Nuclear activity and star formation properties of Seyfert 2 galaxies

TL;DR

This study analyzes the stellar populations in Seyfert 2, star-forming, and passive galaxies using SDSS data, revealing differences in star formation activity and stellar features in their nuclear regions.

Contribution

It provides a comparative analysis of stellar components in different galaxy types using spectral fitting, highlighting recent star formation traces in Seyfert 2 galaxies.

Findings

Seyfert 2 and passive galaxies have similar spectral energy distributions.

Star-forming galaxies show a strong blue excess indicating active star formation.

Seyfert 2 galaxies exhibit signs of recent but not ongoing star formation.

Abstract

In order to characterize the amount of recent or ongoing stellar formation in the circumnuclear region of active galaxies on a statistically sound basis, we have studied the stellar component of the nuclear spectra in three different samples of galaxies, namely Seyfert 2 galaxies (hereafter S2G), star-forming galaxies (SFG) and passive normal galaxies (NG), i.e., no emission lines observed, using Sloan Digital Sky Survey data (SDSS) (Adelman-McCarthy, 2008). The stellar component of the observed spectra has been extracted using STARLIGHT (Cid Fernandes et al., 2004), which fits an observed spectrum with a model (template) spectrum obtained by combining a library of pre-defined simple stellar populations spectra, with distinct ages and metallicities. The resulting template spectra for the different samples of galaxies have been compared to determine the features of the stellar emission component and to evaluate the presence and intensity of the star formation in the nuclear regions of different families of galaxies. From a first qualitative analysis it results that the shape of the Spectral Energy Distribution (SED) of S2G and NG is very similar, while that of SFG is characterized by a strong blue excess. The presence of the 4000 A break in the spectra of S2G and NG together with the lack of a strong blue continuum clearly indicate the absence of ongoing star formation in the circumnuclear regions of S2G and obviously of NG. Anyway traces of a recent star formation history are evident in the spectra of S2G galaxies, which show a 4000 A break systematically shallower than in NG.