arXiv subjects

Thaisa Storchi-Bergmann

Publications and source records attributed to Thaisa Storchi-Bergmann.

157 records · Page 9Linked to original sources

arXiv2004

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.

astro-ph↗

arXiv2003

Long Term Profile Variability of Double-Peaked Emission Lines in AGNs

An increasing number of AGNs exhibit broad, double-peaked Balmer emission lines, which are thought to arise from the outer regions of the accretion disk which fuels the AGN. The line profiles are observed to vary on a characteristic timescales of 5-10 years. The variability is not a reverberation effect; it is a manifestation of physical changes in the disk. Our group has monitored a set of 20 double-peaked emitters for the past 8 years (longer for some objects). Here, we characterize the variability of the double-peaked H alpha line profiles in five objects from our sample. By experimenting with simple models, we find that disks with a single precessing spiral arm are able to reproduce many of the variability trends that are seen in the data.

astro-ph↗

arXiv2002

Stellar population gradients in Seyfert 2 galaxies

We study the variation of the stellar population properties as a function of distance from the nucleus for a sample of 35 Seyfert 2 galaxies using the technique of stellar population synthesis. We sample regions at the galaxies with dimensions in the range 200 x 200 to 400 x 400 pc and compare the synthesis results with those of a control sample of non-Seyfert galaxies. We find that both at the nucleus and up to 3 kpc from it the oldest age component (10 Gyr) presents a smaller contribution to the total flux in the Seyfert than in the non-Seyfert galaxies of the same Hubble type, while the components younger than 100 Myr present a larger contribution in the Seyfert's than in non-Seyferts. In addition, while for the non-Seyferts clear gradients are present, in which the contribution of the oldest components decrease with distance from the nucleus and the contribution of the 1 Gyr component increases -- we do not find such gradients in most Seyferts. These results suggest that the AGN-starburst connection is a large scale phenomenon affecting not only the inner few hundred parsecs, but the inner kiloparsecs.

astro-ph↗

arXiv2002

Feeding the active nucleus of NGC 1097

We discuss the signatures of an evolving accretion disk around the nucleus of NGC 1097, based on observations of its double-peaked H-alpha emission-line profile which cover a time span of 10 yrs. The monitoring of the profile has allowed us to better constrain the inner radius of the accretion disk, and to propose a scenario in which the ionizing source is getting dimmer and making the peak of the disk emissivity to drift inwards.

astro-ph↗

arXiv2001

Diagnostics of composite starburst+Seyfert 2 nuclei: Hints on the starburst-AGN connection

We present a simple population synthesis scheme which recognizes composite starburst+Seyfert 2 nuclei from a few easy-to-obtain optical measurements. Composite systems seem to evolve towards less luminous Seyfert 2's which do not harbor detectable circum-nuclear starbursts. We encourage applications of this cheap diagnostic tool to large samples of Seyfert 2's, as well as its extension to other activity classes, in order to test and refine this evolutionary scenario.

astro-ph↗

arXiv2001

The frequency of nuclear star-formation in Seyfert 2 galaxies

We investigate the detectability of starburst signatures in the nuclear spectrum of Seyfert 2 galaxies by constructing spectral models in the wavelength range 3500-4100A, combining the spectrum of a bulge population (of age ~10Gyr) with that of younger stellar populations, spanning ages from ~3 Myr to 1 Gyr. We also construct models combining the bulge template with a power-law (PL) continuum, which is observed in some Seyfert 2's in polarized light, contributing with typically 10-40% of the flux at 4020A. We conclude that such continuum cannot be distinguished from that of a very young stellar population (age < 10 Myr), contributing with less than ~0.02% of the mass of the bulge. The models are compared with nuclear spectra - corresponding to a radius of 200-300 pc at the galaxy - of 20 Seyfert 2 galaxies, in which we specifically look for the signatures above of young to intermediate age stellar populations. We find them in ten galaxies, thus 50% of the sample. But only in six cases (30% of the sample) they can 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 due to intermediate age stars (~1 Gyr). We find a tendency for the young stars to be found more frequently among the late type Seyfert's. This tendency is supported by a comparison between the equivalent widths (W) of absorption lines of the nuclear spectra of the Seyfert 2's with those of normal galaxies of the same Hubble type.

