1,721,658 research outputs found
SPECTROSCOPY OF THE CIRCUMNUCLEAR EMISSION LINE REGIONS IN SEYFERT GALAXIES: I. NGC 1667
No full textIn this paper we investigate the physical properties in the circumnuclear and extranuclear regions of the Seyfert 2 galaxy NGC 1667. Long-slit, medium-resolution spectra are used to derive the ionization properties of the gas in nuclear and circumnuclear regions with the aid of a grid of photoionization models computed with the code CLOUDY 84. In the nuclear regions the high [NII] λ6583/Hα and [SII] λ6725/Hα ratios are best fitted assuming an overabundance of nitrogen and sulphur relative to the solar value in clouds photoionized by a 'hard' power-law continuum. The Seyfert nucleus is surrounded by extended HII regions the ionization of which is only weakly influenced by the central engine. The star formation rates (SFR) in these regions are evaluated from the measured Hα luminosities and compared with the values derived from infrared data. The equivalent stellar content in terms of O5 stars estimated in the extranuclear regions, N(O5)~700 - 3000, is a strong evidence that an intense star formation activity is occurring in the inner 10kpc. The observed velocity curve is well fitted by a model of spherical gravitational potential and purely circular motions. We derived a mass M~1.3E11 Solar Masses within a 30'' (~9kpc) radius
Optical spectral evolution of Nova Cygni 1992 = V 1974 Cygni
No full textWe report the results of the optical spectrophotometric observations of N Cyg 1992 = V 1974 Cyg performed at Asiago between 6 and 670 days after its visual maximum (1992 February 22, m_v approximately 4.4). On 1992 February 28, during the first decline the spectrum of the nova was characterized by the presence over a strong continuum of wide emission lines of HI (Balmer), FeII, NaI, CaII, flanked by two systems of P Cyg absorptions with mean radial velocities of -1250 and -1900 km/s. The transition phase was reached at the beginning of April with a gradual fading of the lines of lower ionization potential and the emerging of the HeI, NII, NIII lines. P Cygni absorption features with radial velocity higher than in February were still perceptible. The nova entered the nebular stage some weeks later, at the end of April. The presence in the spectrum, besides (OIII) λλ 4959, 5007, of strong forbidden lines of (NeIII) at λλ 3869, 3968, left no doubt of its classification as a 'neon nova' similar to QU Vul. The evolution of the spectrum in the next months was characterized by the progressive growth of the ionization degree. Following the forbidden lines of (NeIII) and (OIII), also those of (NeIV), (FeVI), (FeVII) and (NeV) became outstanding. The highest degree of ionization was attained on 1993 July, about 500 days from visual maximum, when the coronal lines of (FeX), (FeXI), (AX) reached their highest strength. The steadily decline of the degree of ionization began about one-hundred days later, with the rapid disappearance of the coronal lines and the progressive fading of the lines of highest ionization potential. The intensities of the emission lines during the nova evolution are reported. The discussion concerns the determination of: color excess, absolute magnitude and distance (2.8 kpc) of the nova; effective temperature of the contracting photosphere; and abundance in the ejecta of He, O, N, and Ne, relative to H
Spectroscopy of the circumnuclear emission lines regions in Seyfert Galaxies II. NGC 7214
No full textThis paper is the second of a series (Radovich & Rafanelli 1996) devoted to the investigation of the emission line properties of the circumnuclear and extranuclear regions in a sample of nearby Seyfert galaxies. Our aim is to identify star formation regions around the active nucleus and to derive their properties like star formation rate and dust content, and compare them with those observed in non-active galaxies. We show that photoionization models may fit the observed line ratios both in the nuclear and circumnuclear regions of the Sy 1.2 galaxy NGC 7214. In the nucleus, components with different densities must be considered. In the circumnuclear regions, line ratios to projected distances of ~ 2 kpc from the nucleus are typical of ionization by a power law. At higher distances ( ~ 5-8 kpc) they are fitted either by a thermal continuum or assuming that both stars and the non thermal source contribute to the ionization of the gas. A simple interpretation is suggested, in the framework of the unified model for Seyfert galaxies. Evidence for enhanced star formation, maybe related to the ongoing interaction with a close spiral galaxy, is found in only one region. Based on observations collected at ESO - La Silla (Chile)
A review of diagnostic emission line ratios in the narrow line region of active galactic nuclei
No full textThe availability of new observing facilities both from ground and space such as the Keck 10 m telescope and the Hubble Space Telescope is casting new light on the spectroscopic investigation of emission line galaxies. In particular, it is now possible to detect spectra with a significantly improved signal to noise ratio in a very wide wavelength range, from the ultraviolet to the near infrared (HST, Keck) and beyond (ISO is unveiling the far infrared domain). As one moves to high redshifts, however, it is more likely that the observed spectra are given by the contribution of different components in the galaxy: in particular, this may be the case if an active galactic nucleus (AGN) emitting a power-law continuum is surrounded by regions with strong star formation activity. The identification of the source which ionizes the line emitting regions is then complicated by the fact that one is observing the integrated spectrum from regions which are ionized by different sources. In this paper the authors wish to review which line ratios may be used in order to discriminate between photoionization by young stars and power-law continuum in the wavelength range from 1200 Å to 100 μm. To this aim they used the photoionization code Cloudy (Ferland 1996) to present a series of diagnostic diagrams showing the dependence of emission line ratios on the main input parameters of photoionization models in the case of one component models with gaseous clouds ionized by (1) a stellar continuum typical of an H II region and (2) a power-law continuum typical of an AGN: these line ratios are plotted as isoratio maps for different values of the hydrogen density, ionization parameter and slope of the power-law continuum
Spectroscopic analysis of the nuclear and circumnuclear regions of the Seyfert 2 galaxy NGC 7130
No full textWe discuss here the spectroscopic properties of the Seyfert 2 galaxy NGC 7130 (= IC 5135). Emission line regions were isolated and line ratios were measured and fitted with photoionization models; this allowed to discriminate between thermal and non thermal ionization in the circumnuclear regions. Massive star formation is likely to occur at projected distances from the nucleus >= 2 kpc: line profiles in these regions suggest the presence of outflows of gas due to stellar winds from hot massive stars
The interpretation of the emission spectra of the Seyfert 2 galaxy NGC 7130: determination of the active galactic nucleus and starburst contributions
No full textDetailed modeling of the different regions of NGC 7130 is presented, accounting for its composite nature of an active galactic nucleus (AGN) and a starburst galaxy. Shock waves, created by stellar winds from hot massive stars and by supernova ejecta, are evident in the continuum and line spectra emitted from the clouds. Therefore, the SUMA code, which accounts consistently for photoionization and shocks, is adopted in model calculation. The results show that the nuclear region is dominated by gas ionized by a power-law radiation flux from the active center (AC). High-velocity (V = 1000 km/s) clouds, which account for the broad FWHM component of the line profile, are found close to the AC and are characterized by a high dust-to-gas ratio (>1E-12, while the dust-to-gas ratio is about 1E-14 throughout the galaxy). Massive stars with temperatures of (5-7)×1E4 K photoionize and heat the gas in the outer regions, and old star population (T = 3000 K) background radiation contributes to the fit of the continuum in the optical-near-IR range. The AGN-starburst connection is discussed on the basis of model results, considering, particularly, the distribution of densities and velocities throughout the galaxy
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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