Starburst Galaxies (2) ...
The following figure shows
M82 at visual, infrared, and X-ray wavelengths.
The IR and X-ray observations, coupled
with the previously discussed RF observations, permit a relatively clear understanding
of the strange optical appearance of M82.
The M82 Starburst
The inner 400 parsecs of M82 is undergoing an intense starburst that is thought to have
started approximately 10-100 million years ago. Because of the starburst, the central
region of M82 contains many OB stars and remnants of recent supernovae. These hot
stars and expanding bubbles from supernovae are expelling gas from the central
region.
Much of this gas is being ejected in a "starburst wind" perpendicular to the
plane of the galaxy, presumably because it encounters less resistance to expand in that
direction. The adjacent right figure illustrates schematically the formation of such a
wind.
It is estimated that at least 10 million solar masses of hot gas have
already been
ejected by the continuing starburst in M82. Because many supernova events have contributed to
this wind, it is rich in heavier elements expelled by supernova
explosions. Therefore, such starburst winds may be a major way in which heavy elements get
distributed through the Universe.
X-Ray Emission
The Chandra X-Ray Observatory image on the right of the above figure contains two pieces
of evidence in support of this premise for M82.
The hot gas of the starburst wind has a temperature of millions of degrees
and so glows in X-rays. The diffuse clouds in the X-ray image chart
this emission. In
addition, Chandra is able to resolve various point-like sources of
X-rays lying more in the plane of the galaxy (the bright dots in the X-ray image).
These are thought to be
supernova remnants in some cases, and very bright X-ray binaries in others.
Some of the binaries probably contain
neutron stars and some black holes, with the intense X-ray emission associated with
accretion onto the condensed object from the primary star (many of these binaries
may be remnants of an earlier period of starburst activity).
In either case, we would expect such binaries
and the supernova remnants
to be a product of intensive starmaking in a starburst, since neutron stars, black
holes, and supernova remnants are expected byproducts of core-collapse in massive stars.
Infrared Emission
One mark of starburst galaxies is that they tend to be more blue in the visual region of
the spectrum because of the high concentration of new stars. However, as was
confirmed extensively by observations of the IRAS satellite, strong starbursts actually
tend to emit much of their energy in the infrared spectrum. This is because the
new star formation is cloaked initially in dust, which tends to absorb the UV and visible light
from the new stars and re-emit it as infrared. Therefore, strong IR emission is
also a
telltale sign of a starburst galaxy. The above left IR image of M82 confirms strong
infrared emission from the central region of the galaxy where the starburst is taking
place.
