Evolving nomenclature for galaxies

[Ciel Extreme]

Hey all

I was trying to identify some recently observed galaxies using NED. One of the “anonymous” galaxies I had picked up was the small Im galaxy between NGC2633 and NGC2634 in Camelopardalis. NED provided the ID as: 2MASXJ08482381+7402176. Ahh, just rolls off the tongue, doesn’t it??? At any rate, it got me to thinking about the recent propensity to catalogue galaxies by their celestial coordinates. 2MASS, SDSS and GALEX all do this but interestingly there are ALWAYS slight differences in the coordinates (of course, always with the last couple of digits in the RA and DEC coordinates, a byproduct of each catalogue defining the centre of a galaxy slightly differently). For instance, here are the IDs for IC4554:

2MASX J15350481+2328457
SDSS J153504.83+232845.4
GALEXASC J153504.88+232844.3

I'm wondering: does anyone know if there is any interest in the professional community to standardize the coordinates so that they would be identical no matter which survey catalogue was used? I know this involves literally millions of galaxies,but.... Also, these coordinates are all for epoch 2000.0. What happens in 2050 (and beyond) as precession slowly causes changes in the coordinates for each object (and big changes for objects near the celestial poles)? The CGCG used epoch 1950 as its standard, but this catalogue doesn’t seem to be used much anymore. Wouldn’t it have been wiser for the professionals to use a coordinate system using the Milky Way as the reference? We could at least go a 100,000 years or so before we would start seeing significant drift in coordinates. And by then, I mean, really, who amongst us would really care?

Anyone out there willing to straighten me out on this?

[Clear Skies]

I trust a new, leading catalogue will pop up before 2050, containing all non NGC-IC-PGC objects. Current designations will then probably just become synonyms. Designations such as 2MASX, SDSS, etc. are just designations based on surveys. In the end, coordinates (+ epoch) will be leading as DSO's do not tend to move a lot in reference to our observing sites. Centuries from now, that may be a different story.

[svdwal]

As I understand it (not being a professional astronomer), astronomers have abandoned the idea that your coordinate systems needs to be tied to a rotating frame. They have now fixed the coordinate system to its place at J2000.0: http://en.wikipedia.org/wiki/International_Celestial_Reference_System.

Regarding surveys, as you do not know if a source in one survey is the same as a source in a different one, looking in a different part of the spectrum, all the sources need their own designator. If you want to merge them, you need to check whether all these sources do in fact are about the same object. And what do you do when you see the core of a galaxy in one part of the spectrum, and the spiral arms in a different part? Different parts of the galaxy with different emission mechanisms.

[Dragan]

Having no profession experience to help with Marks original question, I thought I'd ask a friend of ours, Dr Steve Odewahn, resident astronomer at McDonald Observatory. Here was his reply to my email:

This comes up a lot actually. I recall once sitting
through a 2 hour conversation in Paris about
whether a new numbering system should be established
for the RC3. Some folks wanted a new "running
integer" system, some wanted names based on sky
position. In the end, the RC3 included the new names
that George Paturel was using: the PGC numbers used
in (what became) LEDA.

The problem with RA_DEC based names is not so much the
equinox issue. Most everyone would adopt a unique equinox
(i.e. 2000.0 these days). The problem is usually with the variation
in positional accuracy. Even for the same survey (i.e. SDSS) the
sources vary in magnitude and surface brightness variation, as
well as the brightness of the sky when the images are taken, hence
the algorithms used to define "an object" and the position of
that object are all subject to change. How is that reflected in the
name? Do you assign a name using different numbers of decimals
in the proper part of the RA,DEC string? That makes parsing such
names as fixed strings a lot more complicated. So, most catalog
builders just adopt a fixed format and go with that.

I guess I'm pretty middle of the road on this. I like some aspects
of using catalogs with RA,DEC-based names. However, I will almost
always have to use the real (full accuracy) position for most things
in the end. Using short (<8 digits) integer-based names makes life
much easier in many applications. So, like most things, it all depends
in the end of the ultimate intended goal for the catalog.

My comments are not particularly insightful, but if you want to
post them on your web site, please feel free to do so.

[Ciel Extreme]

Thank you all, and especially Dragan and Steve, for your explanations. I'm wondering if the final solution to something like this wouldn't be to use a supercomputer to scan a complete sky deep survey of the entire sky and renumber the millions of galaxies, star clusters and nebulae to a single standard. I doubt JLE Dreyer would be available, or up to the task...

[svdwal]

What would be the point? With 200 billion galaxies all numbering systems are akward, imho.