about galaxy "velocity cubes"

[Arnold Rots]

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
> 
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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/
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