paper

The frequency of nuclear star-formation in Seyfert 2 galaxies

arXiv:astro-ph/0007027 · doi:10.1086/317247

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.

18 figures, revised version published in ApJ, December 2000, vol. 544, p. 747

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