A morphological comparison between the central region in AGN and normal galaxies using HST data

Abstract

We study the morphology of the central region of a sample of Active Galactic Nuclei (AGN) and a ``control'' sample of normal galaxies using archival observations of the WFPC2 instrument onboard the Hubble Space Telescope (HST). We use the ellipse fitting technique in order to get a good description of the inner ``smooth'' light distribution of the galaxy. We then divide the observed galaxy image by the artificial image from the fitted ellipses in order to detect morphological signatures in the central region around the nucleus of the galaxy. We perform quantitative comparisons of different subgroups of our sample of galaxies (according to the Hubble type and the nuclear activity of the galaxies) by calculating the average amplitude of the structures that are revealed with the ellipse fitting technique. Our main conclusions are as follows: 1) All AGNs show significant structure in their inner 100 pc and 1 kpc regions whose amplitude is similar in all of them, independent of the Hubble type of the host galaxy. 2) When considering early-type galaxies, non-AGN galaxies show no structure at all, contrary to what we find for AGN. 3) When considering late-type galaxies, both AGN and non-AGN galaxies show significant structure in their central region. Our results are consistent with the hypothesis that all early-type galaxies host a supermassive black hole, but only those that have enough material in the central regions to fuel it show an active nucleus. The situation is more complicated in late-type galaxies. Either not all of them host a central black hole, or, in some of them, the material inside the innermost 100 pc region is not transported to the scales of the central engine for some reason, or the large amount of gas and dust hides the active nucleus from our sight.