HST /WFPC2 IMAGING OF THE CIRCUMNUCLEAR STRUCTURE OF LOW-LUMINOSITY ACTIVE GALACTIC NUCLEI. I. DATA AND NUCLEAR MORPHOLOGY (original) (raw)
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The Stellar Populations of Low-Luminosity Active Galactic Nuclei. I. Ground-based Observations
Astrophysical Journal, 2004
In a recently completed survey of the stellar populations properties of LINERS and LINER/HII Transition Objects (TOs), we have identified a numerous class of galactic nuclei which stand out because of their conspicuous 10 8−9 yr populations, traced by high order Balmer absorption lines and other stellar indices. These objects were called "Young-TOs", since they all have TO-like emission line ratios. In this paper we extend this previous work, which concentrated on the nuclear properties, by investigating the radial variations of spectral properties in Low Luminosity Active Galactic Nuclei (LLAGN). Our analysis is based on high signal to noise long-slit spectra in the 3500-5500Å interval for a sample of 47 galaxies. The data probe distances of typically up to 850 pc from the nucleus with a resolution of ∼ 100 pc (∼ 1 ′′ ) and S/N ∼ 30. Stellar population gradients are mapped by the radial profiles of absorption line equivalent widths and continuum colours along the slit. These variations are further analyzed by means of a decomposition of each spectrum in terms of template galaxies representative of very young (≤ 10 7 yr), intermediate age (10 8−9 yr) and old (10 10 yr) stellar populations.
Monthly Notices of the Royal Astronomical Society, 2005
In a recently completed survey of the stellar populations properties of LINERS and LINER/HII Transition Objects (TOs), we have identified a numerous class of galactic nuclei which stand out because of their conspicuous 10 8−9 yr populations, traced by high order Balmer absorption lines and other stellar indices. These objects were called "Young-TOs", since they all have TO-like emission line ratios. In this paper we extend this previous work, which concentrated on the nuclear properties, by investigating the radial variations of spectral properties in Low Luminosity Active Galactic Nuclei (LLAGN). Our analysis is based on high signal to noise long-slit spectra in the 3500-5500Å interval for a sample of 47 galaxies. The data probe distances of typically up to 850 pc from the nucleus with a resolution of ∼ 100 pc (∼ 1 ′′ ) and S/N ∼ 30. Stellar population gradients are mapped by the radial profiles of absorption line equivalent widths and continuum colours along the slit. These variations are further analyzed by means of a decomposition of each spectrum in terms of template galaxies representative of very young (≤ 10 7 yr), intermediate age (10 8−9 yr) and old (10 10 yr) stellar populations.
Astrophysical Journal, 2004
We present a study of the stellar populations of low-luminosity active galactic nuclei (LLAGNs). Our goal is to search for spectroscopic signatures of young and intermediate-age stars and to investigate their relationship with the ionization mechanism in LLAGNs. 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 Å) STIS spectra of 28 nearby LLAGNs 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 LLAGNs. Our main findings are as follows: (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-[O I] ([OI]/Hα<=0.25) 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 [O I]. Weak-[O I] LLAGNs that have young stars stand out for their intermediate-age population. (4) Most of the strong-[O I] LLAGNs have predominantly old stellar population. A few of these objects also show a featureless 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 LLAGNs with strong [O I]. However, the ionization in weak-[O I] LLAGNs with young and/or intermediate-age populations could be due to stellar processes. A comparison of the properties of these objects with Seyfert 2 galaxies that harbor a nuclear starburst suggests that weak-[O I] LLAGNs are the lower luminosity counterparts of the Seyfert 2 composite nuclei. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Based on observations made with the Nordic Optical Telescope (NOT), operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.
Nuclear Gas and Dust Disks in Nearby 3CR Elliptical Galaxies
Astrophysical Journal Supplement Series, 2000
Using WFPC2 V , R, I, and Ha][N II] images, we examine the morphologies, environments, colors, and line emission of dust and gas disks located in the cores of seven low-redshift elliptical galaxies hosting 3C radio sources : NGC 383 (3C 31)/NGC 382, NGC 547 (3C 40), NGC 3862 (3C 264), NGC 4261 (3C 270), NGC 5532 (3C 296), NGC 7720 (3C 465)/NGC 7720A, and UGC 12064 (3C 449). The color maps are generally consistent with absorption by dust in Ñattened conÐgurations viewed at intermediate inclinations. In most cases the disks are nonuniform in continuum light and are surrounded by arcs, Ðlaments, and di †use absorbing clumps, suggesting that the dust in the cores of these galaxies is not dynamically settled. Extended, clumpy or smooth Ha ] [N II] line emission is detected in all four cases (NGC 383, NGC 3862, UGC 12064, and NGC 7720) for which narrowband images are available. Line emission is found at projected locations from the nuclei to the edges of the disks.
