The frequency of nuclear star formation in Seyfert 2 galaxies

We investigate the detectability of starburst signatures in the nuclear spe ctrum of Seyfert 2 galaxies by constructing spectral models in the waveleng th range lambda lambda 3500-4100, combining the spectrum of a bulge populat ion (of age approximate to 10 Gyr) with that of younger stellar populations , spanning ages from approximate to3 Myr to 1 Gyr. The major constraints in the analysis are (1) the continuum ratio lambda lambda 3660/4020, which ef ficiently discriminates between models combining a bulge spectrum with a st ellar population younger than approximate to 50 Myr and those with older st ellar populations; (2) the presence of the Balmer lines H8, H9, and H10 in absorption, which are unambiguous signatures of stellar populations with ag es in the range 10 Myr-1 Gyr for the relevant metallicities. Their detectab ility depends both on the age of the young component and on its contributio n to the total flux relative to that of the bulge. We also construct models combining the bulge template with a power-law (PL) continuum, which is obs erved in some Seyfert 2 galaxies in polarized light, contributing with typi cally 10%-40% of the flux at 4020 Angstrom. We conclude that such continuum cannot be distinguished from that of a very young stellar population (age less than or equal to 10 Myr), contributing with less than approximate to0. 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 galax ies, in which we specifically look for the signatures above of young- to in termediate-aged stellar populations. We find them in 10 galaxies, thus 50% of the sample. But only in six cases (30% of the sample) can they be attrib uted to young stars (age < 500 Myr): Mrk 1210, ESO 362-G8, NGC 5135, NGC 56 43, NGC 7130, and NGC 7582. In the remaining four cases, the signatures are caused by intermediate-aged stars (<approximate to>1 Gyr). We find a tende ncy for the young stars to be found more frequently among the late-type Sey fert galaxies, a well-known effect in the nuclei of normal galaxies. This t endency is supported by a comparison between the equivalent widths (W) of a bsorption lines of the nuclear spectra of the Seyfert 2 galaxies with those of normal galaxies of the same Hubble type. For the late-type galaxies, th e W values of the Seyfert galaxies are within the observed range of the nor mal galaxies, suggesting a similar stellar population. On the other hand, t he W values are lower than those of the normal galaxies for seven out of th e 11 Seyferts in early-type galaxies.