GALAXIES - SPIRAL GALAXIES
What are spiral galaxies?
Spiral galaxies are flattened disc-shaped systems
incorporating spiral arms of perhaps up to 100 billion bright young stars
embedded in interstellar matter winding out from a dense central
nucleus. Although two-armed spirals are the most common, one- or even
three-armed systems have been observed. There exist normal spirals, with
arms emerging directly from the nucleus and barred spirals, in which the
arms emanate from the ends of a bright central bar across the nucleus. Both
of these types exist in a wide range of forms from large dominant nuclei
with thin, tightly bound spiral arms (Sa,SBa), to tiny nuclei with large,
loosely bound spiral arms (Sc, SBc). Spiral galaxies are rich in gas and
dust, distributed mostly in clouds along the spiral arms. The nuclei are
seen to contain older stars, which extend in a fairly smooth
axially-symmetric distribution beyond the nucleus, but because of their
intrinsic faintness, are much less conspicuous than the arms.
IMAGE:- M83 is thought to be very like our own Milky
Way galaxy, but seen from above one of its poles and at a distance of
about 20 million light years. Composed of billions of stars and huge
clouds of dust and gas, this object is one of the finest examples of a
spiral galaxy and shows a concentration of older, yellow stars in its
central nucleus with younger, blue stars and patchy red clouds of
glowing gas and dark dust lanes in the trailing spiral arms
(copyright: Anglo-Australian Observatory)
How do Spiral galaxies appear in X-rays?
IMAGE:- X-ray emission from the spiral galaxy
NGC300. Contours of X-ray emission are shown superimposed on an
optical image. Many point sources are visible, including a possible
black hole binary and many supernova remnants. NGC300 is a very quiet
galaxy, and shows little in the way of diffuse gaseous X-ray emission.
More active spiral galaxies show a larger diffuse X-ray contribution
from the hot interstellar medium.
Spiral galaxies are spatially extended, complex
X-ray sources with X-ray luminosities ranging from 1031 to a few
times 1034 watts. Although this is only a small fraction of the
total power output of a galaxy, the X-ray data can be used to study the end
products of stellar evolution, such as compact objects (e.g. close
accreting binaries and black holes) and supernova remnants, and also the
hot interstellar medium (ISM), the gas which surrounds and encompasses all
the stars within the galaxy. The distribution of X-ray sources can also be
studied, especially in nearby galaxies, placing constraints on binary
evolution theories, and also the structure of the ISM can be investigated
using observations of diffuse X-ray emission from the planes of spiral
galaxies. Studies of this hot gas are of great help in the development of
supernova remnant evolution theories.