Organic matter in Seyfert 2 nuclei: Comparison with our Galactic center lines of sight ^*

¹ Institut d'Astrophysique Spatiale, UMR-8617, Université Paris-Sud, Bâtiment 121, 91405 Orsay, France e-mail: emmanuel.dartois@ias.u-psud.fr
² European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile e-mail: o.marco@eso.org
³ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile

Received: 13 January 2004
Accepted: 19 April 2004

Abstract

We present ESO – Very Large Telescope and ESA – Infrared Space Observatory 3 to 4 μm spectra of Seyfert 2 nuclei as compared to our galactic center lines of sight. The diffuse interstellar medium probed in both environments displays the characteristic 3.4 μm aliphatic CH stretch absorptions of refractory carbonaceous material. The profile of this absorption feature is similar in all sources, indicating the CH₂/CH₃ ratios of the carbon chains present in the refractory components of the grains are the same in Seyfert 2 inner regions. At longer wavelengths the circumstellar contamination of most of the galactic lines of sight precludes the identification of other absorption bands arising from the groups constitutive of the aliphatics seen at 3.4 μm. The clearer continuum produced by the Seyfert 2 nuclei represents promising lines of sight to constrain the existence or absence of strongly infrared active chemical groups such as the carbonyl one, important to understand the role of oxygen insertion in interstellar grains. The Spitzer Space Telescope spectrometer will soon allow one to investigate the importance of aliphatics on a much larger extragalactic sample.