The frequency of nuclear star-formation in Seyfert 2 galaxies

Abstract

We investigate the detectability of starburst signatures in the nuclear spectrum of Seyfert 2 galaxies by constructing spectral models in the wavelength range 3500-4100A, combining the spectrum of a bulge population (of age ~10Gyr) with that of younger stellar populations, spanning ages from ~3 Myr to 1 Gyr. We also construct models combining the bulge template with a power-law (PL) continuum, which is observed in some Seyfert 2's in polarized light, contributing with typically 10-40% of the flux at 4020A. We conclude that such continuum cannot be distinguished from that of a very young stellar population (age < 10 Myr), contributing with less than ~0.02% of the mass of the bulge. The models are compared with nuclear spectra - corresponding to a radius of 200-300 pc at the galaxy - of 20 Seyfert 2 galaxies, in which we specifically look for the signatures above of young to intermediate age stellar populations. We find them in ten galaxies, thus 50% of the sample. But only in six cases (30% of the sample) they can be attributed to young stars (age < 500 Myr): Mrk 1210, ESO 362-G8, NGC 5135, NGC 5643, NGC 7130 and NGC 7582. In the remaining four cases, the signatures are due to intermediate age stars (~1 Gyr). We find a tendency for the young stars to be found more frequently among the late type Seyfert's. This tendency is supported by a comparison between the equivalent widths (W) of absorption lines of the nuclear spectra of the Seyfert 2's with those of normal galaxies of the same Hubble type.