The Quasar 3C309.1
Scott E. Aaron, John F.C. Wardle, & David H. Roberts
We have observed the quasar 3C309.1 at many frequencies between 1.4
and 43 GHz with the Very
Long Baseline Array between 1995 and 1998, mapping the total intensity
and linear polarization structure at angular resolution of the order of
milliarcseconds. The source is for the following reasons
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It is a prominent example of the Compact Steep Spectrum (CSS) class of
radio sources. These are sources with a steep radio spectrum and
small linear size, and are interesting because they may be very young radio
sources which have not yet evolved into the giants usually associated with
flat spectra. These sources usually show strong depolarization at
longer observing wavelengths, which is interpreted as indicating dense,
highly ionized gas in the environment of the radio emitting plasma, which
constrains the growth of the source, accounting for the relatively small
linear size.
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It is believed to lie at the center of an extreme cooling flow. These
are clusters of galaxies with hot gas flowing toward the center as it cools.
This hot gas would be ionized and manifest itself in the Rotation Measure
and depolarization properties of the radio emission.
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The radio jet shows dramatic bending between small and large angular scales.
It has been found in samples of core dominated radio sources that there
is a bimodal distribution of the misalignment angle between large and small
jets, one peak at 0 degrees (i.e. aligned jets) and 90 degrees.
It is believed that such bending is due to hydrodynamic instabilities in
the jet plasma, specifically Kelvin-Helmholtz instabilities.
Our observing projects allow us to examine and test all of these properties.
Our primary results are summarized below.
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For this source, we have demonstrated that the depolarization properties
of the integrated radio emission is accounted for, not by depolarization
of the jet emission, but by the polarized emission at different frequencies
arising in physically different portions of the source. At 5 GHz,
the radio core is unpolarized and the integrated polarized emission arises
in the jet. At 15 GHz, the core is moderately polarized and the integrated
polarized emission arises from the core. This suggests that comparison
of the integrated polarization emission at different frequencies, which
has provided the basis of the idea that CSS sources are embedded in dense
environments, may not be physically meaningful.
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The steep spectrum for the integrated emission may be accounted for by
the relatively weak core at moderate (< 5 GHz) frequencies. Flat
spectrum sources tend to be core dominated at these frequencies, and the
core of a radio source has a flat spectrum. For 3C309.1, the integrated
emission is dominated by the jet, which shows a steep spectrum, as do jets
in flat spectrum objects. This suggests the difference between flat
and steep spectrum objects is simply the dominance of the core emission
to the total emission. It has been noted by others that CSS objects
tend to have weaker cores than flat spectrum objects.
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The source shows generally low, possibly Galactic in origin, rotation measure
across the source. This is contrary to the proposition that the source
is embedded in a massive cooling flow.
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The jet is disrupted about 50 mas from the core, most likely by a collision
with a dense cloud of gas in the ambient medium. The dramatic bend
of the jet is associated with this disruption. Such disruptions in
the flow have been noted by us in other highly bent sources, e.g.
Mrk 501 and 0814+425. This suggests that the bending is not due to
fluid instabilities, but to local interactions with the ambient material.
It is not yet clear how these sharp bends generate a preference for 90
degree misalignments between small and large scale jets.