Other
Scientific paper
Jun 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986apj...305..109b&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 305, June 1, 1986, p. 109-130.
Other
51
Astronomical Models, Brightness Distribution, Galactic Structure, Plasma Jets, Surface Properties, Turbulent Jets, Adiabatic Flow, Density Distribution, Depolarization, Faraday Effect, Hubble Constant, Integral Equations, Mach Number, Shock Waves
Scientific paper
The approach, initiated in the first paper in this series, of using jet spreading rate data to infer the velocity variation of extragalactic jets is generalized to jets of arbitrary density ratio. The dissipative heating of the jet plasma by turbulence is neglected and the circumstances under which the assumption is valid are examined. In this adiabatic model the surface brightness variation of a Fanaroff-Riley class I jet is due to the compression and expansion of plasma and magnetic field as the jet decelerates and accelerates under the opposing influences of entrainment and buoyancy. The model is applied to the main jet in NGC 315, and it is shown that good fits to the surface brightness of this jet can be obtained from low Mach number, low density ratio models with the temperature of the background medium approximately equal to the virial temperature. For Mach numbers at the field transition point > 1.5 dissipation could be as important as adiabatic processes in determining the surface brightness in the first 50". An estimate of the amount of dissipation occurring along the length of the jet shows that this is comparable to the radiative losses. The inferred variations of velocity. Mach number, density ratio, and mass flux together with their correlations with the pressure gradient provide an appealing simple explanation for the collimation behavior of this jet. The initial expansion of the jet is due to turbulence but the jet becomes laminar due to its increasing Mach number and density ratio and the favorable pressure gradient. It then collimates until the pressure starts to flatten out to the background pressure of the IGM and is no longer favorable to laminar flow. This causes the jet to become turbulent again, reexpand, decelerate, and maintain an approximately constant surface brightness. A corollary of the theory is that the parallel component of magnetic field arises from the processing of an initially perpendicular magnetic field through oblique shocks and strong shear in the dissipative region where the jet "turns on" close to the core. A likely range of velocities of this jet at the field transition point 24" from the core is between 1600 and 3800km s-1 and the maximum possible velocity is ˜ 16000km s-1. An important implication of the model is that NGC 315 does not have a massive dark halo and that its atmosphere is confined by a combination of the potential associated with the luminous matter and the pressure of the surrounding IGM. If the physics of jet propagation envisaged in this model is qualitatively correct, then the surface brightness variations of other long jets in class I radio sources could be used to determine whether the associated giant ellipticals possess massive dark halos.
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