^{| | | |}

 

 

 

"Soft X-ray observations of nearby galaxies (PhD thesis)"

Our understanding of the soft X-ray properties of nearby galaxies has been revolutionised as a result of the Rontgensatellit (ROSAT) mission. This thesis considers a number of aspects of the properties of nearby galaxies on the basis of recent ROSAT PSPC and HRI observations.

Firstly, the extragalactic contribution to the X-ray background in the softest accessible X-ray band (0.1 - 0.4 keV) is investigated using the shadows cast by the H I discs of nearby galaxies. A preliminary investigation considers the case of NGC 55, a nearby edge-on galaxy with an extensive H I disc. The technique is then refined and employed on a sample of nearby galaxies, resulting in the best constrained measurement currently available of the extragalactic X-ray background intensity at 0.25 keV of 24.9 +/- 5.0 keV/s/sr/cm^2/keV. This result has implications for the source populations which compose the extragalactic X-ray background below 1 keV.

Detailed studies of the X-ray properties of two nearby galaxies, NGC 55 and NGC 4736, are then presented. The X-ray emission from NGC 55 is dominated by a population of point sources, with some diffuse emission evident in and around its disc. NGC 4736, on the other hand, is dominated by the emission from an extended nuclear source. X-ray data strongly suggests that this region contains a low-luminosity AGN, a conclusion that is supported by observations at other wavelengths.

The incidence of low-luminosity AGN in nearby galaxies is probed by correlating ROSAT PSPC point source catalogues with an optically-identified sample of nearby galactic nuclei. A sample of 90 ``nuclear X-ray sources'' is defined, and general trends in their X-ray properties are identified; for example, low-luminosity Seyfert nuclei are generally much more X-ray luminous than nuclear H II regions, and soft X-ray hardness ratios are seen to differentiate between optical broad-line and narrow-line AGN.

This page written and maintained by Tim Roberts. Last updated 15/08/00