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Using the VLT together with the near infrared instrument ISAAC, we have obtained medium spectral and high spatial resolution observations of a sample of nearby Seyfert galaxies in the H-band. This band is particularly suited for stellar population studies since the stellar component dominates over the AGN nucleus. The H-band also includes the peak...
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... 1-D spectra were then extracted from the 2-D frames and corrected for telluric absorption using the early type stel- lar templates, taking into account the differential airmass. An example of the telluric spectrum is shown in Fig. 1. The over- all shape of stellar spectra of the early type and of cool stars we observed were compared to the model spectra taken from Hauschildt et al. (1999a,b) in order to derive relative flux cal- ibrations. The final stellar spectra were found to be in agree- ment within a few percent, over the full wavelength range, with those ...
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... The dynamically decoupled core could be the result of inflow of gas into the central regions of the galaxy via, for example, a past minor merger event. For our target, the majority of the stellar population is old and fairly homogeneous, not showing any variations with radius [2]. There is nevertheless indication of a series of previous star formation bursts [3]. ...
Aim. Numerous studies of the host galaxy of Seyfert nuclei are being conducted in the optical/visible range. However, in the case of Seyfert 1, the spectra of the inner galactic core can be dominated by broad emission lines coming from the nucleus that totally flood the underlying galactic spectrum, preventing any study of stellar populations. Methods. Because part of the IR H-band is free of the strongest AGN emission lines, we developed a method that allows the stellar population of the very internal regions of the Seyfert 1 galaxies to be studied in the presence of a diluting continuum. Results. A new inverse method using the flux as observables was developed and deeply tested. The specificity of the method is to take the non stellar parameters ( reddening, dust emission, and non-stellar continuum) into account directly in the synthetic distance to be minimised. Conclusions. The method is powerful for deriving the stellar content of the very central part of AGN. However, the results for the stellar population are still tentative, as the incompleteness of the stellar base ( lack of supermetallic giant stars) could lead to large uncertainties. New observations of stars in the infrared at high spectral resolution, in particular metal-rich stars, are mandatory to build a complete stellar library that can be used to synthesise the host galaxies of AGN with as much accuracy as possible.
Aims.We present a near-infrared spectral atlas of 47 active galactic nuclei (AGN) of all degrees of activity in the wavelength interval of 0.8-2.4 mum, including the fluxes of the observed emission lines. We analyze the spectroscopic properties of the continuum and emission line spectra of the sources. Methods: . In order to exclude aperture and seeing effects we used near-infrared spectroscopy in the short cross-dispersed mode (SXD, 0.8-2.4 mum), taking the JHK-bands spectra simultaneously. Results: .We present the most extensive NIR spectral atlas of AGN to date. This atlas offers a suitable database for studying the continuum and line emission properties of these objects in a region full of interesting features. The shape of the continuum of QSOs and Sy 1's are similar, being essentially flat in the H and K bands, while a strong variation is found in the J band. In Seyfert 2 galaxies, the continuum in the Flambda × lambda space smoothly decreases in flux from 1.2 mum redwards in almost all sources. In J, it smoothly rises bluewards in some sources, while in others a small decrease in flux is observed. The spectra are dominated by strong emission features of H I, He I, He II, [S III] and by conspicuous forbidden lines of low and high ionization species. Molecular lines of H{2} are common features of most objects. The absence of O I and Fe II lines in Seyfert 2 galaxies and the smaller FWHM of these lines relative to that of H I in the Seyfert 1 give observational support to the fact that they are formed in the outermost portion of the broad-line region. The[P II] and coronal lines are detected for all degrees of activity. The [Fe II] 12 570 Å/16 436 Å line ratio becomes a reliable reddening indicator for the narrow-line region of Seyfert galaxies.
In this paper we apply the stellar population synthesis method previously described in Boisson et al. (\cite{Boisson2000}) to five more AGN. The analysis of these new data strengthen our previous conclusions: i) homogeneity of the stellar population within a class of nuclear activity regardless of the morphological type of the host galaxy; ii) populations within the nuclear regions of LINERs and Seyfert 2s are different: LINERs have a very old metal-rich population while in the Seyfert 2s a contribution of a weak burst of star formation is observed together with the old high metallicity component; iii) in the circum-nuclar region (200 pc
We present the first near-IR integral field spectroscopy observations of the galaxy MCG–6-30-15. The H-band data studied in this paper cover the central 500 pc of the galaxy at the best resolution (0.1 arcsec) so far. The spectra
of the innermost regions are dominated by broad Brackett series emission lines and non-stellar continuum, under which we are
able to trace the distribution and kinematics of the stars and also the [Fe ii] line emission. We find that there is a counter-rotating stellar core extending out to 125 pc, which appears to be associated
with the [Fe ii] emission. Based on the mass-to-light ratio, and the presence of this emission line, we estimate the age of the central stellar
population to be of the order of 65 Myr. We show that the gas needed to fuel the black hole is, at most, only 1 per cent of
that needed to form these stars. We derive independent constraints on the black hole mass using the dynamical information
and determine an upper limit for the black hole mass, MBH < 6 × 107 M⊙, that is consistent with other estimates.
