In the optical, the double lobed Fanaroff Riley type II
(FR II) radio
galaxy Cygnus A (or 3C405, 1957+405) has been identified with a giant
elliptical cD galaxy at a luminosity distance of about 170 Megaparsec
(z = 0.0562, assuming H0 = 100 km/sec/Mpc). The three images show the
radio morphology of Cygnus A on arcsecond (top) and sub-milliarcsecond
(bottom) scales. The technique of radio interferometry allows to trace
and study the jets from scales of hundred thousand of light years down
to scales of light days.
The upper image was made with the VLA at wavelength
lambda = 6 cm (frequency nu = 5 GHz) (Perley et al. 1984,
Astrophysical Journal 285, L35), and shows two faint jets
emerging from the central nucleus, feeding the two extended radio lobes
and embedded hot spots. On the sky, the separation of the radio lobes
is 130 arcseconds, which translates into a spatial separation of about
100 kiloparsec.
The image in the middle show a VLBI map of the inner
jets of Cygnus A made
with a global VLBI array of 13 stations (including the 100m Effelsberg
radio telescope, the phased VLA and the VLBA) at lambda = 1.3 cm (nu =
22 Gigahertz). Clearly visible is a two-sided wiggly jet, emerging
symmetrically from an unresolved central core. The overall length of
the jet and counter-jet shown in this image is 100 milli-arcseconds,
corresponding to 74 parsec. Repeated VLBI imaging between 1992 and 1994
allowed to determine the jet kinematics and from this the inclination
of the jets with respect to the observer's line
of sight. Jet velocities in the range of 0.1 to 0.7 c (velocity of
light) are
found (see Krichbaum et al. 1998, Astronomy and Astrophysics 329,
873).
The apparent acceleration with slower speeds near the core and higher
speeds further out
was interpreted by phase velocirties, which are related to
Kelvin-Helmholtz instabilities in the highly relativistic jet. The jet
itself is inclined by 70 to 90 degrees relative to the observer, lying
almost in the sky plane.
The VLBI image at the bottom shows the inner 3
milli-arcseconds with an angular resolution of 0.15 milli-arcseconds,
corresponding to a spatial resolution of 0.11 parsec (or 130 light
days!). The map was made with an array of 8 stations (including the
100m radio telescope) operating at the short wavelength of lambda = 7
mm (nu = 43 Gigahertz). Comparison of the brightness ratio between
western jet and eastern counter-jet at the different wavelengths yields
strong evidence for a gaseous torus of absorbing material surrounding
the core,
which partially blocks the radiation from the counter-jet, but not from
the jet. Future VLBI observations at millimeter wavelength wil allow to
study the base of the jet and possibly the central engine with
even higher resolution. The accurate determination of a possible
velocity difference between jet and counter-jet could even facilitate a
measurement of the distance of Cygnus A independent from the Hubble
constant.
Thomas
Krichbaum et al., 1998, A&A 329, 873, Copyright MPIfR)
(VLA Image: Rick Perley et al. 1984, ApJ 285, L35,
Copyright NRAO)
public_at_mpifr-bonn.mpg.de
