Determination of nuclear SB ages in Seyfert galaxies

1. Determination of nuclear SB ages in Seyfert galaxies Torres-Papaqui J.P., INAOE Mexico, Terlevich R.J., INAOE Mexico, Terlevich E., INAOE Mexico, and Bressan A., Osservatorio Astronomico di Padova, Italy. And Estrada-Piedra T. INAOE, Mexico. papaqui@inaoep.mx, rjt@ast.cam.as.uk, and et@ast.cam.as.uk, bressan@pd.astro.it, and trilce@ccc.inaoep.mx AbstractWe have obtained optical/near UV spectra of nuclear regions of Seyfert galaxies over the wavelength region 3600-5300 Angstroms. We find that many Seyferts, both type 1 and type 2 show the high-order Balmer lines in absorption. The detection of these Balmer absorptions constitute a strong evidence of the presence of recent star formation in the nuclear region of these active galaxies. While this finding supports some kind of connection between starburst and AGN activity it is not yet clear that starburts are present in all AGNs and what ages do they have. If such a connection exists, it would bear important implications for theories of AGN formation and their role in galaxy formation and evolution.We present a new method to determine ages of the stellar populations in the nuclei of Seyfert galaxies were dilution plays a major role. Our method is based on the comparison of the profiles of the high-order Balmer and Ca K absorption lines with our high spectral resolution spectral synthesis models. The profile comparison method is insensitive to the dilution that affects the traditional spectral analysis methods. The new method was calibrated using population synthesis models combining two different age stellar populations. (1) a young population with age ranging from 10^7.0 to 10^8.6 yrs, representing a starburst component, and (2) an old population with age ranging from 10^8.0 to 10^9.6 yrs,representing the bulge component. We also included Gaussian noise with a range of levels. The application of our algorithm to the synthetic models recovers the input ages with high accuracy and turns out to be capable of obtaining the correct ages even with noise levels higher than those of our observed spactra. It has to be pointed out, that given that our method depends only on the width and shape of the absorption lines, it is insensitive to both dilution by a featureless continuum and to dust reddenning. On the other hand, alas, it is not free from the metallicity-age degeneracy that affects all population dating methods. ResultsWe present in Figure 2 the results of applying the method to our sample of Seyfert galaxies and the Solar abundance synthesis models. The abscissa shows the logarithmic ages from the CaII K line while the ordinate show the ages derived from the upper Balmer series. No interpolation between models was attempted at this stage as seen form the discretness of the results. The stellar populations are young but not extremely so with ages ranging from 40Myr to 500 Myrs in the Balmer estimator, i.e. the starburst component, and from 400 to 4000 Myr in the CaII estimator, i.e. the bulge component. The age shift reflects the difference in the mean ages of the central starforming region and the older galaxian bulge. Globally, there seems to be a systematic shift between type 1 and type 2 Seyferts, with the type 1 having shorter ages than the type 2. The data base Our data base consist of spectra of nearby Seyfert galaxies taken by us at the 2.12m telescope of the OAGH (Cananea, Mexico) and at the 3.5m ESO NTT. The sample was selected from Veron-Cetty and Veron (2000) and from Nelson and Whittle (1995) catalogs. At the OAGH we used a Boller and Chivens spectrograph equipped with a 1024x1024 pixels CCD with a spatial sampling of 0.44" per pixel and a grating of 300 lines/mm providing a dispertion of 1.6 A per pixel. A slit width of 1.5 arcsec was used and the seeing was between 1.3 and 2.5 arcsec FWHM. At the ESO NTT we used EMMI equipped with a 1024x1024 pixels CCD; the grating provided a dispersion of 1.8 A per pixel and again a slit width of 1.5 arcsec was used. During all observations we took special care to minimize the effect of atmospheric differential refraction by observing at parallactic angle and at low airmasses. In figure 1 we show both some of the type 1 and type 2 Seyfert galaxies with detected high-order Balmer absorption lines. Fig 2: Result of derive age to both balmer and CaII k population age estimators. ConclusionsThe main contribution of this work is the finding of direct evidence for massive star formation in the central regions of type 1 Seyfert galaxies and its confirmation in type 2 Seyferts. We postulate that it is posible to determine the age of the nuclear stellar population in these galaxies using new methods that are insensitive to the presence of a diluting nuclear continuum and/or the effects of reddenning. Applying the new method we find a systematic difference in the age of the nuclear stellar populations between type 1 and type 2 Seyferts. The type 1Seyferts seem to be systematically younger than the type 2. We are improving the method both at the level of synthesis models and fitting algorithms; forthcoming work will include a wider sample of AGN. ReferencesGonzalez-Delgado, R., Leitherer, C. and Heckman, T. 1999,125,489Nelson C.H. and Whittle M., 1995, Astrophysical Journal Supplement v.99,p.67Veron-Cetty, M.P. and Veron P., 2000, VizieR On-line Data Catalog: VII/215 Fig 1: Spectra of both of the type 1 and 2 Seyfert galaxies with detected high-order Balmer absorption lines. Discussion When performing population analysis to the nuclei of Seyfert galaxies, a main difficulty encountered is the dilution of stellar absorptions by the nuclear non-stellar component. This dilution makes the use of traditional methods based on absorption line strength, extremely difficult or impossible to use. We have developed an alternative method that uses the profile of those absorption lines whose main broadening mechanism is related to the surface gravity of the stars. The hydrogen Balmer series and the CaII H and K lines are excellent examples of this effect. Both sets of lines becoming incresingly broad as the stellar population ages. Within our observed spectral range the main characteristic of recent star formation are strong emission lines plus Balmer absorptions while strong Ca II absorption is typical of the older bulge population.