astro-ph↗

arXiv2000

Extended Gas in Seyfert Galaxies: Near-Infrared Observations of 15 Active Nuclei

Results from an analysis of low resolution (R~250) near-IR long-slit spectra covering simultaneously the I, J, H, and K bands, for a sample of 15 Seyfert galaxies and the N5253 starburst nucleus, are presented. The Seyfert galaxies were selected as presenting `linear' or cone-like high excitation emission line in the optical, most probably due to the collimation of the central source's radiation by a dusty molecular torus. Our goal was to look for signatures of this torus, and to investigate the gaseous distribution, excitation and reddening. The IR emission lines are spatially extended in most cases, and we have used the [FeII]/Pa(beta) ratio as a measure of the gaseous excitation in Mrk573, N1386, and N7582. Values for this ratio between 1.5 and 6 are found, suggesting excitation of [FeII] by X-rays or shock waves in some regions. Nuclear Pa(beta) in N1365, and possibly nuclear Br(gama) in Mrk573, are broad. From analysis of the spatial distribution of the continuum (J-H) and (H-K) colours derived from our spectra, we find redder colours for the nucleus than the nearby bulge in most of the Seyfert 2s observed. Comparison with models including emission from dust and stars shows that hot (T~1000 K) dust emission dominates the nuclear continuum in N1365, N2110, N3281, N7582, and ESO362-G18. In N1386, N5643, and N5728 the main contributor is the underlying stellar population, combined with some foreground reddening and/or cool dust emission. In a few cases, the (J-H) colours on opposite sides of the nucleus differ by 0.3-0.8 mag, an effect that we interpret as partly due to differences in the local stellar population, and possibly extinction gradients.

astro-ph↗

arXiv1999

Nuclear Activity in Nearby Galaxies

I discuss some recent observational results in the research of nearby active galactic nuclei (AGN). These results cover three main topics: (i) evidences for the current paradigm for AGN's, which include a nuclear supermassive blackhole (SMBH) fed via an accretion disk; (ii) evidence that this paradigm may also apply to LINER's, the lowest luminous AGN's and to normal galaxies; (iii) evidences of how the fueling of the SMBH occurs and its relation to recent and intermediate age (10^6 to 10^8 yrs old) episodes of star formation.

astro-ph↗

arXiv1998

The Nature of the UV-Optical Continuum in Seyfert 2 Galaxies

The nature of the UV-Optical continuum in Seyfert 2 galaxies is a matter of current debate, fundamental to which is the issue of characterizing and quantifying the stellar population contribution to the nuclear spectra. Using high S/N long-slit spectroscopy of a sample of 20 Seyfert 2 galaxies, we have applied a novel approach to investigate the nuclear stellar population in these galaxies. Our main results are: (1) the stellar populations in Seyfert 2 galaxies are varied, and in most cases cannot be adequately represented by an elliptical galaxy template, as done in previous works; (2) the central kpc of most Seyfert 2's contain substantially larger proportions of 100 Myr stars than either elliptical galaxies or normal spirals of the same Hubble type. One important consequence of our findings is that the controversial nature of the so called ``second'' featureless continuum (FC2) in Seyfert 2's is most likely a result of inadequate evaluation of the stellar population.

astro-ph↗

arXiv1998

Spectral Synthesis of the Nuclear Region of Seyfert 2 and Radio Galaxies

We present the results of an optical spectral synthesis analysis for the nuclei of 20 Seyfert 2 and 4 Radio Galaxies, using a base of stellar population templates of different ages and metallicities and a power-law continuum. Compared with the stellar population of elliptical galaxies, we find that Seyfert 2s usually have a smaller contribution from old metal rich stars (10 Gyr, Z>=Z_o), and a larger contribution from stars with 100 Myr. We also find that the contributions from stars with age <=10 Myr and from a power law continuum are small, rarely exceeding 5 per cent. These results show that the general assumption of elliptical galaxies as stellar population templates for these objects is incorrect, also implying that the excess blue continuum frequently found in their nuclear spectra is probably due to this template mismatch. We find a considerable contribution from 100 Myr stars (~5 per cent), which can be interpreted from the point of view of models where the fueling of the AGN is done by interactions/mergers.

