272 research outputs found
Stellar population gradients in Seyfert 2 galaxies. Northern sample
We use high signal-to-noise ratio long-slit spectra in the 3600-4700A range
of the twenty brightest northern Seyfert 2 galaxies to study the variation of
the stellar population properties as a function of distance from the nucleus.
In order to characterize the stellar population and other continuum sources
(e.g. featureless continuum FC) we have measured equivalent widths Ws of six
absorption features, four continuum colours and their radial variations, and
performed spectral population synthesis as a function of distance from the
nucleus. About half the sample has CaIIK and G-band W values smaller at the
nucleus than at 1 kpc from it, due to a younger population and/or FC. The
stellar population synthesis shows that, while at the nucleus, 75% of the
galaxies present contribution > 20% of ages younger or equal than 100Myr and/or
of a FC, this proportion decreases to 45% at 3 kpc. In particular, 55% of the
galaxies have contribution > 10% of the 3 Myr/FC component (a degenerate
component in which one cannot separate what is due to a FC or to a 3 Myr
stellar population) at the nucleus, but only 25% of them have this contribution
at 3 kpc. As reference, the stellar population of 10 non-Seyfert galaxies,
spanning the Hubble types of the Seyfert (from S0 to Sc) was also studied. A
comparison between the stellar population of the Seyferts and that of the
non-Seyferts shows systematic differences: the contribution of ages younger
than 1 Gyr is in most cases larger in the Seyfert galaxies than in
non-Seyferts, not only at the nucleus but up to 1 kpc from it.Comment: 23 pages, 18 figures, MNRAS in pres
An atlas of Calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering the
region around the 8498, 8542, 8662 Calcium triplet (CaT) lines. The sample
comprises 78 objects, divided into 43 Seyfert 2s, 26 Seyfert 1s, 3 Starburst
and 6 normal galaxies. The spectra pertain to the inner ~300 pc in radius, and
thus sample the central kinematics and stellar populations of active galaxies.
The data are used to measure stellar velocity dispersions (sigma_star) both
with cross-correlation and direct fitting methods. These measurements are found
to be in good agreement with each-other and with those in previous studies for
objects in common. The CaT equivalent width is also measured. We find average
values and sample dispersions of W_CaT of 4.6+/-2.0, 7.0 and 7.7+/-1.0
angstrons for Seyfert 1s, Seyfert 2s and normal galaxies, respectively. We
further present an atlas of [SIII]\lambda 9069 emission line profiles for a
subset of 40 galaxies. These data are analyzed in a companion paper which
addresses the connection between stellar and Narrow Line Region kinematics, the
behaviour of the CaT equivalent width as a function of sigma_star, activity
type and stellar population properties.Comment: 18 pages, 10 figures, accepted for publication in MNRA
The Nuclear and Circum-nuclear Stellar Population in Seyfert 2 Galaxies: Implications for the Starburst-AGN Connection
We report the results of a spectroscopic investigation of a sample of 20 of
the brightest type 2 Seyfert nuclei. Our goal is to search for the direct
spectroscopic signature of massive stars, and thereby probe the role of
circumnuclear starbursts in the Seyfert phenomenon. The method used is based on
the detection of the higher order Balmer lines and HeI lines in absorption and
the Wolf-Rayet feature at 4680 \AA in emission. These lines are strong
indicators of the presence of young (a few Myrs) and intermediate-age (a few
100 Myrs) stellar populations. In over half the sample, we have detected HeI
and/or strong stellar absorption features in the high-order (near-UV) Balmer
series together with relatively weak lines from an old stellar population. In
three others we detect a broad emission feature near 4680 \AA that is most
plausibly ascribed to a population of Wolf-Rayet stars (the evolved descendants
of the most massive stars). We therefore conclude that the blue and near-UV
light of over half of the sample is dominated by young and/or intermediate age
stars. The ``young'' Seyfert 2's have have larger far-IR luminosities, cooler
mid/far-IR colors, and smaller [OIII]/H flux ratios than the ``old''
ones. These differences are consistent with a starburst playing a significant
energetic role in the former class. We consider the possibility that there may
be two distinct sub-classes of Seyfert 2 nuclei (``starbursts'' and ``hidden
BLR''). However, the fact that hidden BLRs have been found in three of the
``young'' nuclei argues against this, and suggests that nuclear starbursts may
be a more general part of the Seyfert phenomenon.Comment: To be published in ApJ, 546, Jan 10, 200
Circumnuclear stellar population, morphology and environment of Seyfert 2 galaxies: an evolutionary scenario
We investigate the relation between the characteristics of the circumnuclear
stellar population and both the galaxy morphology and the presence of close
companions for a sample of 35 Seyfert 2 nuclei. Fifteen galaxies present
unambiguous signatures of recent episodes of star formation within 300
pc from the nucleus. When we relate this property with the Hubble type of the
host galaxy, we find that the incidence of recent circumnuclear star formation
increases along the Hubble sequence, and seems to be larger than in non-Seyfert
galaxies for the early Hubble types S0 and Sa, but similar to that in
non-Seyfert galaxies for later Hubble types. Both in early-type and late-type
Seyferts, the presence of recent star-formation is related to the galaxy
morphology in the inner few kiloparsecs, as observed in HST images through the
filter F606W by Malkan et al., who has assigned a late ``inner Hubble type'' to
most Seyfert 2s with recent nuclear star-formation. This new classification is
due to the presence of dust lanes and spiral structures in the inner region.
