about galaxy "velocity cubes"

[Juan de Dios Santander Vela]

I think that trying to put "spatially local" coordinates makes it much
worse, because that "size" is a function of distance. Plus, you are
having a lot of effects due to the angular inclination of the object.
You are indeed observing ra,dec, and frequency, and that is what you
actually have. The conversion to velocities is bidirectional, no
losses, and depends on the selection of a suitable 0 value for the
velocity.

So, we might argue if having them as ra, dec, and frequency is a more
natural representation, but the velocity one is equivalent to that.

On Tue, Dec 7, 2010 at 07:46, Igor Chilingarian <chil at sai.msu.ru> 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
>



-- 
Juande Santander Vela
Applied Scientist, Archive Management Group
Archive Department, Data Management & Operations Division
European Southern Observatory (Germany)