Diapositiva 1

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_at_inaoep.mx, rjt_at_ast.cam.as.uk, and
et_at_ast.cam.as.uk, bressan_at_pd.astro.it, and
trilce_at_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 107.0 to 108.6 yrs,
representing a starburst component, and (2) an
old population with age ranging from 108.0 to
109.6 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.