Stellar population synthesis in the presence of diluting components

Application to the infrared range for MCG-6-30-15

LUTH, UMR 8102 du CNRS, associé à l'Université Denis Diderot, Observatoire de Paris, Section de Meudon, 92195 Meudon Cedex, France e-mail: catherine.boisson@obspm.fr

Received: 10 November 2006
Accepted: 24 May 2007

Abstract

Aims.Numerous studies of the host galaxy of Seyfert nuclei are being conducted in the optical/visible range. However, in the case of Seyfert 1, the spectra of the inner galactic core can be dominated by broad emission lines coming from the nucleus that totally flood the underlying galactic spectrum, preventing any study of stellar populations.

Methods.Because part of the IR H-band is free of the strongest AGN emission lines, we developed a method that allows the stellar population of the very internal regions of the Seyfert 1 galaxies to be studied in the presence of a diluting continuum.

Results. A new inverse method using the flux as observables was developed and deeply tested. The specificity of the method is to take the non stellar parameters (reddening, dust emission, and non-stellar continuum) into account directly in the synthetic distance to be minimised.

Conclusions. The method is powerful for deriving the stellar content of the very central part of AGN. However, the results for the stellar population are still tentative, as the incompleteness of the stellar base (lack of supermetallic giant stars) could lead to large uncertainties. New observations of stars in the infrared at high spectral resolution, in particular metal-rich stars, are mandatory to build a complete stellar library that can be used to synthesise the host galaxies of AGN with as much accuracy as possible.