GALAXIES - STARBURSTS AND MERGERS
What are starburst galaxies?
Starbursts are sites of large-scale, extremely
high-rate star formation. These systems experience star formation at a rate
that can only be maintained in equilibrium if the duration of the star
formation activity (perhaps 10 million years) is short compared to the life
of the galaxy. Though our understanding of their causes, properties, and
evolution is not complete, a large fraction of high-luminosity starbursts
are thought to be triggered by interactions between galaxies. It is also
believed that the bars within barred spirals may be the cause of some
starbursts. Effects such as these cause interstellar gas to be compressed
and heated, and converted rapidly into stars. Often this occurs within the
galaxy nucleus itself.
How do starburst galaxies appear in X-rays?
IMAGE:- X-ray emission from the famous
starburst galaxy NGC253. Low temperature X-ray emission is shown in
red and high temperature X-ray emission is shown by the contours. Most
striking are the huge plumes extending above and below the galaxy
spiral disc (shown schematically by the ellipse - i.e. we are seeing
the galaxy almost edge-on). This galactic wind is due to supernova
explosions within the starburst nucleus. Also very visible in contours
are many point sources, the brightest of which are seen at the
Starbursts tend to have very enhanced X-ray
emission compared to more normal galaxies, and their X-ray emission usually
originates from spatially extended regions, the largest of which is often
at the galaxy nucleus. As these massive stars in the starburst nucleus go
supernova, the combined mass of hot supernova gas can blow out of the plane
of the galaxy in huge plumes, and can be visible in X-rays extending for
perhaps 30,000 light years. Hydrodynamical computer models have been
performed to try and mimic these features.
IMAGE:- A numerical
simulation of a starburst-driven wind. The picture shows the gas
density (a logarithmic plot, red represents high density, blue
represents low density). Energy and mass deposited into the
interstellar medium of the galaxy by the starburst region (in the
bottom centre of the image) heats the gas up to millions of degrees,
where it emits X-rays. The hot gas expands and eventually breaks out of
the disk of the galaxy, escaping into the halo of the galaxy at
approximately 1000 km per second.
Merging Galaxies (next page)