superluminal motions & H_0 - comments from Ajk - Bartek's IAP talk

[Bartosz Lew]

hi

On Mon, 30 Aug 2004, Boud Roukema wrote:

> hi,
>
> On Mon, 30 Aug 2004, Bartosz Lew wrote:
>
> > witam
> >
> > On Fri, 27 Aug 2004, Boud Roukema wrote:
>
> > > -> on month scale changes very rapidly - some blobs just stop for
> > > some time, and then start up again.
> > >
> >
> > plus it often happens that they fade away very quickly or change shape so
> > within a few months the recognition of the same blob mae sometimes
> > be difficult.
>
> OK, but these don't seem to affect the statistics as far as i understand.

sure - it's just a problem with measurments.

>
> It seems to me that blobs that fade are no longer detected, hence no
> longer relevant; and if a blob changes shape without changing speed,
> then it does increase the error bar (determining the centre is difficult),
> or drop out from detection statistics.

sure - I agree. but if the blob fades out the whole sequence of previous
observations might be useless resulting in having one source less in the
sample.

>
> > > My initial reaction is that in a statistical sample these should
> > > average out, provided that the errors are random and symmetrically
> > > distributed.
> > >
> > > Alternatively: can we try to statistically correct for this?
> > >
> > > Given that blobs stop and then start again, or change direction, but
> > > are only *detected* when they are moving superluminally, then it would
> > > seem to me that the true average speed is slower than estimated from
> > > the doppler factor.
> > hmm this  I quite don't understand ? why do you say so, that they are only
> > detected when they are superliuminal. in fact superluminality isn't a
> > condition "sine qua non" for doing this thing.
>
> It seems to me that they are more *likely* to be detected when they
> are superluminal - and the doppler boosting has a big role in this.
>

I agree, thats why we try to play only with these guys, but it's a trade
of choosing between luminosity (superliminality) and angular resolution
ofthe VLBI capabilities measuring proper motions.

> > > Most practical cosmology is about statistical corrections - it requires
> > > careful modelling and correct statistical analysis, but it can be done.
> > >
> > > Is it reasonable to use the observational data which *shows* the
> > > erratic behaviour of these objects to statistically model this?
> > >
> > > If yes, then we would have the necessary correction factor.
> > >
> >
> > from what I've done and played with the real data, I see that the final
> > value is quite sinsitive on precise measurments from the maps !! so
> > true - the big sample is needed and this should work.
>
> :)
>
> pozdr
> boud
>
>

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