Stellar Population In LLAGN. II: STIS Observations
Arxiv preprint astro-ph/ …, 2004
We present a study of the stellar population in Low Luminosity Active Galactic Nuclei (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 (2900-5700Å) 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 8 Jansky Fellow
Physical Parameters and Classification of Eight Galactic Nuclei from the Second Byurakan Survey
Publications of the Astronomical Society of the Pacific, 2007
Accurate spectral parameters estimates are essential to investigate the physical conditions in AGNs. Particularly, intrinsic reddening can be estimated from the Hα/Hβ ratio of the narrow line components. The mass of the central black hole can be obtained from the stellar velocity dispersion in the bulge, which can be estimated from the [O III] λ5007 width, or by the relation between the AGN luminosity and the width of the broad Hβ component. Two different luminosity values are important: one derived from the monochromatic 5100Å, and the integrated luminosity for the broad component of Hβ. The spectral parameters are also a fundamental key to obtain a reliable classification. Spectroscopic observations with a resolution of 4.5Å were performed for a sample of eight galactic nuclei extracted from the Second Byurakan Survey, and one companion galaxy of SBS 1204+505B. The EW and FWHM of the emission lines were measured, and useful line ratios and diagnostic diagrams were used for object classification and reddening estimates. Intrinsic reddening quantities were calculated for all non QSO, i.e. seven objects. Particularly the amount of reddening, B −V color excess, extinction coefficient and optical depths in the V band, at Hα and Hβ wavelengths, and at 5100Å. The broad line region size was also estimated for seven objects, as well as the central black hole masses. Three mass estimates were usually performed for each object and compared. A peculiar line, probably He I λ5048, is detected in the QSO SBS 1626+554. Evidence for a stratified narrow line region is found for the two narrow line Seyfert 1 galaxies included in the sample. A revised classification is proposed for two objects, and the companion galaxy of SBS 1204+505B is reported as a nuclear starburst galaxy.
Stellar Population in LLAGN. I: Ground-based observations
Arxiv preprint astro-ph/ …, 2004
We present a spectroscopic study of the stellar populations of Low Luminosity Active Galactic Nuclei (LLAGN). Our main goal is to determine whether the stars 10 Jansky Fellow 11 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which are operated by AURA, Inc., under a cooperative agreement with the National Science Foundation.
Monthly Notices of the Royal Astronomical Society, 2000
We present photometry, in V,R,I continuum bands and in the Hα + [NII] emission lines, for a sample of four circumnuclear star forming regions (CNSFR), located in galaxies with different kinds of activity in their nuclei: NGC 7469 (Sy 1), NGC 1068 (Sy 2), NGC 7177 (LINER) and NGC 3310 (Starburst).
Nuclear stellar discs in low-luminosity elliptical galaxies: NGC 4458 and 4478
Monthly Notices of the Royal Astronomical Society, 2004
We present the detection of nuclear stellar discs in the low-luminosity elliptical galaxies, NGC 4458 and 4478, which are known to host a kinematically decoupled core. Using archival Hubble Space Telescope imaging, and available absorption line-strength index data based on ground-based spectroscopy, we investigate the photometric parameters and the properties of the stellar populations of these central structures. Their scalelength, h, and face-on central surface brightness, µ c 0 , fit on the µ c 0-h relation for galaxy discs. For NGC 4458, these parameters are typical for nuclear discs, while the same quantities for NGC 4478 lie between those of nuclear discs and the discs of discy ellipticals. We present Lick/Image Dissector Scanner (IDS) absorption line-strength measurements of Hβ, Mg b and Fe along the major and minor axes of the galaxies. We model these data with simple stellar populations that account for the α/Fe overabundance. The counter-rotating central disc of NGC 4458 is found to have similar properties to the decoupled cores of bright ellipticals. This galaxy has been found to be uniformly old despite being counter-rotating. In contrast, the cold central disc of NGC 4478 is younger, richer in metals and less overabundant than the main body of the galaxy. This points to a prolonged star formation history, typical of an undisturbed disc-like, gas-rich (possibly pre-enriched) structure.