arXiv subjects

Geoffrey V. Bicknell

Publications and source records attributed to Geoffrey V. Bicknell.

40 records · Page 3Linked to original sources

arXiv1996

A Varying Mass-To-Light Ratio in the Galactic Center Cluster?

We reanalyze published kinematic and photometric data for the cool star population in the central 10 pc (240 arcsec) of the Galaxy, while (a) isolating the photometric data appropriate to this population, and (b) properly allowing for projection effects. Under the assumptions that the system is spherical and isotropic, we find that M/L_K varies from <=1 outside a radius of 0.8 pc to >2 at 0.35 pc. We suggest that such a varying M/L_K could be due to an increasing concentration of stellar remnants towards the Galactic center. Our derived mass-radius curve confirms the existence of ~3x10^6 M_sol within 0.35 pc of the Galactic Center, and ~1.5x10^6 M_sol with 0.2 pc. However, the latter estimate is subject to the uncertain distribution of cool stars in the region. We also consider the dynamics of the hot star population close to the Galactic center and show that the velocity dispersion of He I stars and the surface brightness distribution of the hot stars are consistent with the mass distribution inferred from cool stars.

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arXiv1994

Relativistic Jets and the Fanaroff-Riley Classification of Radio Galaxies

Recently, Owen(1993) and Owen and Ledlow (1994) have shown that the dividing line between Fanaroff-Riley class~I and class~II Radio Sources is very sharp when the sources are represented as points in the radio-optical luminosity plane. It is shown that if one accepts the propositions that the sources in the vicinity of this dividing line are characterized by (1) transonic Mach numbers and (2) mildly relativistic velocities (consistent with deceleration of an initially moderately relativistic or ultrarelativistic jet), then the slope of the dividing line is readily obtained using simple physical arguments and established empirical relationships between the X-Ray luminosities, core radii, velocity dispersions and absolute magnitudes of elliptical galaxies. The intercept of the dividing line depends upon parameters which are known perhaps to within factors of order unity and agrees with the data to within an order of magnitude. ROSAT observations of elliptical galaxies will be important in constraining the central pressures and X-Ray core radii of radio ellipticals. Knowledge of these two parameters will assist in a more detailed assessment of the physics which is proposed here as being relevant. High resolution observations of jets within a kpc of the radio core will also be useful for determining the spreading rates of jets in this region.

astro-ph↗

arXiv1993

Beamed Radiation in Radio Galaxies

This paper focuses on two aspects of the Physics of Radio Galaxies in which it has been proposed that relativistic beaming of radiation is necessary to account for the observations. Physical arguments are presented which show that it is quite plausible that the jets in FRI radio galaxies are relativistic on the parsec scale and decelerate to subrelativistic flow on the kiloparsec scale. On the other hand it is shown that beaming of the optical continuum from the core of Centaurus~A is not necessary to account for the excitation of the emission line filaments in the ISM of this galaxy. Models involving autoionizing shocks produced by the interaction between a low Mach number jet and dense clouds along its path are capable of explaining the morphology of the filaments and the simultaneous high excitation and low excitation lines in their spectra.

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arXiv1993

On the Relationship between BL-Lacertae Objects and FRI Radio Galaxies

The idea that Fanaroff-Riley Class~I Radio Galaxies contain core jets with Lorentz factors of the order of a few and are the parent population for BL~Lac Objects is examined, with particular reference to the data on two FRI Radio Galaxies, NGC~315 and NGC~6251. Conservation laws for an entraining relativistic jet are derived and are used to determine a relationship between Mach number and velocity for initially relativistic jets. One immediate consequence of this relationship is that, if an initially relativistic jet is decelerated to a transonic Mach number between, say 0.5 and 1.5, then its velocity at this point is between 0.3 and 0.7 times the speed of light. Analysis of the milliarcsecond and arcsecond data on NGC~315 and NGC~6251 shows that the Lorentz factors of the core jets in these galaxies can be as high as 2-4 provided that the jet pressures are not much more than an order of magnitude higher than the minimum pressures. Such Lorentz factors are more plausible if the jets are inclined at $\sim 30$ degrees to the line of sight, although larger angles would also be possible. For core Lorentz factors in the range 2-4, the jets are mildly relativistic on the large scale and their velocities are consistent with the jet brightness asymmetries being caused by Doppler beaming. The Lorentz factors required for significant beaming of core jets may be reconciled with subluminal motions of knots in the core if these are reverse shocks advected by the jet. Moderate jet Lorentz factors are possible if the shocks are normal and higher Lorentz factors are possible if the shocks are oblique. This paper provides support for the idea that the transition from two-sided to one-sided jets across the FRI/II break is due to the transition from

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