Black hole
'bully' blasts galaxy
By Paul Rincon
Science reporter, BBC News
Infographic, BBC
A powerful jet of particles from a "supermassive" black hole has
been seen blasting a nearby galaxy, according to the US space
agency (Nasa).
Galaxies have been seen colliding before, but this form of galactic
violence is rarely witnessed by astronomers.
The jet could have a profound effect on any planets in the jet's
path and could also trigger a burst of star formation.
The findings are to be published in the Astrophysical
Journal.
They were obtained using Nasa's space-based Chandra X-ray
Observatory, its Hubble Space Telescope, and Spitzer Space
Telescope, as well as the Very Large Array (VLA) and Merlin radio
telescopes on the ground.
The event took place in a system called 3C321, which lies 1.4
billion light-years from Earth. It contains two galaxies in orbit
around one another which are in the process of merging.
Most, if not all, galaxies - including our own Milky Way - are
thought to host supermassive black holes at their galactic centres.
A handful of these galaxies eject powerful jets from the vicinities
of their black holes, and are known as radio galaxies - because
jets are very "visible" at radio wavelengths.
The larger of the two galaxies in 3C321 - dubbed the "death star
galaxy" by the astronomers - has a jet emanating from the vicinity
of the black hole at its centre. The unfortunate smaller galaxy has
apparently swung into the jet's line of fire.
Destructive force
A bright spot in some images shows where the jet has slammed into
the side of the companion galaxy, dissipating some of its energy.
After striking it, the jet has become disrupted and
deflected.
Jets can race out at close to the speed of light and can travel
vast distances. The jet in 3C321 was about 1,000 light-years across
and might have travelled one or two million light-years from its
origin.
These jets consist of high energy particles and magnetic fields.
They produce enormous amounts of radiation, especially in the form
of high-energy X-rays and gamma-rays.
"This jet could be causing all sorts of problems for the smaller
galaxy it is pummeling," said Dan Evans, lead author from the
Harvard-Smithsonian Center for Astrophysics in Cambridge, US.
This type of event is rarely seen. But astronomers Steve Croft and
Wil van Breugel have observed a gas-rich jet in setting off star
birth in an arc of stars known as Minkowski's Object.
The combined effects of radiation and particles travelling at
almost the speed of light could have disastrous consequences for
the atmospheres of any Earth-like planets lying in the path of the
jet.
For example, protective layers of ozone in the planet's upper
atmosphere could be destroyed, which could result in the mass
extinction of any life that had evolved on the planet.
Neil Tyson, director of the Hayden Planetarium in New York
commented: "Black holes are famous for wreaking havoc on their
environment. This particular black hole is disrupting its local
region by dining on matter that wanders too close - which is the
source of the energy for this jet.
"It also fires a jet out of the galaxy. So it is like a black hole
bully, punching the nose of a passing galaxy."
Basic properties
"There are still basic unanswered questions about how these jets
work," said co-author Martin Hardcastle of the University of
Hertfordshire, UK.
"We don't know how exactly they're generated close to the black
hole, what they're made of, how fast they're going, or how they
evolve with time. So an object like 3C321 can act as an experiment
which can give us an insight into the inner working of the
jet."
The effect of the jet on the companion galaxy is likely to be
substantial, because the galaxies in 3C321 are extremely close to
one another. At only about 20,000 light years apart, these galaxies
lie approximately the same distance as the Earth is from the centre
of the Milky Way.
It is possible that it would not all be bad news for the galaxy
struck by the jet. The massive influx of energy and radiation from
the jet may induce the formation of large numbers of stars and
planets once its initial wake of destruction is complete.
"Although we call it a death star galaxy, in the end it might be a
source of new life in the more distant galaxy," said Dr
Hardcastle.
Features seen in images from the VLA and Chandra indicate that the
jet started hitting the smaller galaxy about one million years ago.
This is a blink of the eye in the lifetime of 3C321, which marks it
out as an important opportunity to study a rare astronomical
phenomenon, say the astronomers.