UAT08.01 Scavenger Hunt #1: Utilities for ALFALFA
The purpose of this scavenger hunt is to introduce you to the ALFALFA
stand-alone FORTRAN which allow you to anticipate
or check what galaxies ALFA swept across during an observing period.
To participate in the scavenger hunt,
you need to keep track of how you found your answers; Your method is
as important as the answers!
- Be sure you have answered all the questions in Hunt #0
before you tackle this set.
- The Arecibo General Catalog. 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
- If you misuse it, you may become the proud wearer of a pair of
cement shoes. What are the conditions of use?
here for the answer and follow the link to answer the next questions.
- 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.
- Try out the ALFALFA stand-alone utilities. These are executable FORTRAN
codes that are aliased in the "alfalfa" account at Arecibo on the LINUX
(not SOLARIS!) side. (Note: If you don't have these installed locally on your
own machine, that's ok. Skip this part and go to Hunt #2.)
Notice that coordinates are usually entered by us (as they are entered
in the AGC), i.e., without decimal places or spaces in format
hhmmsss+ddmmss. The seconds of RA is entered in 0.1sec, as an integer.
Let's take a look at the part of the sky that ALFA will cross during our
planned observations for the run 08.01.14. "plotd" plots the ALFA beam
tracks and the distribution of galaxies in a 3x3deg region of the sky
starting at a given (RA,Dec). The galaxies are color coded by recessional
velocity, larger ones with diameter and position angle information are
shown as properly oriented polygons (but the scaling is for
illustrative purposes only!). You can interactively retrieve a summary
of the basic AGC entry information by clicking on each galaxy in the
display. Run "plotd" on the drift beginning
at R.A. = 12h36m, and Decl. = +26d02'12".
- What is the largest galaxy in the frame?
- What ALFA beam will sweep across the center of that gelaxy?
- What is the position angle of that galaxy?
- Has the galaxy been detected in HI before?
"skyd" generates an HTML file containing a set of links to SDSS, DSS1
(via Skyview), DSS2 (red and blue) and NED for a given position (entered
in "AGC style": hhmmsss+ddmmss).
NED entries with 10 arcmin are returned; images of 2 sizes can be
examined for the DSS sets. The HTML
file is called "skylinks.html" which you then need to load into your
browser. It is created in the local directory where
you are working so you need to keep track of
where you are working; be sure not to write over someone else's
file! Run "skyd" on the position
RA = 12h41m30.0s, Dec=+26d02'12" and experiment with the links.
If you do not have much familiarity with NED, SDSS and DSS1/2, you
should certainly learn more about them by visiting their own
- What do you find there?
- At what zooming scale can you see the large galaxy discussed
in the previous exercise.
- What do you notice about the availability of SDSS spectroscopy
in this region (at least as of Jan 2008)? How did you figure
- "plot30" (superceded by idl_alfa routine "plotalfa", see below,
but useful if you don't want to open IDL)
Now let's take a look at the small group of galaxies you noted in the observing
proposal. "plot30" plots the location of the ALFA beams at the instant Beam 0
sweeps across the input position. The beam tracks are shown, and you can interactively
retrieve a summary of the basic AGC entry information by clicking on each galaxy in the
display. Run "plot30" on the position RA = 12h41m30.0s, Dec=+26d02'12" (just as above).
- We might expect the HI signal from UGC7852=NGC4615 to be detected in which beams?
- Which beam will detect the galaxy FIRST (in the LST time of our observations)?
- Will we be able to distinguish the signals from the three galaxies? What is
the problem? Discuss the issues involved.
- Try another position: RA = 11h22m00.0s and same center declination as the drift. Discuss
what you find.
The ALFALFA observing setup uses the WAPP spectrometers which deliver specta which
cover 100 MHz total, centered on 1385 MHz; each spectrum has 4096 "channels" (or "pixels",
"array elements", however you want to think of them). "a2010fv" calculates the
frequency and recessional velocity (sans heliocentric correction -- it's not exact! --
assume simply that v = c*z)
corresponding to each channel in an ALFALFA spectrum.
- What frequency/velocity correspond to channel 2048?
- What are the units of frequency and velocity?
- Does velocity increase or decrease with increasing channel number? What about frequency?
- What channel corresponds to a redshift of 0.03?
- Write down the formula used to convert from frequency to velocity. Hint: assume the
simple expression v = c*z; but use the "optical convention" for the doppler shift.
Now you are ready to try out Hunt #2!
This page created by and for the members of the
ALFALFA Survey Undergraduate team
Last modified: Sun Jun 22 13:21:35 EDT 2008