High-Energy Astrophysics American and Soviet Perspectives/Proceedings from the U.S.-U.S.S.R. Workshop on High-Energy Astrophysics (1991) / Chapter Skim
Currently Skimming:
OPTICAL OBSERVATIONS OF ACTIVE GALACTIC NUCLEI
OPTICAL OBSERVATIONS OF ACTIVE GALACTIC NUCLEI
Pages 91-107
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 91... ...
V FTLIPPENKO University of California, Berkeley ABSTRACT I describe the most important observed characteristics of active galactic nuclei, concentrating on their optical spectra.
|
From page 92... ...
Photoioni7~tion models imply that Te ~ 10, 000-20, 000 K Comparable temperatures are also found from various intensity ratios among the narrow lines, although the derived densities are much lower (ne ~ 103 - 105 cm~3~. with such low temperatures, the thermal widths of emission lines are only 1~20 lan sol.
|
From page 93... ...
Although spherical accretion has been considered, it is now generally believed that the gas forms an accretion disk around the black hole. Infalling matter almost certainly has specific angular momentum greater than that of the least stable orbit around a black hole, and accretion disks provide efficient release of energy (up to 0.37 mc2 for a Kerr black hole)
|
From page 94... ...
Ultra-High and Intermediate Densities Another, equally serious problem is that the broad emission lines are observed to vary rapidly in response to changes in the luminosity of the ionizing continuum; see Peterson (1988) for a thorough review.
|
From page 95... ...
without the presence of beamed radiation, it is difficult to reproduce this knot's isolated nature and its very high ionization parameter. The polarized blue continuum detected in this lmot is probably caused by scattering of the beamed radiation (di Serego Alighieri et at 19~.
|
From page 96... ...
Moreover, the luminosity of the ionizing continuum from the nucleus of NGC 106S, deduced from the total flux spectrum and the assumption of spherical symmetry, cannot account for the luminosity of forbidden lines in the NLR. No problem exists, on the other hand, if the NLR clouds have a much more direct view of the compact nucleus.
|
From page 97... ...
The nucleus of the nearby, frequently studied spiral galaxy NGC 4051 was for many years the least luminous known Seyfert 1 (MB ~-16; V6ron 1979~. Owing to contamination by starlight, it is quite difficult to detect the nonstellar blue continuum in very low-luminosity Seyfert 1 nuclei.
|
From page 98... ...
Note that the widths of the forbidden lines span a wide range. normal, inactive galaxies devoid of strong emission lines.
|
From page 99... ...
The narrow lines have FWHM < 60 km s~i smaller than in any other Mown Seyfert nucleus. Weak, broad components are clearly visible in the per~itted-line profiles, but not in the forbidden lines.
|
From page 100... ...
If NGC 4395 were much farther away, so that the spectroscopic entrance aperture included a large amount of extranuclear light, its spectrum would have been dominated by emission lines from H II regions and continuum from OB associations, making the Seyfert activity difficult to detect. At optical wavelengths the featureless continuum of NGC 4395 can be described by a power law with spectral index 1.5, comparable to that of other Seyfert 1 nuclei.
|
From page 101... ...
Such a black hole could be produced in only 200 million years if it started out with one solar mass and always Secreted at the Eddington limit Adopting the formalism of Wandel and Yahil (1985) , on the other hand, we find that the current dynamical mass is about 104 M ,, if gravity accelerates the emission-line clouds to typical observed speeds of ~ 500 lan sol.
|
From page 102... ...
A key point is that stellar mass loss increases with increasing metal abundance in massive stars, so that very massive stars formed in highmetallici~ environments can end their lives as bare He-C-D cores with surface temperatures comparable to those of the hottest lmown nuclei of planetary nebulae but with considerably higher luminosities. Calculations show that after 3 million years, the ionizing continuum of a cluster created during a large burst of star formation (initial mass function slope = 3.0; LRqueux 1979)
|
From page 103... ...
The observed velocity of the broad-line gas Is unlikely to be produced by gravitational effects; a sufficiently compact and massive star cluster would also be very short lived, due to stellar collisions. If, on the other hand, the broad permitted lines are produced by SNe, as suggested by Terlevich et aL (1987)
|
From page 104... ...
Nevertheless, had SN 1987F occurred in the nucleus of a normal galaxy, a low-resolution spectrum undoubtedly would have led to a Seyfert 1 classification for the galaxy, especially if some Warmers were present to produce high-ioni~tion narrow lines and a blue continuum. If we further assume that physical conditions (e.g., a dense interstellar medium, as may have been He case for SN 1987F)
|
From page 105... ...
A two-component broad emission line model. Monthly Notices of the Royal Astronomical Society 232: 539-550.
|
From page 106... ...
1988. Indirect evidence for massive black holes in nearby galactic nuclei.
|
From page 107... ...
1989. Fitting improved accretion disk models to the multiwavelength continua of quasam and active galactic nuclei.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.