about galaxy "velocity cubes"

[Jose Enrique Ruiz]

Dear all

I fully agree with Arnold in the description he provides on the velocity
axis of these "galaxy datacubes", neither do I see any anything wrong with
mixing axes of different nature (e.g. angular, spatial, wavelenght,
velocity, etc.) In this sense I think a common agreement is needed in order
to know where to place this quantities in a DataModel that fully
characterize these objects. Do we need a Char.RedshiftAxis in the CharDM ?
If not ,where to say the method used for the interpretation of this redshift
(radio, optical, "relativistic") ? Does the STC solve this issue ?

Best,


2010/12/7 Arnold Rots <arots at head.cfa.harvard.edu>

> Igor,
>
> I am perfectly well familiar with those galaxy cubes; I did my thesis
> on it and worked on that stuff for more than 20 years.
>
> No, the Doppler velocity or redshift axis is not a data analysis
> result, it is an interpretation of a measurement coordinate which is
> fundamentally different from the true spectral coordinate.
>
> For instance, you can imagine having a 4-D hypercube which is a
> collection of traditional cubes with axes RA, Dec, and Doppler velocity.
> These cubes represent different spectral lines - say, the four OH
> lines, or various molecular lines. The fourth axis is then a true
> spectral one.
>
> I don't think there is anything wrong with mixing angular axes with a
> km/s axis; that last one is only an interpretation based on a
> formalism and should not be confused with a true velocity - after all,
> there are at least three variants: radio, optical, and "relativistic",
> none of them pretending to be a true space velocity.
> That's why STC makes a distinction between redshift/Doppler velocity
> and true space velocity (which is under the spatial coordinates).
>
> Cheers,
>
>  - Arnold
>
> Igor Chilingarian wrote:
> > Hi Arnold,
> >
> > Following our short discussion after Jose Enrique's talk.
> > Unfortunately, since we astronomers are neither mathematicians, nor
> > physicists, sometimes we tend to do quite weird things (here I would also
> > recall to my last ADASS talk).
> >
> > The "galaxy velocity cubes" being a standard practice in radio-astronomy
> is a
> > very good example of such a thing. I have to admin that sometimes I'm
> using it
> > myself and now we even have a service of providing such an output from
> the
> > results of simulations in the GalMer database which I implemented in
> order to
> > fulfill the demand from radio-astronomers.
> >
> > The problem with this data type is that the two *observable axes* (RA and
> Dec)
> > are mixed with the velocity axis (or redshift if you wish) which is a
> data
> > analysis result. My impression is that conceptually it is horribly wrong.
> If
> > one is using radial velocity for the Z-axis, then he/she should be using
> real
> > spatial coordinates in kpc or other physical units (e.g. meters) for the
> other
> > two axes. On the other hand, if obervables are used (e.g. RA/Dec, l/b or
> > whatever) for coordinates, than the spectral coordinate
> > (wavelength/frequency/energy) has to be used for the 3rd dimension.
> >
> > What do you think?
> >
> > With best regards,
> >                                               Igor
> >
> --------------------------------------------------------------------------
> Arnold H. Rots                                Chandra X-ray Science Center
> Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
> 60 Garden Street, MS 67                              fax:  +1 617 495 7356
> Cambridge, MA 02138                             arots at head.cfa.harvard.edu
> USA                                     http://hea-www.harvard.edu/~arots/<http://hea-www.harvard.edu/%7Earots/>
> --------------------------------------------------------------------------
>
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