By combining radio and X-ray observations, astronomers have obtained their
most detailed view yet of the effects a powerful galactic jet has as it
blasts its way through stars and gas on its way out from the centre of a
galaxy. New observations of our nearest neighbouring radio galaxy, Centaurus
A, (10 million light years away in the southern sky) show intense X-rays in
places where the fast-moving jet is apparently running into the gas and
stars that make up the galaxy. Because Centaurus A is the nearest radio
galaxy, it is a key object for understanding how all other radio galaxies
work. Results from the research will be presented at the UK/Ireland National
Astronomy Meeting in Dublin by Dr Martin Hardcastle of the University of
Bristol.

Only a small minority of galaxies have powerful jets of electrically charged
particles but, where they are present, they can have profound effects on the
galaxies they inhabit. They are believed to come from close to a central
massive black hole and in many cases they extend for hundreds of thousands
of light years. Astronomers have been aware of radio emission and visible
light from such jets for many years, but more recently, scientists using the
orbiting observatory Chandra have discovered that X-ray emission from jets
is also common. The X-rays come from electrons carrying large amounts of
energy – comparable with the energies reached in the accelerators used in
particle physics experiments on Earth.

The team working on Centaurus A, led by Dr Hardcastle, a Royal Society
Research Fellow at the University of Bristol, and Dr Ralph Kraft, of the
Harvard-Smithsonian Center for Astrophysics in the USA, observed its jet
with both Chandra and the Very Large Array radio telescope in New Mexico.
The radio observations, taken between 1991 and 2002, showed that parts of
the jet are moving away from the centre of the galaxy at speeds of about
half the speed of light. However, the regions of the jet that are emitting
the most X-rays were stationary. The interpretation the team put on this
finding is that the stationary regions are where the jet is stalled when it
encounters clouds of gas or peculiar stars. The X-ray emission would be
produced by the powerful shock generated as the fast jet flow runs into the
stationary material. If this is true, it would be the first time that
interactions in a jet have been seen in so much detail.

The team believe that their research will have implications for the study of
other, more distant galactic jets. “Because Cen A is the closest radio
galaxy to us, and we can image its jet so clearly, it’s a key object,” said
Dr Hardcastle. “We really need to understand it if we’re going to understand
any of the others.”

IMAGE

A picture showing the radio and X-ray images of the inner 4,000 light years
of the jet in Cen A can be found at
http://www.star.bris.ac.uk/~mjh/cena_press/

CONTACT

Dr Martin Hardcastle, University of Bristol
Phone: (+44) (0)117 928 9051 Fax: (+44) (0)117 925 5624
E-mail: M.Hardcastle@bristol.ac.uk

Dr Hardcastle will be at the NAM in Dublin 8 – 11 April

NOTES

1. The full Centaurus A collaboration comprises Drs Martin Hardcastle and
Diana Worrall of the University of Bristol and Drs Ralph Kraft, Steve
Murray, Bill Forman and Christine Jones of the Harvard-Smithsonian Center
for Astrophysics.

2. A paper giving more details of the team’s findings is shortly to appear
in the Astrophysical Journal.

3. This research was funded by the Royal Society, PPARC, and NASA.