The presence of recent star formation in Seyfert 2 nuclei is also related to
interactions: among the 13 galaxies of the sample with close companions or in
mergers, 9 have recent star formation in the nucleus. These correlations
between the presence of companions, inner morphology and the incidence of
recent star formation suggest an evolutionary scenario in which the interaction
is responsible for sending gas inwards which both feeds the AGN and triggers
star-formation. The starburst then fades with time and the composite Seyfert 2
+ Starburst nucleus evolves to a ``pure'' Seyfert 2 nucleus with an old stellar
population.Comment: 12 pages, 6 figures, 2 table
Is a minor-merger driving the nuclear activity in the Seyfert 2 galaxy NGC 2110?
We report on a detailed morphological and kinematic study of the isolated
non-barred nearby Seyfert 2 galaxy NGC 2110. We combine Integral Field optical
spectroscopy, with long-slit and WFPC2 imaging available in the HST archive to
investigate the fueling mechanism in this galaxy. Previous work (Wilson &
Baldwin 1985) concluded that the kinematic center of the galaxy is displaced
\~220 pc from the apparent mass center of the galaxy, and the ionized gas
follows a remarkably normal rotation curve. Our analysis based on the stellar
kinematics, 2D ionized gas velocity field and dispersion velocity, and high
spatial resolution morphology at V, I and Halpha reveals that: 1) The kinematic
center of NGC 2110 is at the nucleus of the galaxy. 2) The ionized gas is not
in pure rotational motion. 3) The morphology of the 2D distribution of the
emission line widths suggests the presence of a minor axis galactic outflow. 4)
The nucleus is blue-shifted with respect to the stellar systemic velocity,
suggesting the NLR gas is out-flowing due to the interaction with the radio
jet. 5) The ionized gas is red-shifted ~100 km/s over the corresponding
rotational motion south of the nucleus, and 240 km/s with respect to the
nuclear stellar systemic velocity. This velocity is coincident with the HI
red-shifted absorption velocity detected by Gallimore et al (1999). We discuss
the possibility that the kinematics of the south ionized gas could be perturbed
by the collision with a small satellite that impacted on NGC 2110 close to the
center with a highly inclined orbit. Additional support for this interpretation
are the radial dust lanes and tidal debris detected in the V un-sharp masked
image. We suggest that a minor-merger may have driven the nuclear activity in
NGC 2110.Comment: Full resolution images at
http://www.iaa.csic.es/~rosa/preprints/preprints.html or at
http://www.journals.uchicago.edu/ApJ/future.htm
Resolving galaxies in time and space: II: Uncertainties in the spectral synthesis of datacubes
In a companion paper we have presented many products derived from the
application of the spectral synthesis code STARLIGHT to datacubes from the
CALIFA survey, including 2D maps of stellar population properties and 1D
averages in the temporal and spatial dimensions. Here we evaluate the
uncertainties in these products. Uncertainties due to noise and spectral shape
calibration errors and to the synthesis method are investigated by means of a
suite of simulations based on 1638 CALIFA spectra for NGC 2916, with
perturbations amplitudes gauged in terms of the expected errors. A separate
study was conducted to assess uncertainties related to the choice of
evolutionary synthesis models. We compare results obtained with the Bruzual &
Charlot models, a preliminary update of them, and a combination of spectra
derived from the Granada and MILES models. About 100k CALIFA spectra are used
in this comparison.
Noise and shape-related errors at the level expected for CALIFA propagate to
0.10-0.15 dex uncertainties in stellar masses, mean ages and metallicities.
Uncertainties in A_V increase from 0.06 mag in the case of random noise to 0.16
mag for shape errors. Higher order products such as SFHs are more uncertain,
but still relatively stable. Due to the large number statistics of datacubes,
spatial averaging reduces uncertainties while preserving information on the
history and structure of stellar populations. Radial profiles of global
properties, as well as SFHs averaged over different regions are much more
stable than for individual spaxels. Uncertainties related to the choice of base
models are larger than those associated with data and method. Differences in
mean age, mass and metallicity are ~ 0.15 to 0.25 dex, and 0.1 mag in A_V.
Spectral residuals are ~ 1% on average, but with systematic features of up to
4%. The origin of these features is discussed. (Abridged)Comment: A&A, accepte
Probing the near infrared stellar population of Seyfert galaxies
We employ IRTF SpeX NIR (0.8-2.4 microns) spectra to investigate the stellar
population (SP), active galactic nuclei (AGN) featureless continuum (FC) and
hot dust properties in 9 Sy 1 and 15 Sy 2 galaxies. Both the starlight code and
the hot dust as an additional base element were used for the first time in this
spectral range. We found evidence of correlation among the equivalent widths
(W) Si I 1.59 microns x Mg I 1.58 microns, equally for both kinds of activity.
