NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/sci/astro/8170

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From: [e-mail] (James Annis)
Newsgroups: sci.astro
Subject: Re: grav lensing (MG1654)
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Date: 21 Jul 92 14:44:34 GMT
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In a previous post I argued that, on the basis of relative simplicity,
it is possible to choose the gravitational lens model over any
"ring associated with galaxy" model.  Tom Van Flandern responded by saying
that I had used a strawman model and that he would tend to prefer a "quasar
radio lobe spraying the galaxy" model. Well, ok, but my argument stands:
the lensing model naturally predicts a ring, while a "spraying" model must
invoke a mechanism for illuminating only a ring, and not the whole galaxy.
The gravitational lens model seems simpler.

But the real counterthrust to the lensing model was the questioning of
whether MG1654 fits the observed characteristics of double lobed radio sources:

    Bert Rust asks for information on the frequency of double lobed radio
sources that are not linear, and on the frequency of angles when not linear.

    and Tom brings up a list of "abnormal features":

     * The inferred position of the invisible lobe is not linearly opposite
         the visible lobe.
     * The inferred position of the invisible lobe is not as close to either
         the galaxy or the ring center as it should be in order for a
         gravitational lens to produce a symmetric ring.
     * The reconstructed image of the invisible lobe is not at all like that
         of the visible lobe.

The major objection seems to be that the ring is seriously off-axis
from the lobe-core axis. A secondary objection is that the inferred lobe
may not be similar to the observed lobe.

But, once again, MG1654 has a lot going for it. This time its strengths
lie in the manner in which it was found.

MG1654 was found during a systematic survey of radio sources.  Lawrence and
company published the radio survey in ApJ Supp 62:105 1986.  The survey has
632 objects. As they are good observers, they published contour maps
of 460 of the extended source in the survey.  One can go look at that paper
and see page after page of the radio objects.  As each of these objects was
selected in the same way as MG1654, this collection is the very best
comparison sample to look at.

So I looked.

There are 50 double lobed radio sources that have a hotspot. Lawrence and co.
call these core-double objects.

I laid a straight edge on each core-double. If the core and lobes lined up,
I counted it as straight. If they did not, I counted it as "bent". If there
was uncertainity, I assigned it to the straight class (for an uncertain one,
see 2156+023). The results:
    50 core-double lobe sources.
    33 of which are straight.
    17 of which are bent.
        7 of these are seriously bent, roughly of the same magnitude
        as MG1654. (Good examples of classic double lobe radio sources
        that are very bent are 1210+121 and 2025+026.

So, the rough average was 67% straight, 33% bent.

A respectable fraction of double lobed radio sources are therefore bent.
Interestingly, one can look at the image of the archetype of the double lobed
radio source, Cygnus A, (Sky and Tel, Aug 1992) and see that it, too, is bent.
Perhaps by only 10 degrees, but the bend is obvious.

The idea that double lobe radio sources are linear is basicly a Platonic ideal
to which the observed sources fall short. The idea that the lobes should be
mirror images of each other is similar ideal.  One has to look at the sources
to get an idea of the variaty they actually show.  As Tom pointed out, nature
is rarely as simple as the concepts we hold of it.

One of the pleasing things about MG1654 is how close to the Platonic ideal
it actually is, compared with the other double lobed radio sources in the
survey.  Except, of course, for the ring, and the galaxy inside the ring.

To summarize, then, I suggest that the the lensing model for MG1654 requires
nothing out of the ordinary.  It remains the most simple model for this system
that I know of.

--
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James Annis                                [e-mail]
Institute for Astronomy    University of Hawaii, Honolulu, Hawaii USA
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