astro-ph↗

arXiv1998

The Stellar Content of Active Galaxies

We present results of a long-slit spectroscopic study of 39 active and 3 normal galaxies. Stellar absorption features, continuum colors and their radial variations are analyzed in an effort to characterize the stellar population in these galaxies and detect the presence of a featureless continuum underlying the starlight spectral component. Spatial variations of the equivalent widths of conspicuous absorption lines and continuum colors are detected in most galaxies. Star-forming rings, in particular, leave clear fingerprints in the equivalent widths and color profiles. We find that the stellar populations in the inner regions of active galaxies present a variety of characteristics, and cannot be represented by a single starlight template. Dilution of the stellar lines by an underlying featureless continuum is detected in most broad-lined objects, but little or no dilution is found for the most of the 20 type 2 Seyferts in the sample. Color gradients are also ubiquitous. In particular, all but one of the observed Seyfert 2s are redder at the nucleus than in its immediate vicinity. Possible consequences of these findings are outlined.

astro-ph↗

arXiv1998

The Nature of the Optical Light in Seyfert 2 Galaxies with Polarized Continuum

We investigate the nature of the optical continuum and stellar population in the central kpc of the Seyfert 2s Mrk 348, Mrk 573, NGC 1358 and Mrk 1210 using long-slit spectra obtained along the radio or extended emission axis. These galaxies are known to have polarized continuum-including polarized broad lines in Mrk 348 and Mrk 1210--and previous studies indicate featureless continuum (FC) contributions in the 20-50% range at 5500 A. Nevertheless, our measurements of the equivalent widths of absorption lines and continuum ratios as a function of distance from the nuclei show no dilution of the lines nor bluening of the spectrum, as expected if a blue FC was present at the nucleus in the above proportions. We investigate one possibility to account for this effect: that the stellar population at the nucleus is the same as that from the surrounding bulge and dominates the nuclear light. A spectral analysis confirms that this hypothesis works for Mrk 348, NGC 1358 and Mrk 1210, for which we find stellar contributions at the nucleus larger than 90% at all wavelengths. We find that a larger stellar population contribution to the nuclear spectra can play the role of the ``second FC'' source inferred from previous studies. Stellar population synthesis shows that the nuclear regions of Mrk 348 and Mrk 1210 have important contributions of young to intermediate age stars (0--100 Myr), not present in templates of elliptical galaxies. In the case of Mrk 1210, this is further confirmed by the detection of a ``Wolf-Rayet feature'' in the nuclear emission-line spectrum.

astro-ph↗

arXiv1997

Chemical Abundance Calibrations for the Narrow-Line-Region of Active Galaxies

We investigate two chemical abundance calibrations for the narrow-line-region (NLR) of active galaxies in terms of three easily observable optical emission-line ratios, namely, [OIII]$λλ$4959,5007/H$β$, [NII]$λλ$6548,84/H$α$ and [OII]$\lambda3727/$[OIII]$λλ$4959,5007. The calibrations are obtained from a grid of models on the assumption that the main process responsible for the production of these lines is photoionization by a ``typical'' active galactic nucleus continuum. The chemical elements vary their abundance together with oxygen, except nitrogen, which is assumed to be a product of secondary nucleosynthesis. The calibrations are calculated for the range $8.4 \le 12+log(O/H) \le 9.4$, and tested using NLR data for a sample of Seyfert's and LINER's having HII regions in the vicinity of the nucleus. The gaseous abundances of these HII regions have been determined in previous works, and the NLR abundances are obtained on the assumption that they can be extrapolated from those of the HII regions. The calibrations work very well for the Seyfert's, giving abundance values which agree with those obtained from the HII regions, and can thus be used for quick estimates of the chemical abundances of the NLR's. For the LINER's, the calibrations give lower values than those derived from the corresponding HII regions, suggesting that the assumptions of the models do not apply for them, and that there are different physical processes at work in the NLR of the LINER's.

astro-ph↗