Narrow-line Region Of Active Galaxies Research Articles

Modelling the narrow-line regions of active galaxies in the Sloan Digital Sky Survey – I. Sample selection and physical conditions

We systematically determine the electron density and electron temperature of active galaxies and star-forming galaxies using spectroscopy from the SDSS DR7, while mainly focusing on the NLRs. Density and temperature are determined through the I[S II] 6716/6731 and I[O III] 5007/4363 ratios, respectively, in our [O III] 4363 emission sample of 15 019 galaxies. We find two sequences and the typical range of density in the NLRs of AGNs is 100-1000 /cm3. The temperatures in the NLRs range from 10 000 to 20 000 K for Seyferts, and the ranges were even higher and wider for LINERs and composites. We also propose that Y_LINER ~ Y_Seyfert > Y_composite > Y_star-forming, where Y is the characteristic present-day star-formation time-scale. While in the AGN case, we find several strong lines of evidence suggest that some supplementary energy source(s) should be responsible for high ionization potential.

A Comparison of Stellar and Gaseous Kinematics in the Nuclei of Active Galaxies

To investigate the relationship between black holes and their host galaxies, many groups have used the width of the [O III] 5007 line as a substitute for the stellar velocity dispersion (sigma*) of galaxy bulges. We directly test this assumption with a large and homogeneous sample of narrow-line active galactic nuclei from the Sloan Digital Sky Survey. We consider multiple transitions ([O II] 3727, [O III] 5007, and [S II] 6716, 6731) and various techniques for quantifying the line width in order to obtain a calibration between the gas velocity dispersion, sigma_g, and sigma*. We find that sigma_g of the low-ionization lines traces sigma*, as does sigma_g for the core of [O III] after its asymmetric blue wing is properly removed, although in all cases the correlation between sigma_g and sigma* has considerable scatter. While the gas kinematics of the narrow-line region of active galaxies are primarily governed by the gravitational potential of the stars, the accretion rate, as traced by the Eddington luminosity ratio, seems to play an important secondary role. Departures from virial motions correlate systematically with accretion rate. We discuss the implications of these results for previous studies that use [O III] line widths to infer stellar velocity dispersions in quasars and narrow-line Seyfert 1 galaxies.

Ultraviolet Continuum, Physical Conditions, and Filling Factor in Active Galactic Nuclei

The narrow-line region of active galaxies is formed by gas clouds surrounded by a diluted gas. Standard one-dimensional photoionization models are usually used to model this region in order to reproduce the observed emission lines. Since the narrow-line region is not homogeneous, two major types of models are used: (1) those assuming a homogeneous gas distribution and a filling factor less than unity to mimic the presence of the emitting clouds and (2) those based on a composition of single-cloud models combined in order to obtain the observed spectra. The first method is largely used but may induce misleading conclusions, as shown in this paper. The second one is more appropriate but requires a large number of observed lines in order to limit the number of single models used. After discussing the case of an extragalactic H II region, for which the ionizing radiation spectrum is better known, we show that one-dimensional models for the narrow-line region with a filling factor less than unity do not properly mimic the clumpiness but just simulate an overall lower density. Multicloud models lead to more reliable results. Both models are tested in this paper, using the emission-line spectra of two well-known Seyfert galaxies, NGC 4151 and NGC 1068. It is shown that ionizing radiation spectra with a blue bump cannot be excluded by multicloud models, although excluded by Alexander et al. using homogeneous models with a filling factor less than unity.

A Grid of Composite Models for the Simulation of the Emission‐Line Spectra from the Narrow‐Line Regions of Active Galaxies

A grid of composite models for the narrow-line region of active galaxies, which consistently account for both the effect of a photoionizing radiation from the active center and of a shock front, is presented. Theoretical results, calculated with the SUMA code, are given for different values of shock velocities, preshock densities, geometrical thickness of the clouds, and, particularly, for the ionizing radiation intensity in a large range. The input parameters are chosen within the ranges indicated by previous fits of several observed emission-line and continuum spectra from active galaxies. Shock velocities from 100 to 1500 km s-1 and preshock densities from 100 to 1000 cm-3 are considered. The line intensities of the most important ultraviolet, optical, and infrared transitions are obtained and are listed in several tables.

