The properties of discrete X-ray sources in star-forming galaxies

Abstract

Observations of starburst galaxies have revealed a large number of point-like X-ray sources located within these systems, including active galactic nuclei (AGN), X-ray binaries (XRBs), and utraluminous X-ray sources (ULXs). In this thesis we investigate the physical properties of a variety of these objects using both their X-ray emission characteristics and their optical counterparts, in order to better understand the accretion physics of such objects, and the environments in which they are found.

This work begins with a study of 3 moderate-redshift (z ~ 0.1) X-ray bright (> 10^42 erg s^-1) galaxies, all of which display no clear signs of the presence of an AGN in the optical band. Given the high X-ray luminosities of these objects, they must either be the most X-ray luminous starburst galaxies known; or they must harbour a hidden AGN. We use new, pointed observations of the galaxies to determine their detailed X-ray characteristics, and demonstrate that each X-ray source is consistent with an AGN. The most likely explanation for the lack of AGN signatures in the optical spectra of these galaxies is that the AGN emission lines are being diluted by star formation signatures from within the host galaxies.

Next, we present age constraints on 13 bright XRBs located within the high-metallicity drop-through ring galaxy NGC 922. While less than half the X-ray sources are located close to recent star formation as traced by Hα emission, the majority of sources are associated with star clusters, that we are able to age on the basis of their optical colours. We find that the sources that are closest to clusters tend to also have the youngest counterpart clusters and, in most cases, are close to H emission, placing a limit of < 10 Myrs on their age, while those with greater distances to star clusters are also significantly older. We also investigate the possibility that the X-ray sources were ejected from their parent clusters, either by dynamical interactions, or by supernova kicks. An important caveat to the results of this work is that the a number of sources may have associated clusters that are highly extinguished and are therefore undetectable in the optical band.

Finally, we present a study of the 8 brightest X-ray point-sources in the prototypical starburst galaxy M82, using the rich data set afforded to us by a ~ 480 ks Chandra observation of the galaxy. From our investigation, we find that the sources with X-ray luminosities < 10^39 erg s^-1 are heterogeneous, but all display X-ray properties that are typically observed in canonically accreting back hole binaries. A possible bi-modality is seen in accretion states between the more luminous and less luminous sources in this subset of our sample. The majority of these sources show significant long-term variability, with one of the sources being identified as a transient candidate. The three brightest sources in our sample, all of which are known ULXs, display long-term variability and spectral characteristics consistent with previous observations. This work demonstrates the rich potential for future studies of the diverse X-ray binary populations in nearby galaxies.