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README file for GASS rotation curves:

Sean Moran, Mar30 2012
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Rotation curves are plotted in files G[gass_id]_rot.eps, with the
plotted points tabulated in the associated G[gass_id]_rot.txt files.

In the .eps files, velocities (in km/s) relative to the redshift at
galaxy center are plotted as black or red points as a function of slit
position in arcseconds (where +/- X is defined such that X becomes
more negative as you move outward in the direction of the SDSS
Position Angle (PA) of the galaxy.) Black points are velocities
measured via cross-correlation of the absorption spectrum with
templates, while red points are measured via Gaussian fits to the
Halpha emission line. The solid and dotted lines, respectively, are
the best-fit rotation curve models to the given points. The fitted
maximum velocities of the flat part of the rotation curves are printed
on the plot. Fits are only valid for smoothly rising rotation curves,
and often do fail, so use the reported numbers with caution.

In the .txt files, I list X in arcseconds, velocity and velocity error
from the absorption line fits, and then velocity and velocity error
for the emission line fits. When no significant Halpha is detected in
a bin, we set this velocity = -1 and error = 99.0

Some rotation curves exhibit small velocity offsets between the
absorption and emission line rotation curves. If the offsets are small
(<~50km/s) and uniform across the galaxy, then they are likely small
systematic differences arising from the differences in the two
measurement methods, and should be ignored. (In some cases these could
be due to uncorrected atmospheric dispersion resulting in a slight
velocity/spatial drift from the blue to red end of the
spectrum). Remember, though the offsets look significant in some
curves, anything less than 50km/s represents < 1 pixel.  For curves
with offset that varies with position, or which is radically different
from the absorption line curve, there probably is a real kinematic
de-coupling between the stars and the gas in this galaxy.