History of XRay Observations of Galaxies

X-ray observations of galaxies were started with the first X-ray satellite Uhuru, which carried a collimated proportional counter instrument sensitive for photons in the energy range 2-10 keV and was launched in 1970. Uhuru discovered intense X-ray emission from compact stellar remnants (white dwarfs, neutron stars, black holes) in binary systems in the Milky Way, from AGN and from hot plasma in clusters of galaxies. Also X-ray emission from the Local Group galaxies M 31 and the Magellanic Clouds (MC) were detected; however, emission from other normal galaxies was below the Uhuru detection threshold (see [27] for a review of Uhuru results). The first satellite with a focusing X-ray telescope was the Einstein Observatory launched in 1978. Its much higher sensitivity and spatial resolution resulted in the detection and study of a significant number of normal galaxies. For a review of the Einstein observations of galaxies see e.g. [17] and the catalogs of Einstein images and spectra of galaxies [20,42,43].

More than a decade later, the focal plane instruments aboard the ROSAT observatory provided much higher sensitivity at low X-ray energies and slightly better angular resolution, its telescope had a wider field of view, and the mission lasted significantly longer compared to the Einstein Observatory. All these points have led to a better understanding of the spatial morphology of the X-ray emission of galaxies, and to the detection and study of the soft X-ray-emitting ISM of spiral galaxies.

ASCA and BeppoSAX extended the observable energy band to 10keV and above. In addition, the CCD detectors as focal plane instruments aboard ASCA provided improved energy resolution by factors of 5-10 compared to previous instruments. This allowed the study of different spectral components of the X-ray emission. However, the significantly inferior angular resolution of these satellites compared to Einstein and ROSAT prohibited in most cases detailed studies of individual galaxies.

The situation drastically chanced with the launch of the Chandra and XMM-Newton observatories in June and December 1999. These satellites represent a big step forward in sensitivity, imaging capabilities, and spectral resolution compared to the previous generation of X-ray observatories and provide a new and much deeper look at galaxies in X-rays. For a more detailed discussion of the history of the X-ray observations of galaxies see [19].

With the increased sensitivity of the X-ray instrumentation, galaxy science has developed to an important topic of X-ray astronomy. This is best demonstrated by the fact that observations of fields in the Large Magellanic Cloud (LMC) were selected as first light targets for ROSAT and XMM-Newton (see Fig. 20.1), and pointings

ROSAT PSPC LMC X-1 Region 05-2.0 keV

Right Ascension (JBQOO)

Fig. 20.1 ROSAT PSPC (left) and XMM-Newton EPIC pn (right) first light images pointing at a LMC area slightly west of 30 Dor. The XMM-Newton image [14] is a zoom-in by about a factor of six compared to the ROSAT image

Right Ascension (JBQOO)

ROSAT PSPC LMC X-1 Region 05-2.0 keV

Fig. 20.1 ROSAT PSPC (left) and XMM-Newton EPIC pn (right) first light images pointing at a LMC area slightly west of 30 Dor. The XMM-Newton image [14] is a zoom-in by about a factor of six compared to the ROSAT image to nearby galaxies (e.g., LMC, Small Magellanic Cloud SMC, M 31, NGC 253) were used as calibration and performance verification targets for XMM-Newton and Chandra.

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