CENSIMENTO DI NUCLEI GALATTICI ATTIVI DI BASSA LUMINOSITA'

Aim of this thesis is to improve the census of low-luminosity AGN in nearby galaxies by using the analysis of optical emission lines. In fact, recent results suggest that the line emission, usually ascribed to the active nucleus, suffers from a substantial contamination from stellar processes. We started from the best census currently available, provided by the Palomar spectroscopic survey (PSS). We analyzed the spectra taken with the Hubble Space Telescope, that has a smaller aperture (with an area reduced by a factor of ~160) than the Hale telescope at Mount Palomar. What we expect to obtain with the reduced aperture is to better isolate the central region of galaxies, dominated by the presence of the AGN, rejecting the stellar continuum that contaminates the real nuclear emission. We selected all the galaxies from the PSS, having data in the HST Archive and with an Equivalent Width of the [OIII] line, EW([OIII]), measured from the PSS smaller than 3 Å (in order to exclude obviously active galaxies), forming a sample of 28 galaxies. The modeling returns measurements of line fluxes and EW. We also re-processed the data from the Palomar survey with our more refined method of subtraction of the stellar continuum. Genuine AGN should show an increase of the line EW in the HST spectra when compared to the values obtained from the Palomar data, due to the reduced contribution of the stellar emission. We find that the many of the galaxies in the sample do not show this effect and cannot be qualified as AGN. We built the luminosity function for this sample of galaxies, comparing it with the one obtained from the Palomar data and considering the 'false' AGN: we found that the AGN luminosity function from the PSS must be strongly revised particularly at the lowest luminosities. We finally considered the X-rays luminosities measured from Chandra data and available in the literature. In fact the X-ray data also provide us with a robust tool to find AGN. Crossing the HST and Chandra data results in an even more reliable census: we obtained a substantial confirmation of the results obtained with our analysis, with a few interesting exceptions.