling techniques are discussed. Although one of these seems to have significantly more potential than the other two, at this stage work should be done on all three. A major effort is needed to decide how to make a model consistent with a catalogue such as that which Gaia will produce. Given the complexity of the problem, it is argued that a hierarchy of models should be constructed, of ever increasing complexity and quality of fit to the data. The potential that resonances and tidal streams have to indicate how a model should be refined is briefly discussed.

Abstract:

The effect of a third, active electrode placed inside the ceramic chamber of a Hall thruster is analyzed. Both electron-collecting and electron-emitting modes are considered. Significant efficiency enhancement with respect to the single-stage operation can be obtained for a good electron-emitting electrode, placed in an intermediate location of the acceleration region, and for an anode-to-electrode (inner-stage) potential significantly larger than the ionization potential. Optimum values of the electrode location and voltage are determined. The performance improvement is due to a reduction of the energy losses to the chamber walls. This is the consequence of lower Joule heating and thus lower electron temperature in the outer stage. When the ionization process is efficient already in the single-stage operation, (i) two-stage operation does not affect practically the propellant and voltage utilizations and (ii) thrust efficiency decreases when the intermediate electrode works as an electron collector. © 2005 American Institute of Physics.

Abstract:

This is the second in a series of papers dedicated to unveiling the mass structure and orbital content of a sample of flattened early-type galaxies in the Coma cluster. The ability of our orbit libraries to reconstruct internal stellar motions and the mass composition of a typical elliptical in the sample is investigated by means of Monte Carlo simulations of isotropic rotator models. The simulations allow a determination of the optimal amount of regularization needed in the orbit superpositions. It is shown that under realistic observational conditions and with the appropriate regularization, internal velocity moments can be reconstructed to an accuracy of ≈15 per cent; the same accuracy can be achieved for the circular velocity and dark matter fraction. In contrast, the flattening of the halo remains unconstrained. Regularized orbit superpositions are applied to a first galaxy in our sample, NGC 4807, for which stellar kinematical observations extend to 3 r eff. The galaxy seems dark-matter dominated outside r > 2 r eff. Logarithmic dark matter potentials are consistent with the data, as well as NFW profiles, mimicking logarithmic potentials over the observationally sampled radial range. In both cases, the derived stellar mass-to-light ratio Υ agrees well with independently obtained mass-to-light ratios from stellar population analysis. The achieved accuracy is ΔΥ ≈ 0.5. Kinematically, NGC 4807 is characterized by mild radial anisotropy outside r > 0.5 r eff, becoming isotropic towards the centre. Our orbit models hint at either a distinct stellar component or weak triaxiality in the outer parts of the galaxy. © 2005 RAS.

Abstract:

We investigate the central regions of 23 spiral galaxies using Space Telescope Imaging Spectrograph (STIS) spectroscopy and archival Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) imaging. The sample is taken from our program to determine the masses of central massive black holes (MBHs) in 54 nearby spiral galaxies. Stars are likely to contribute significantly to any dynamical central mass concentration that we find in our MBH program, and this paper is part of a series to investigate the nuclear properties of these galaxies. We use the Nuker law to fit surface brightness profiles, derived from the NICMOS images, to look for nuclear star clusters and find possible extended sources in three of the 23 galaxies studied (13%). The fact that this fraction is lower than that inferred from optical Bubble Space Telescope studies is probably due to the greater spatial resolution of those studies. Using R - H and J - H colors and equivalent widths of Hα emission (from the STIS spectra), we investigate the nature of the stellar population with evolutionary models. Under the assumption of hot stars ionizing the gas, as opposed to a weak active galactic nucleus (AGN), we find that there are young stellar populations (∼ 10-20 Myr); however, these data do not allow us to determine what percentage of the total nuclear stellar population they form. In addition, in an attempt to find any unknown AGN, we use [N II] and [S II] line flux ratios (relative to Hα) and find tentative evidence for weak AGNs in NGC 1300 and NGC 4536. © 2005. The American Astronomical Society. All rights reserved.