  In this Master's project, the X-ray nuclear properties of a sample of bright
nearby galaxies are explored. This is done by matching their comprehensive
optical spectroscopic classification to the latest available XMM-Newton
catalogue - 3XMM-DR4. The good coverage (approx. 38 per cent) ensures that a
statistically representative sample is investigated. All nuclear and
morphological subsets found within the original sample of 486 galaxies are
encompassed, but early-type galaxies and galaxies with optical features
characteristic for active galactic nuclei (AGN) are favoured. The results from
the investigation of the properties of our cross-matched sample are overall
consistent with the presence of a large fraction of X-ray - detected
low-luminosity AGN (LLAGN). The X-ray - detected galaxies within our HII and
transition-LINER subsets are of particular interest, as they could harbour
LLAGN missed by optical spectroscopic selection. The properties of these nuclei
are explored by X-ray spectral fitting of available XMM-Newton observations.
The majority of the examined spectra are found to be consistent with a thermal
plasma model and an underlying power-law continuum, and the relative
contributions of a potential LLAGN and of thermal emission to the source
luminosity are presented. In the case that these spectra are not highly
contaminated, as confirmed for some of the nuclei by examining available
higher-resolution Chandra images, we classify approx. 43 per cent of the HII
nuclei and 40 per cent of the transition-LINER nuclei as optically undetected
LLAGN-candidates. This classification is found to be reasonably successful,
given the project limitations, when compared to X-ray and multi-wavelength
results found in literature for many of the nuclei.
