R.A.LaingSpace Science and Technology Department,CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX and University of Oxford, Department of Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH
A.H.BridleNational Radio Astronomy Observatory520 Edgemont Road, Charlottesville, VA 22903-2475, U.S.A. Monthly Notices of the Royal Astronomical Society, in press |
![]() 3C31: Radio jets at 3.6cm (red) on WFPC2 optical image (blue) |
We present a dynamical analysis of the flow in the jets of the low-luminosity
radio galaxy 3C31 based on our earlier geometrical and kinematic model
(Laing and Bridle 2002) and on estimates of
the external pressure and density distributions
from Chandra observations (Hardcastle et al. 2002). We apply conservation of
particles, energy and momentum to derive the variations of pressure and density
along the jets and show that there are self-consistent solutions for
deceleration by injection of thermal matter. We initially take the jets to be
in pressure equilibrium with the external medium at large distances from the
nucleus and the momentum flux to be =
/c, where
is the energy
flux; we then progressively relax these constraints. With our initial
assumptions, the energy flux is well determined:
9 -- 14 x
1036 W. We infer that the jets are over-pressured compared with the
external medium at the flaring point (1.1 kpc from the nucleus) where they
start to expand rapidly. Local minima in the density and pressure and maxima in
the mass injection rate and Mach number occur at
3 kpc. Further out,
the jets decelerate smoothly with a Mach number
1. The mass injection
rate we infer is comparable with that expected from stellar mass loss throughout
the cross-section of the jet close to the flaring point, but significantly
exceeds it at large distances. We conclude that entrainment from the galactic
atmosphere across the turbulent boundary layer of the jet is the dominant mass
input process far from the nucleus, but that stellar mass loss may also
contribute near the flaring point. The occurrence of a significant
over-pressure at the flaring point leads us to suggest that it is the site of a
stationary shock system, perhaps caused by reconfinement of an initially free
jet. Our results are compatible with a jet consisting of e-e+ plasma on
parsec scales which picks up thermal matter from stellar mass loss to reach the
inferred density and mass flux at the flaring point, but we cannot rule out an
e-p+ composition with a low-energy cut-off.