Normal galaxies were first observed in the X-rays with the Einstein Observatory, the first X-ray telescope to observe the deep universe, launched by NASA in 1978 and developed under the leadership of the CfA HEA. This first look at the X-ray emission of galaxies revealed bright point-like sources and diffuse emission. With the Chandra X-ray Observatory we are now studying the X-ray emission of galaxies in exquisite details. We detect populations of luminous and variable point-like X-ray sources, diffuse emission from hot gas and in some cases emission from active nuclei.
To know more, see:
Fabbiano, G. 1989, Ann. Rev. A&A, 27, 87 and
Fabbiano, G. 2006, Ann. Rev. A&A, 44, 323
Populations of X-Ray Sources in Galaxies
G. Fabbiano, D.-W. Kim, A. Zezas, A. Prestwhich, C. Jones, R. DiStefano, Jonathan McDowell