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The Physical Nature of the Continuum in Active Galaxies


Malkan, Matthew Arnold (1983) The Physical Nature of the Continuum in Active Galaxies. Dissertation (Ph.D.), California Institute of Technology.


We have measured the intrinsic nonstellar continuum from the nuclei of a wide range of active galaxies. We find little or no continuum reddening (from the 2175Å absorption bump) above that expected from the interstellar medium of the Milky Way. Nor do optical and ultraviolet emission from permitted Fe II lines alter the overall appearance of the energy distribution very much. After accounting for hydrogen recombination radiation, we find the continuum consists of a power law (fV∝V-1.1), closely related to the X-ray emission, and an optically thick thermal component. Some bright quasars produce as much or more thermal power as nonthermal. The thermal energy has an average characteristic temperature of 20--30,000 K. It is perfectly described by relativistic accretion disk models which include the gravitational redshifting and focusing of emerging light. These models are completely specified by two well-determined parameters: the mass of the central black hole and the accretion rate. The hole masses range from 0.2--0.5 billion M☉ in 3C 273 to 1--3 billion in the most luminous quasars, depending on the disk inclination and the angular momentum of the hole. All the bright quasars are shining at within a factor of two of their Eddington limits, while the Seyfert 1 galaxies are well below theirs.

Unlike most Seyfert 1 galaxies, those of type 2 have a large portion of their total power re-radiated in the infrared by warm dust grains. The intrinsic continuum in the Seyfert 2 galaxies may have a nonthermal power-law, but appears to lack the hot thermal component seen in quasars. The host galaxies around Seyfert nuclei of types 1 and 2, and those around low-redshift quasars are normal spirals of early to intermediate type.

We calculated the level populations for the 0+ and S+ ions, to use the [0 II] (7320+7330)/(3726+3729) and [S II] (4069+4076)/(6716+6730) line ratios as reddening indicators (accurate to ±0.1 mag. in EB-V). Seyfert 2 galaxies are typically reddened by 0.2--0.5 mag. Most Seyfert 1 galaxies have less than half as much reddening. The Hα/Hβ ratio in the broad-line region is often much larger than the Case B value, and it is significantly larger than 2.86 in some narrow-line regions. The intrinsic Balmer continuum/H[alpha] ratio in the narrow-line region is 1.4, consistent with the Case B prediction. But in the broad-line region it is two to three times larger, because of the effects of high electron density and optical depth.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:quasar, accretion disk, black hole, emission line, Seyfert galaxy, reddening
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astrophysics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Sargent, Wallace L. W.
Thesis Committee:
  • Sargent, Wallace L. W. (chair)
  • Oke, J. Beverly
  • Blanford, Roger D.
Defense Date:23 May 1983
Non-Caltech Author Email:malkan (AT)
Record Number:CaltechETD:etd-09102008-091935
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3436
Deposited By: Imported from ETD-db
Deposited On:18 Sep 2008
Last Modified:02 Mar 2016 16:20

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