Introduction to the AGC database
Members of the ALFALFA team have access to the private database dubbed the
"AGC" which is compiled and maintained by Martha and Riccardo at Cornell
(for more years than we will admit!). Hours and hours of time are spent updating
and maintaining it, and it contains a lot of really useful information. We hope
you will find it helpful in your ALFALFA-related research. However:
- there are conditions for its use; READ THEM;
- you need to understand its limitations; that's YOUR
RESPONSIBILITY! Follow the link on the AGC conditions page (above) to read the AGC
documentation page. Make sure you understand what all of the entries are.
From Scavenger Hunt #1 of the UAT Jan2008 workshop:
- What is the "UGC"?
- When is the AGC entry number equal to the UGC entry number?
- What is the "position angle"?
- What is the "TELCODE" for Arecibo+ALFA?
- Some galaxies have two velocity measurements: "VOPT" and "V21".
In that case, what should you adopt as the recessional velocity
for the galaxy?
- Sometimes, "V21" is entered but does not indicate the redshift
of the galaxy. Why not? Find us an example.
- Sometimes "VOPT" is actually a 21 cm velocity. Some of these are easy
to identify. How? Find us an example.
Practice using the IDL utilities that allow you to look at the AGC:
AGCBROWSE and PLOTALFA.
More advanced:
Examine the global properties of the AGC, exploring its completeness and
uniformity. You will easily discover its warts and limitations!
- Use a simple program to count the number of galaxies in the AGC in
different regions of the sky, the ones with redshifts, the ones larger (or
smaller) than 1 arcminute, ones with HI fluxes, ones with HI velocity
widths greater (or less) than some value, etc.
- Learn about different map projections in IDL and then plot subsets
of the AGC on such an equal area projection (Mollweide, Aitoff) using
(separately) (a) equatorial and (b) galactic coordinates.
- Explore N(m) (distribution of galaxies as function of apparent
magnitude as entered in the AGC -- not the magnitudes are very
inhomogeneous!) for different subsets of the AGC
- Consider how and why different regions of the sky show different distributions
in your results for the previous exercises.
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Last updated Sun May 16 11:09:28 EDT 2008 by martha