Part of the W{Na I 2.21 microns} and W {CO 2.3 microns} strengths may be
related to galaxy inclination. Our synthesis shows significant differences
between Sy 1 and Sy 2 galaxies: the hot dust component is required to fit the
K-band spectra of ~90% of the Sy 1 galaxies, and only of ~25% of the Sy 2;
about 50 % of the Sy 2 galaxies require a component contribution >20%,
while this fraction increases to 60% in the Sy 1; also, in about 50 % of the
Sy2, the combined FC and young components contribute with more than 20%, while
this occurs in 90% of the Sy1, suggesting recent star formation in the central
region. The central few hundred parsecs of our galaxy sample contain a
substantial fraction of intermediate-age SPs with a mean metallicity near
solar. Our SP synthesis confirms that the 1.1 micron CN band can be used as a
tracer of intermediate-age SPs. The simultaneous fitting of SP, FC and hot dust
components increased in ~150% the number of AGNs with hot dust detected and the
mass estimated. The NIR emerges as an excellent window to study the stellar
population of Sy 1 galaxies, as opposed to the usually heavily attenuated
optical range. Our approach opens a new way to investigate and quantify the
individual contribution of the three most important NIR continuum components
observed in AGNs.Comment: The paper contains 14 figures and 5 tables. Accepted for publication
in MNRA
Stellar Population in LLAGN.I: Ground-based observations
(Abridge): We present a spectroscopic study of the stellar populations of Low
Luminosity AGN (LLAGN). Our main goal is to determine whether the stars who
live in the innermost (100 pc-scale) regions of these galaxies are in some way
related to the emission line properties, which would imply a link between the
stellar population and the ionization mechanism. High signal to noise, ground
based long-slit spectra in the 3500--5500 A interval were collected for 60
galaxies.Our main findings are: (1) Few LLAGN have a detectable young (< 10
Myr) starburst component, indicating that very massive stars do not contribute
significantly to the optical continuum. In particular, no features due to
Wolf-Rayet stars were convincingly detected. (2) High Order Balmer absorption
lines of HI (HOBLs), on the other hand, are detected in 40% of LLAGN.
These features, which are strongest in 100 Myr--1 Gyr intermediate age
stellar populations, are accompanied by diluted metal absorption lines and
bluer colors than other objects in the sample. (3) These intermediate age
populations are very common (50 %) in LLAGN with relatively weak [OI] emission,
but rare (10 %) in LLAGN with stronger [OI]. This is intriguing since LLAGN
with weak [OI] have been previously hypothesized to be ``transition objects''
in which both an AGN and young stars contribute to the emission-line
excitation. Massive stars, if present, are completely outshone by intermediate
age and old stars in the optical. This happens in at least a couple of objects
where independent UV spectroscopy detects young starbursts not seen in the
optical. (4) Objects with predominantly old stars span the whole range of
[OI]/Halpha values, but (5) sources with significant young and/or intermediate
age populations are nearly all (90%) weak [OI] emitters.Comment: To be published in ApJ (April issue). Paper with full resolution
images is at: http://www.iaa.csic.es/~rosa/preprints/preprints.html
http://www.journals.uchicago.edu/ApJ/journa
Stellar Population in LLAGN.II: STIS observations
We present a study of the stellar population in Low Luminosity AGN (LLAGN).
Our goal is to search for spectroscopic signatures of young and intermediate
age stars, and to investigate their relationship with the ionization mechanism
in LLAGN. The method used is based on the stellar population synthesis of the
optical continuum of the innermost (20-100 pc) regions in these galaxies. For
this purpose, we have collected high spatial resolution optical (2900-5700 A)
STIS spectra of 28 nearby LLAGN that are available in the Hubble Space
Telescope archive. The analysis of these data is compared with a similar
analysis also presented here for 51 ground-based spectra of LLAGN. Our main
findings are: (1) No features due to Wolf-Rayet stars were convincingly
detected in the STIS spectra. (2) Young stars contribute very little to the
optical continuum in the ground-based aperture. However, the fraction of light
provided by these stars is higher than 10% in most of the weak-[OI] LLAGN STIS
spectra. (3) Intermediate age stars contribute significantly to the optical
continuum of these nuclei. This population is more frequent in objects with
weak than with strong [OI]. Weak-[OI] LLAGN that have young stars stand out for
their intermediate age population. (4) Most of the strong-[OI] LLAGN have
predominantly old stellar population. A few of these objects also show a
feature-less continuum that contributes significantly to the optical continuum.
These results suggest that young and intermediate age stars do not play a
significant role in the ionization of LLAGN with strong [OI]. However, the
ionization in weak-[OI] LLAGN with young and/or intermediate age population
could be due to stellar processes.Comment: To be published in ApJ (April issue). Paper with full resolution
images is at: http://www.iaa.csic.es/~rosa/preprints/preprints.html
http://www.journals.uchicago.edu/ApJ/journa
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