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, [O III] λλ4959, 5007/Hβ; [N II] λλ6548, 6584/Hα; and [O II] λ3727/[O III] λλ4959, 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 elemental abundances vary with oxygen, except nitrogen, which is assumed to be a product of secondary nucleosynthesis. The calibrations are calculated for the range 8.4 ≤ 12 + log (O/H) ≤ 9.4 and tested using NLR data for a sample of Seyfert galaxies and LINERs that have H II regions in the vicinity of the nucleus. The gaseous abundances of these H II 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 H II regions. The calibrations work very well for the Seyfert galaxies, yielding abundance values that agree with those obtained from the H II regions, and can thus be used for quick estimates of the chemical abundances of the NLRs. For the LINERs, the calibrations yield lower values than those derived from the corresponding H II 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 LINERs.

Stability of narrow emission line clouds in active galactic nuclei

view Abstract Citations (29) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stability of Narrow Emission-Line Clouds in Active Galactic Nuclei Mathews, William G. ; Veilleux, Sylvain Abstract Clouds in the narrow-line region of active galaxies can be significantly eroded or destroyed by a variety of instabilities. Narrow-line clouds having column densities N <~ 10^22^ cm^-2^, at rest and in pressure equilibrium with an intercloud medium, are intrinsically unstable because the momentum transferred by continuum radiation is absorbed and raises the internal pressure above the ambient value. This results in a lateral flow of gas away from the sides of the clouds which can erode or ultimately destroy clouds before they reach desired velocities. Lateral flow instabilities can sharply limit the lifetime of clouds even though the radiation absorbed is otherwise dynamically insignificant. If these same clouds are pushed by a wind in the absence of radiation forces, they are quickly destroyed by Rayleigh-Taylor instabilities. Ironically, when radiation and wind forces act together, the clouds can be effectively stabilized against both lateral flow disruption and Rayleigh-Taylor instabilities, even when the dynamical pressure on the accelerating clouds greatly exceeds their internal thermal pressure. Stable clouds with columns N >~ 10^23^ can be accelerated to observed narrow-line velocities before the lateral flow instability becomes important. Very thick clouds having N >~ 10^24^ are not in hydrostatic equilibrium and may be too large to fit into narrow-line regions having modest or small covering factors. Clouds with column densities N ~ 10^23^ are the most likely candidates for the narrow-line region; they can be accelerated without appreciable disruption to observed velocities in just a few radial sound crossing times. Winds in the narrow-line intercloud medium provide the best means of rapid cloud acceleration. Narrow-line clouds falling radially inward under the influence of gravity are unable to reach appropriate velocities before the disruptive instabilities set in. Clouds in the narrow-line region, like their broad-line cousins, are more fragile and transient than was formerly thought. Publication: The Astrophysical Journal Pub Date: January 1989 DOI: 10.1086/166996 Bibcode: 1989ApJ...336...93M Keywords: Active Galactic Nuclei; Emission Spectra; Line Spectra; Molecular Clouds; Taylor Instability; Gas Flow; Hydrodynamics; Interstellar Gas; Seyfert Galaxies; Astrophysics; GALAXIES: NUCLEI; GALAXIES: SEYFERT; HYDRODYNAMICS; INSTABILITIES full text sources ADS |

THEORETICAL HYDROGEN-LINE RATIOS FOR THE NARROW-LINE REGIONS OF ACTIVE GALACTIC NUCLEI

ABSTRACT The authors present calculations of the relative strengths of Lα, Hα, Hβ, Hγ, and Pα for low-density gas clouds photoionized by a continuum such as might be found in active galactic nuclei. They confirm the finding of Halpern (1982) that the Hα/Hβ ratio is insensitive to the ionization parameter. The ratio also depends only weakly on the electron density. Hα/Hβ is mostly a function of metallicity and the shape of the UV to X-ray continuum. It is recommended that Hα/Hβ should be "reinstated" as an indicator of reddening for the narrow-line regions of active galaxies, but with an assumed intrinsic value of about 3.0, as proposed by Halpern, rather than the previous case B value of 2.85.

REDDENING OF THE NARROW-LINE REGIONS OF ACTIVE GALAXIES AND THE INTRINSIC BALMER DECREMENT

ABSTRACT The reliability of the narrow line Balmer decrement as an indicator of reddening is examined. Estimates of the reddening and electron density of the narrow line regions of a number of active galaxies are made by using the trans-auroral/nebular line ratios of forbidden line S II and O II. The derived densities agree with published estimates from the S II forbidden line 6717 A/6731 A ratios, and reddenings derived by this and other methods agree with those predicted from Balmer decrements. It is concluded that the intrinsic narrow-line Balmer decrement is close to the Menzel-Baker case B values, and that observations of the narrow line Balmer decrement do give a reliable indication of the reddening.