The Nuclear Region of Low Luminosity Flat Radio Spectrum Sources. I. Stellar Content

Abstract

We have examined the spectroscopic properties of a sample of 19 optically bright, low luminosity Flat Radio Spectrum (LL FRS) sources. Our study focuses on the properties of their host galaxies, namely the nuclear stellar populations and dust content. In the optical, the objects in the sample are mainly dominated by the host galaxy starlight, which strongly dilutes the non-thermal continuum as well as possible emission-line features related to the active nucleus. We have computed the nuclear stellar populations contributing to the spectra of our objects. The stellar population synthesis has been performed by using a very reliable mathematical method, which yields a Global Principal Geometrical solution. Our results show that, for most of the objects in the sample, the populations are composed of old stars of solar metallicity, or lower; the populations are mainly composed of late-type stars, i.e. G, K and M spectral types, the young component coming thus from supergiant stars; the dust content is weak. Both the stellar populations and the dust content are in agreement with what is usually observed in "normal" elliptical galaxies. Similar stellar content has equally been found in the nuclear regions of galaxies hosting a Low Ionization Nuclear Emission Line Region, or LINER. The present work is important in illustrating the different applications of stellar population synthesis in the study of low luminosity radio sources. In fact, the synthesis allows us not only to obtain valuable information about the stellar populations and dust content of the host galaxies, therefore providing material for further studies on the connection between host galaxy and active nucleus, but also to reveal the so-far unstudied optical emission-line features present in the spectrum of our objects.