In order to understand the fueling mechanism that powers low level AGNs,
a large effort was made by the DEGAS consortium to collect the necessary
observational material for a sample of isolated Seyferts and a well
matched control sample of normal spirals. Based on the analysis of their
NIR images, on long slit spectroscopy at various slit position angles in
the Halpha emission and Calcium triplet absorption lines we conclude
that: (a) the morphology and large scale kinematics of Seyfert hosts and
normal spirals are equivalent; (b) the differences claimed to be related
to the mechanisms driving AGN activity may only be found in details
related to circumnuclear regions still unresolved by our observations;
(c) the presence of drops in the stellar velocity dispersions seems
related to the existence of a central elongated structure, probably an
inner disk, where the CaT equivalent width also reaches a maximum. We
have started a project to study a much larger sample of AGN and normal
host spirals with better spatial and spectral resolutions to see whether
inner disks, velocity drops and young stellar populations are related
phenomena, as suggested by present day numerical simulations. Optical
and NIR HST images of all these galaxies have already been analyzed, and
the spectroscopic data is being collected.
One of the main issues concerning Nuclear Activity in galaxies (AGNs) is
to understand the triggering mechanisms for the onset of non-thermal
emission in their nuclei. Both the origin of the gas accreted onto the
black hole and the physical mechanisms for the loose of angular momentum
required for this funnelling to be effective, have to be elucidated. In
other words, the goal is to understand the needed conditions to switch
on the AGN activity. But still many aspects of the investigation are a
matter of debate. Among them, the role played by gravitational
interactions and the relevance of the host galaxy need to be clarified.
In this review, the different relationships between AGN activity, the
morphological type of the host galaxy and the environment are discussed,
in order to understand whether the AGN activity is more related to
interacting effects or otherwise can be due to the secular evolution in
the hosting galaxies.
To understand the nuclear stellar populations and star formation histories of the nuclei of spiral galaxies, we have obtained K-band nuclear spectra for 41 galaxies and H-band spectra for 20 galaxies in the ISO Atlas of Bright Spiral Galaxies. In the vast majority of the subsample (80%), the near-infrared spectra suggest that evolved red stars completely dominate the nuclear stellar populations and that hot young stars are virtually non-existent. The signatures of recent star formation activity are only found in 20% of the subsample, even though older red stars still dominate the stellar populations in these galaxies. Given the dominance of evolved stars in most galaxy nuclei and the nature of the emission lines in the galaxies where they were detected, we suggest that nuclear star formation proceeds in the form of instantaneous bursts. The stars produced by these bursts comprise only ~2% of the total nuclear stellar mass in these galaxies, but we demonstrate how the nuclear stellar populations of normal spiral galaxies can be built up through a series of these bursts. The bursts were detected only in Sbc galaxies and later, and both bars and interactions appeared to be sufficient but not necessary triggers for the nuclear star formation activity. The vast majority of galaxies with nuclear star formation were classified as HII galaxies. With one exception, LINERs and transition objects were dominated by older red stars, which suggested that star formation was not responsible for generating these galaxies' optical line emission. Comment: AJ, 2004, in press
For applications in population synthesis, libraries of theoretical stellar spectra are often considered an alternative to template libraries of observed spectra, because they allow a complete sampling of stellar parameters. Most attention in published theoretical spectral libraries has been devoted to the visual wavelength range. We present a detailed comparison of theoretical spectra in the range 1.57-1.67$\mu$m, for spectral types from A to early M and for giants and dwarf stars, with observed stellar spectra at resolutions around 3000, which would be sufficient to disentangle the different groups of late type stars. We have selected the NeMo grids of stellar atmospheres to perform such a comparison. We first demonstrate that after combining atomic and molecular line lists, it is possible to match observed spectral flux distributions with theoretical ones very well for almost the entire parameter range covered by the NeMo grids at moderate resolution in the visual range. In the infrared range, although the overall shape of the observed flux distributions is still matched reasonably well, the individual spectral features are reproduced by the theoretical spectra only for stars earlier than mid F type. For later spectral types the differences increase and theoretical spectra of K type stars have systematically weaker line features than those found in observations. These discrepancies are traced back to stem primarily from incomplete data on neutral atomic lines, although some of them are also related to molecules. Improving atomic data in the near infrared is a key element in making the construction of reliable libraries of stellar spectra in the infrared feasible. Comment: 17 pages, 19 figures, accepted for publication in A&A
Near-infrared (hereafter NIR) data may provide complementary information to the traditional optical population synthesis analysis of unresolved stellar populations because the spectral energy distribution of the galaxies in the 1-2.5\mum range is dominated by different types of stars than at optical wavelengths. Furthermore, NIR data are subjected to less absorption and hence could constrain the stellar populations in dust-obscured galaxies. We want to develop observational constraints on the stellar populations of unresolved stellar systems in the NIR. To achieve this goal we need a benchmark sample of NIR spectra of ``simple'' early-type galaxies, to be used for testing and calibrating the outputs of population synthesis models. We obtained low-resolution (R~1000) long-slit spectra between 1.5 and 2.4\mum for 14 nearby early-type galaxies using SofI at NTT and higher resolution (R~3000) long-slit spectra, centered at the MgI at ~1.51\mum for a heterogeneous sample of 5 nearby galaxies observed with ISAAC at VLT. We defined spectral indices for CO, NaI, CaI and MgI features and measured the strengths of these features in the sample galaxies. We defined a new global NIR metallicity index, suitable for abundance measurements in low-resolution spectra. Finally, we present an average NIR spectrum of an early-type galaxy, built from a homogenized subset of our sample. The NIR spectra of the sample galaxies show great similarity and the strength of some features does correlate with the iron abundance [Fe/H] and optical metal features of the galaxies. The data suggest that the NIR metal features, in combination with a hydrogen absorption feature may be able to break the age-metallicity degeneracy just like the Mg and Fe features in the optical wavelength range. Comment: 11pages, 11 figures, accepted for publication in A&A
