Integral Field Spectroscopy of Seyfert Galaxies

Abstract

The present work is based on a spectroscopically selected sample of Seyfert galaxies and consists of the observation, reduction, and analysis of three-dimensional data, obtained through the MPFS, the integral field spectrograph designed and developed by the scientists of the Special Astrophysical Observatory (Russia) for the 6 m telescope. Our aim is focused on a study of the circumnuclear and extranuclear environment of our sample of Seyfert galaxies, within the more general frame of finding clues to the “unified model,” such as the existence of the ionization cones, by applying a modern technique that has proved to be very powerful and advantageous in the case of extended sources. In chapter 1 an overview of active galactic nuclei (AGNs) and the unified model is presented and in particular a description of the main characteristics, similarities, and differences of the Seyfert galaxies. Chapter 2 contains a description of the radiation processes in AGNs, the mechanisms at the origin of the formation of the emission lines, and the diagnostic line ratios that allow one to discriminate between photoionization by thermal and nonthermal sources. Chapter 3 is an introduction to the method of two-dimensional spectroscopy and describes the properties of our observed data, the necessary starting point for developing a reduction procedure. In particular, we can appreciate the advantages and disadvantages in using a simple lenslet array or a more complex combination of a lenslet array and a bundle of optical fibers to decompose the image of a source into several elements. The following chapters present the results obtained through the analysis of the reduced data of three Seyfert galaxies: Mrk 917, NGC 4388, and IRAS 04502 0317. Mrk 917 shows a thick luminous bar, evident both in the images and in the distorted pattern of the velocity field of the gas. The nucleus of the galaxy has a typical spectrum of a Seyfert 2, with index of the power-law continuum . a p 1.5 The excitation map [O iii] l5007/Ha does not show any indication of possible ionization cones. Moreover, the emissionline spectrum of a region close to the active nucleus, classified as AGN-like but showing a high star formation rate, is probably produced by the combined effects of a local starburst and of not absorbed nonthermal radiation from the nucleus. In NGC 4388 we have observed part of one of the two ionization cones known in this galaxy, the so-called southwest cone. The regions inside the cone show spectra of an AGNlike nature. The FWHM of the lines is ∼500 km s , typical of the narrow-line region of the Seyfert galaxies, even if a broad Ha component (FWHM ∼2000 km s ) can be seen, suggesting the idea of scattered photons from the broad-line region. A high degree of ionization, almost constant even at large distances from the active nucleus, cannot be explained only by nuclear photons but is also probably due to the interaction between the known radio jet, aligned with the cone, and the gas within the cone. In IRAS 04502 0317 we have revealed a previously unknown one-sided ionization cone. In spite of its narrow emission lines (FWHM ∼ 300 km s ) more common in a LINER, the nuclear spectrum is typical of a Seyfert galaxy and is modeled by a nonthermal continuum with . A bright a p 1.2 region very close to the nucleus and partially crossed by the ionization cone is present in both the images and the spectra. Some evidence, such as tidal tails (or faint spiral arms), two non-coplanar dust lanes, an offset of the kinematical center with respect to the nucleus, and a negligible circumnuclear star formation, suggests that this bright region could be a secondary nucleus resulting from a minor merger between a gas-poor galaxy and a nucleated companion.