about galaxy "velocity cubes"

[Igor Chilingarian]

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