Subject: Re: FTL Quasar Radio Blobs?

From: Martin Hardcastle <[email-masked]>
Date: 12 Mar 1997 13:48:19 -0500

In article <[email-masked]>,
Ross Tessien <[email-masked]> wrote:
>This means that we ought to be able to measure the included angle of 
>quasars that exhibit FTL behaviors, and to make a correlation between 
>their apparent sizes.  Secondary information about this could be gleaned 
>from the apparent sizes of the radio lobes at the end, since these seem to 
>be a crude spherical shape.  So the orientation of the jets relative to 
>our line of sight would not appreciably alter the diameter of the lobes.  
>Thus, we have a couple of ways of guessing the angles of the jets relative 
>to our line of sight.

Ross is asking about unified models for radio galaxies and quasars
(though he doesn't realise it.) The number of objects with observed
superluminal motion is pretty small, actually, and this is because the
measurements are difficult to make; you need more than one epoch of
VLBI measurement. Some objects, such as 3C273 or 3C345, are very well
studied, but in general the sample size is small and selection effects
are a problem. So superluminal motion, while a good way of telling
whether an object is relativistic and close to the line of sight, is
not the most common diagnostic.

For convenience, let's divide the most powerful radio sources into
(classical double) radio galaxies, lobe-dominated quasars and
core-dominated quasars. When observed on kiloparsec scales in the
radio, the first two classes look similar; they both have radio lobes,
jets, and a central core or nucleus centred on the host object (an
elliptical galaxy in the case of radio galaxies and a quasar in an
elliptical in the case of radio-loud quasars). Lobe-dominated quasars
invariably have brighter cores and brighter, more one-sided jets,
however. (In fact, until recently very few jets had been detected in
this type of radio galaxy at all. This has changed with better
observations.) In the late eighties it was realised that this could be
explained if radio galaxies and lobe-dominated quasars were actually
the same sort of object, with the brightness differences in the radio
cores and jets being explained as a relativistic Doppler beaming effect
(radiation from a moving object is beamed in its direction of motion
and suppressed transverse to the direction of motion). In this case
the radio galaxies lie in or near the plane of the sky, and the
lobe-dominated quasars lie at a smaller angle to the line of
sight. Barthel (1989) showed that for a well-defined sample of radio
galaxies and quasars the number statistics and the linear sizes are
consistent with a critical angle of about 45 degrees. Some care needs
to be taken when looking at linear sizes, because of linear size
(apparent) evolution and selection effects, and there is still some
controversy over this, but the basic model seems OK. The
core-dominated quasars, which as their name suggests are dominated in
radio flux by their central components, can then be tacked on to this
model as objects at a very small angle to the line of sight; their
radio structures on the large scale seem to be consistent with this
(i.e. you often see structures which could be the two lobes projected
on top of one another). Various people have then looked at the
statistics of cores and jets to try to determine what the velocities
required are; the answers vary, but it looks like high velocities are
needed in the core (v/c around 0.99) and that lower velocities,
perhaps as low as v/c around 0.6, are adequate to explain the jets
(e.g. Bridle et al 1994; Hardcastle et al, in prep).

Further evidence for this being the case can then be taken from VLBI
measurements and superluminality; the jet seen on parsec-scales is
always similar in sidedness and direction to the kiloparsec-scale jet,
and the apparent superluminal velocities correlate with the degree of
core-dominatedness. (But we are talking about very small samples in
this last case.) Lobe-dominated quasars and, even more so, radio
galaxies are difficult targets for VLBI because their cores are not
all that bright. However, the nearby classical double Cygnus A has
been observed with VLBI and shows _sub_luminal motion, as predicted by
the model. There are various systematic programmes going on to improve
the sample sizes, and with the VLBA this should get easier.

I've no time to say any more at present or to add references, but I
hope this answers the question...

Martin
-- 
Martin Hardcastle             Department of Physics, University of Bristol
                                              Be not solitary, be not idle