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Etude Spectrosopique des Noyaux Actifs de Galaxies à Spectre de Transition

Abstract : Galaxy nuclei often show emission lines; they were classified in several categories on the basis of the intensity ratios of particular optical lines: Seyferts 1 and 2, LINERs and HII regions; these categories correspond to different excitation conditions. However, a number of objects have intermediate line ratios which do not make it possible to classify them unambiguously in one or the other of these categories.

With the aim of clarifying the nature of the "transition spectra" objects, we carried out spectroscopic observations in the visible of a great number of them; the majority of the observations have been made using the spectrograph CARELEC, attached to the Cassegrain focus of the 1,93~m telescope of the Haute--Provence Observatory. The results of this work constitute the bulk of the manuscript presented here.

Our observations of 61 objects with a "transition spectrum" showed that they fit into a simple classification scheme; indeed, the spectrum of the majority of them results from the simultaneous presence on the slit of the spectrograph of two different emissive regions (in general, a HII region and a Seyfert-type cloud) - what we call a "composite spectrum". Being so, no "intermediary" excitation mechanism, nor evolutionary scenario, is necessary to explain their characteristics.

Among the objects with composite spectra studied two, particularly interesting, drew our attention and are presented separately:
NGC 5252 and KUG 1031+398.

Our study of NGC 5252 is based on observations carried out with both the Hubble space telescope and the 1,93~m telescope; it revealed the simultaneous presence, in this object, of a LINER and a Seyfert-type cloud. This suggests that, at least in this galaxy, the LINER-type emission could be due to an ionizing source of non-stellar origin. Nevertheless, the LINERs constitute a class of objects very poorly known: True AGNs or galaxies whose emission is not due to a central engine, but entirely of stellar origin? The debate is still open.

KUG 1031+398 is a Narrow Line Seyfert 1 galaxy for which an emission
region intermediate between the Broad Line Region and the Narrow Line Region was reported. Our observations do not confirm the presence of an intermediate region in this galaxy, the only one for which a "firm" detection has been published. The existence of such an emission region remains, therefore, to be proven.
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Contributor : Anabela Cardoso Gonçalves <>
Submitted on : Saturday, January 13, 2007 - 8:55:03 PM
Last modification on : Monday, May 18, 2020 - 2:30:18 PM
Long-term archiving on: : Friday, November 25, 2016 - 1:41:24 PM


  • HAL Id : tel-00012027, version 2



Anabela Cardoso Gonçalves. Etude Spectrosopique des Noyaux Actifs de Galaxies à Spectre de Transition. Astrophysique [astro-ph]. Université de Provence - Aix-Marseille I, 1999. Français. ⟨tel-00012027v2⟩



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