A multiwavelength study of X-ray selected samples of star-forming galaxies

This thesis presents a multiwavelength study of two X-ray selected samples of star-forming galaxies. The first sample is defined by the cross-correlation of ROSAT source catalogues with the IRAS Faint Source Catalogue, the Markarian and the Kiso catalogues. The main objective of this project is to investigate whether X-ray luminous star-forming galaxies exist, and if so to quantify their properties. A number of star-forming galaxies with luminosities > 1041erg s--1 are found. Their X-ray luminosity can be explained in terms of a superwind and an X-ray binary component. Moreover, their multi wavelength properties are very similar to the properties of their lower luminosity counterparts. The second sample studied is drawn from the optical spectroscopic atlas of Ho, Filippenko & Sargent (1997a). It consists of 44 bona-fide star-forming galaxies which are observed either as targets or serendipitously with the ROSAT PSPC. The correlation between their X-ray emission and their emission in the optical, far-infrared and radio bands are studied. These correlations are used in order to probe the origin of their X-ray emission. Also a correlation between the star-formation rate and X-ray luminosity for star-forming galaxies is calculated. This is convolved with the evolution of star-formation rate with redshift in order to estimate the contribution of star-forming galaxies to the XRB. It is found that they can produce between 10% -- 50% of the soft extragalactic (0.1-2.5keV) X-ray background. Finally, two examples of X-ray luminous starburst galaxies, namely Arp299 and NGC3310 are studied in detail using data from the ROSAT and ASCA. Their X-ray properties are similar to the X-ray properties of lower luminosity star-forming galaxies, suggesting that the same mechanisms are responsible for the X-ray emission of star-forming galaxies spanning three orders of magnitude in luminosity. Thirdly Holmberg-II which is an X-ray luminous dwarf star-forming galaxy is also studied. It is found that all of its X-ray emission arises from a single variable source. The fact that its X-ray luminosity is ~ 1040erg s--1 suggests that it could be a black-hole X-ray binary with a mass of ~ 200 M⊙ . This result together, with other recent studies, suggests that there might be a new class of X-ray binary systems with extremely high mass blackholes. If so, this raises intriguing questions concerning their formation and evolution. Finally, the potential of the new major X-ray observatories, XMM-Newton and Chandra is discussed in the light of the studies described in this thesis.