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 jj3500È4100, combining the spectrum of a bulge population (of age B10 Gyr) with that of younger stellar populations, spanning ages from B3 Myr to 1 Gyr. The major constraints in the analysis are (1) the continuum ratio jj3660/4020, which efficiently discriminates between models combining a bulge spectrum with a stellar population younger than B50 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 MyrÈ1 Gyr for the relevant metallicities. Their detectability depends both on the age of the young component and on its contribution to the total Ñux 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%È40% of the Ñux at 4020We conclude that such con-A . tinuum cannot be distinguished from that of a very young stellar population (age ¹ 10 Myr), contributing with less than B0.02% of the mass of the bulge. The models are compared with nuclear spectraÈcorresponding to a radius of 200È300 pc at the galaxyÈof 20 Seyfert 2 galaxies, in which we speciÐcally look for the signatures above of young-to intermediate-aged stellar populations. We Ðnd 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 (B1 Gyr). We Ðnd a tendency for the young stars to be found more frequently among the late-type Seyfert galaxies, a well-known e †ect 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.

The Coevolution of Galaxies and Supermassive Black Holes in the Near Universe

“…These results suggest that we are witnessing the co-evolution of the SMBH and their host galaxies in the near Universe: while the SMBH at the center grows at typical rates of 10 −3 M ⊙ yr −1 , the bulge growths at typical rates of 0.1-1 M ⊙ yr −1 . A similar evolution scenario has been previously proposed by [22].…”

Seyfert Galaxies: Nuclear Radio Structure and Unification

“…These starbursts were originally detected by means of either ultraviolet or optical spectroscopy of the central few 100 pc. Several spectroscopic works have detected features of young and intermediate age stellar population (Heckman et al 1997;González-Delgado et al 1998, 2001Storchi-Bergmann et al 2000), suggesting that these populations are significant, if not dominant, in the nuclear region of many Seyfert 2 galaxies. Therefore, on a local scale, evidence is mounting that star formation and nuclear activity are linked.…”

The nuclear starburst activity in the Seyfert 2 galaxy NGC 7679

“…Following Cid Fernandes et al (2001), we use three components: an old and an intermediate population, with ages >1 Gyr and ∼10 8 yr, respectively, and an f λ ∼ λ −1 power-law component to account for either a scattered AGN continuum or a young starburst component, since it is impossible to disentangle the power-law AGN continuum from the contribution of a young starburst of an age ≤10 Myr (Storchi-Bergmann et al 2000;Gonzalez Delgado et al 2001;Cid Fernandes et al 2001). We normalized all spectra at 4800Å, and we compared parameters of continuum fluxes at 4020, 4510 and 5500Å, as well as EW s of the absorption lines for CaIIK λ3933, CN λ4200 and G-band λ4300.…”