As part of the cataloging, we measure a number of quantities associated with the
detection:
a. the heliocentric velocity, v50 or Vhelio b. the width of the profile, W50 c. the S/N, or SNR d. the HI flux, Stot or HIflux e. the HI centroid coordinates, HIhhmmss.s+ddmmss,
where the hhmmss.s is a right ascension in hours, minutes, and seconds,
and ddmmss is declination in degrees, arcminutes, arcseconds
The upper central window in
galcat provides these parameters and others measured for this galaxy. The
combined data are shown at lower left and the spectrum in the middle bottom.
The optical counterpart of the galaxy is seen in the lower right hand corner. The cross is
the HI centroid and the circle is the optical center. The
optical coordinates, called Opt pos, are given in the central window, and in the top
box of the right hand "Modify Parameters" window. The optical velocity (cz) is
also provided, as derived from measurements of optical spectra from public surveys such as SDSS.
Each detection in is cataloged in this manner. The final set of parameters for Code 1, 2, and 9
objects is published. The measured parameters appear in the tables of ALFALFA publications, e.g.,
Haynes et al. 2011, the α40 catalog
Take a look at the catalog listing. The first part of the file contains information about the parameters
listed and how they appear. There are also notes to explain number and letter codes associated
with each entry. In addition to the above parameters, the catalog also lists
f. the distance, Dist, of the source (often calculated from the redshift and Hubble's law)
g. the HI mass, or log of the HI mass, logMsun, in
units of solar masses (calculated following this procedure.)
h. a flag (last column) to describe how the source matches up with a possible
optical counterpart.
Find AGC 214473 in the catalog. Record its HI mass and optical counterpart status.
Compare the final published values of the HI parameters to those you found in the cataloging window.
(They may not be identical because of further refinement in fitting.)
Identifying the most probable optical counterpart.
One of the most important extra contributions of ALFALFA and an integral part of the source extraction process
is the identification of the most probable optical counterpart of each HI source -- where such
exists. Since ALFALFA galaxies don't care that we are observing them only in the HI 21 cm line, we need
to know if they have a stellar component. And since ALFALFA data cubes have a resolution of about 4 arcminutes,
we need to know when the detection is clearly associated with a galaxy -- or not! If a source does not have an optical counterpart
and is not a high velocity cloud, then it is a a candidate "dark galaxy" or a candidate OH megamaser .... or
an evil spurious detection (aaarrgg!)
The practice of identifying the most probable optical counterpart adds considerably to the effort of
construction ALFALFA source catalogs. It is easier said than done. It requires astronomer intelligence,
which is unfortunately why we cannot train our cats to produce catalogs (otherwise, we sure would have!).
An ALFALFA detection can be centroided to about 20'' (or half a degree), for high S/N, even though the
resolution is 4'. The centroiding accuracy is discussed here.
To give you a feel for how to find optical counterparts,
the table below presents five ALFALFA detections and parameters extracted from the HI measurement.
Using the
SDSS and NED tools, find the most probable optical counterpart.
