about galaxy "velocity cubes"

[Arnold Rots]

Yes, STC fully supports it, since it specifies four kinds of
astronomical coordinate axes (in addition to generic axes): temporal,
spatial (including proper space velocities), spectral, and redshift
(including Doppler).

The redshift axis accomodates two pieces of metadata: redshift or
Doppler velocity, and Doppler definition (optical, radio, relativistic).
In addition, this allows specifying a rest frequency/wavelength.
Both spectral and redshift axes allow, in addition to the ones defined
for time and space, special reference positions: LSRD, LSRK, LSR,
LOCAL_GROUP_CENTER.

If one uses full STC metadata elements, this information automatically
ends up in the right place. It is unfortunate that Char was not made
compatible with STC in this respect - the result of allowing to pick
components of a model, rather than full model stuctures.

Cheers,

  - Arnold


Jose Enrique Ruiz wrote:
[ Charset ISO-8859-1 unsupported, converting... ]
> 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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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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