The Frequency of Nuclear Star Formation in Seyfert 2 Galaxies

We investigate the detectability of starburst signatures in the nuclear spectrum of Seyfert 2 galaxies by constructing spectral models in the wavelength range λλ3500-4100, combining the spectrum of a bulge population (of age ≈10 Gyr) with that of younger stellar populations, spanning ages from ≈3 Myr to 1 Gyr. The major constraints in the analysis are (1) the continuum ratio λλ3660/4020, which efficiently discriminates between models combining a bulge spectrum with a stellar population younger than ≈50 Myr and those with older stellar populations; (2) the presence of the Balmer lines H8, H9, and H10 in absorption, which are unambiguous signatures of stellar populations with ages in the range 10 Myr-1 Gyr for the relevant metallicities. Their detectability depends both on the age of the young component and on its contribution 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 observed in some Seyfert 2 galaxies in polarized light, contributing with typically 10%-40% of the flux at 4020 Å. 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-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 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 caused by intermediate-aged stars (≈1 Gyr). We find a tendency for the young stars to be found more frequently among the late-type Seyfert galaxies, a well-known effect in the nuclei of normal galaxies. This tendency is supported by a comparison between the equivalent widths (W) of absorption 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, the W values of the Seyfert galaxies are within the observed range of the normal galaxies, suggesting a similar stellar population. On the other hand, the W values are lower than those of the normal galaxies for seven out of the 11 Seyferts in early-type galaxies.