Physical Conditions in Low-Luminosity Active Galactic Nuclei

Citation

Filippenko, Alexei Vladimir (1984) Physical Conditions in Low-Luminosity Active Galactic Nuclei. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/34v9-vc35. https://resolver.caltech.edu/CaltechETD:etd-09092008-083654

Abstract

Physical conditions in the nuclei of emission-line galaxies are investigated by analyzing their continua and optical emission lines.

The strength and shape of the nonstellar continuum is derived for nine Seyfert galaxies. As in QSOs, it is described by a power law with slope ~ -1.1, and in the most luminous objects it flattens at blue and UV wavelengths. Even at minimum brightness it has no short-wavelength cutoff near 1 μm, eliminating the possibility that the near-infrared flux is dominated by thermal emission from hot dust grains.

A detailed analysis at optical and X-ray energies demonstrates that gas in the nucleus of NGC 7213 is photoionized by nonstellar radiation, even though it also exhibits the spectral characteristics of galaxies thought to be heated by shocks. After careful removal of the strong stellar component, it is shown how these "shock" features are actually more consistent with photoionization. The key is a large range of densities (~ 10³-10⁷cm^-3) in the narrow-line clouds, and a fairly low ionization parameter.

Similar studies of three additional galaxies confirm these results and strengthen the hypothesis that gas in LINERs ("low ionization nuclear emission-line regions") is photoionized rather than shock-heated. A tight correlation is found between the width of forbidden lines and their critical density. Moreover, much of the ionizing radiation is probably produced by a hot (T ~ 10⁴-10⁵ K) accretion disk.

A sensitive survey indicates that ~ 30-40% of LINERS exhibit weak, broad Hα emission. The emission-line intensity ratios are consistent with photoionization by a nonstellar continuum, indicating the presence of activity similar to (but much milder than) that of QSOs in a significant fraction of all nearby galaxies.

Relative intensities of emission lines in the narrow-line X-ray galaxy NGC 7314 are typical of type 2 Seyferts, but their widths (FWHM ~ 120-150 km s^-1) are remarkably small. Thus, nonstellar radiation need not be associated with rapid motions among the narrow-line clouds. The profile of the broad Hα emission suggests that gas flows ballistically away from the nucleus. Dust, which heavily reddens radiation from broad-line clouds, is probably located within or near the clouds themselves.

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