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Nuclear Structure of Quasars at 329 Megahertz


Simon, Richard Stanley (1983) Nuclear Structure of Quasars at 329 Megahertz. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zyfw-8c64.


In the four chapters in this thesis are presented studies of compact quasars using Very Long Baseline Interferometry at 329 MHz.

The first chapter presents hybrid maps of two quasars, 3C147 and 3C286, made at 329 MHz from VLBI observations taken in 1975 with a three station interferometer. The observations presented in the first chapter were used to make the first detailed maps of compact radio structure at this frequency. Both objects consist of an unresolved core; an extended, asymmetric jet; and an even larger, completely resolved halo. For 3C147 it is possible to decompose the spectrum into individual component spectra; at 329 MHz the core of 3C147 is found to be strongly self-absorbed. For both sources the spectral index decreases monotonically from core to jet to halo.

The second chapter presents further observations of quasars at 329 MHz. These observations were taken with a seven-station interferometer and were used to produce high dynamic range maps of the quasars 3C48, 3C147, 3C309.1, 3C380, and 3C454.3. These maps, made with the most extensive low-frequency VLBI observations to date, are the first reliable, high dynamic range maps at this frequency, and reveal extremely complex source structure in four of these sources. All five of the objects are seen to have asymmetric structure that can be interpreted in the "core-jet" picture of compact extragalactic radio sources. The jets observed in these objects are not straight, but bent; three of these quasars have bends in their structure greater than 90° on scales of 1 - 10 kpc.

Chapter three deals with the analysis of the two maps of 3C147 presented in chapters 1 and 2. Comparison of those maps reveals that the core of 3C147 is a low-frequency variable radio source which has brightened by a factor of two in six years. In combination with X-ray observations this is used to demonstrate that bulk relativistic motion is taking place within the core and leads to the prediction that 3C147 is a member of the class of "superluminal" radio sources. It is also found that the inclination to the line of sight of the underlying bulk relativistic motion is less than 10°.

A self-consistent picture of 3C147 as a low-frequency variable is thus developed which is consistent with the available observations, yet does not require an extreme value of the total energy: the total energy of the core of 3C147 is found to be ~1054 ergs. Together with NRAO 140 there are now two extragalactic low-frequency variable radio sources in which bulk relativistic motion has been demonstrated. Thus it may well be that bulk relativistic motion is responsible for the required time-scales in most, if not all, low-frequency variables.

In chapter four, the observations presented in chapter two of 3C48, 3C147, 3C309.1, 3C380 and 3C4S4.3 are analyzed using standard synchrotron emission theory in order to discover the physical conditions occurring in these objects. For 3C48, the suggestion in chapter 2 of this source as a "core-jet" source is seen to be reasonable with fields energy density and pressure all decreasing with increasing distance from the supposed core. For 3C48, the suggestion in chapter 2 of this source as a "core-jet" source is seen to be reasonable, with fields, energy density, and pressure all decreasing with increasing distance from the supposed core. The pressure in the jet in 3C48 is extremely large, implying that the jet cannot be confined by external gas pressure, unless the surrounding medium has extreme values of the temperature and density. In the case of 3C147 (which was analyzed extensively in chapter 3), the halfwidth of the observed jet implies that collimation of the jet is occurring; the jet in 3C147 is definitely not a "free" jet. For 3C309.1, the 329 MHz observations give an upper limit to the turnover frequency for synchrotron self-absorption significantly lower than that available up to now; this limit, in combination with Einstein X-ray observations of 3C309.1, implies that bulk relativistic motion is occurring within the core of 3C309.1. Thus, the core of 3C309.1 may be very similar to the core of 3C147, and is a candidate for being both a superluminal source and a low-frequency variable. For 3C380 and 3C454.3, the limited spectral information available allows broad limits to be put on their properties.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Astronomy
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astronomy
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Readhead, Anthony C. S.
Group:Owens Valley Radio Observatory (OVRO), Astronomy Department
Thesis Committee:
  • Blandford, Roger D. (chair)
  • Cohen, Marshall H.
  • Schmidt, Maarten
  • Readhead, Anthony C. S.
Defense Date:4 November 1982
Record Number:CaltechETD:etd-09092008-093723
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Chapter 1. adapted for Chapter 3.
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3409
Deposited By: Imported from ETD-db
Deposited On:17 Sep 2008
Last Modified:05 Nov 2021 22:42

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