Introduction to Galaxies

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What are Galaxies?

Galaxies are made up of billions of stars that are gravitationally bound to eachother. Galaxies can be a spiral shape (like our own Milky Way) or elliptical or irregular. Galaxies tend to clump together in the universe, forming clusters of galaxies.

How does X-ray Astronomy Fit In?

Generally, the X-ray emission we see from other galaxies is from discrete sources such as X-ray binaries and supernova remnants. Our own galaxy is typical of this; we see the most and the brightest sources near the galactic center and in the galactic plane.

Recent results have shown that there is also an extended X-ray halo around some galaxies (particulary ellipticals) which extends well beyond the optical limits of these galaxies. This indicates the existence of diffuse hot gas and implies the existence of a halo of dark matter.

M31 in X-rays and optical light

On the left is a ROSAT PSPC mosaic of M31. On the right is an optical image obtained from the digitized sky survey of the Palomar plates using SkyView. Notice the X-ray sources in the ROSAT image that do not appear as optical sources in the Palomar image.

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