Abstract
This thesis reports on the study of a volume-limited sample of 46 galaxies. It forms part of
a continuing, multi wavelength program to assess the prevalence of low luminosity nuclear
activity in galaxies with a wide range of sizes. X-ray observations are used, together with
images at other wavelengths from previous work, to create a list of possible sites of nuclear
activity. Optical spectra of these targets are analysed. The X-ray and optical properties of
the nuclear regions, and their relationship with the host galaxies, are investigated. Finally,
one particular object, NGC4395, is described in detail.
R.OSAT X-ray images were studied for 29 galaxies. Contour maps are presented for all
galaxies with detected emission and fluxes are given for all observed point sources. Upper
limits are given for galaxies with no detected nuclear sources. In general, X-ray nuclear
sources are found to be extremely common. Their fluxes correlate strongly with the host
galaxy luminosity (the analysis includes results from the literature for another 4 galaxies).
Long slit, high resolution, ~ 3600 — 6800 A optical spectra were obtained for all the nuclear
targets identified during careful examination of radio, IR, optical and X-ray images.
Spectra are presented for each target and emission line fluxes are measured. Emission
line diagnostic diagrams are used to investigate the nature of the ionising sources of the
line-emitting targets. Results are compared with the spectral classification given by ? for
galaxies common to both surveys and objects without previous observations are classified.
The fraction of optically identified AGN in the sample is obtained.
Based on the observed correlation between X-ray and host galaxy luminosity, the properties
of the nuclear X-ray sources are investigated. Nuclear Ha and X-ray luminosities
are compared with relationships established for more luminous AGN and with predictions
from starburst models. Correlations are also found between nuclear Lla luminosities and
host absolute magnitudes for nuclei optically classified as sta.rbursts or AGN. Tentative
explanations for the observed correlations are explored.
As part of the multiwavelength project, optical spectra and X-ray images of NGC 4395, the
least luminous known Seyfertl, were obtained. The ROSAT observations show a change
in the X-ray flux by a factor of ~ 2 in 15 days. The spectra show a change in both the level and shape of the continuum, becoming bluer when brighter. A power law fit to the
observations shows that the spectral shape changes between a low state (with a ~ 2), a
medium state (a ~ 1), and a high state (a ~ 0). A week of ground-based optical broad
band monitoring of NGC4395 has also given evidence of a variation in the flux by ~ 20%
in less than 24 hours.
