Jet-Intracluster Medium interaction in Hydra A. I Estimates of jet
  velocity from inner knots
  We present the first stage of an investigation of the interactions of the
jets in the radio galaxy Hydra A with the intracluster medium. We consider the
jet kinetic power, the galaxy and cluster atmosphere, and the inner structure
of the radio source. Analysing radio observations of the inner lobes of Hydra A
by Taylor et al. (1990) we confirm the jet power estimates of about 1e45 ergs/s
derived by Wise et al. (2007) from dynamical analysis of the X-ray cavities.
With this result and a model for the galaxy halo, we explore the
jet-intracluster medium interactions occurring on a scale of 10 kpc using
two-dimensional, axisymmetric, relativistic pure hydrodynamic simulations. A
key feature is that we identify the three bright knots in the northern jet as
biconical reconfinement shocks, which result when an over pressured jet starts
to come into equilibrium with the galactic atmosphere. Through an extensive
parameter space study we deduce that the jet velocity is approximately 0.8 c at
a distance 0.5 kpc from the black hole. The combined constraints of jet power,
the observed jet radius profile along the jet, and the estimated jet pressure
and jet velocity imply a value of the jet density parameter approximately 13
for the northern jet. We show that for a jet velocity = 0.8c and angle between
the jet and the line of sight = 42 deg, an intrinsic asymmetry in the
emissivity of the northern and southern jet is required for a consistent
brightness ratio approximately 7 estimated from the 6cm VLA image of Hydra A.
