The Adopted Radial Velocities Used by the SSPP
The spZbest fits file, which is generated from the SDSS spectroscopic
reduction pipeline, provides two estimated radial velocities. One is an
absorption-line redshift derived from a cross-correlation procedure using
templates that were obtained from SDSS commissioning spectra (Stoughton et al.
2002). Another estimate comes from performing a "best-match" procedure that
compares the observed spectra with externally measured templates (in this case,
the ELODIE library high-resolution spectra, as described by Prugniel &
Soubiran 2001 and Moultaka et al. 2004), degraded to the resolving power of
Previous experience with the analysis of SDSS stellar spectra suggested
radial velocity estimated from the ELODIE template matches is often the best
available estimate, in the sense that it is the most repeatable based on
of "quality assurance'' stars with multiple determinations. However, there are
some cases where the quoted error of an ELODIE spectral match velocity is
larger than expected, so we also make use of the cross-correlation radial
the following manner:
- If the velocity determined by comparison with the ELODIE
templates has a reported error of 20 km s-1 or less,
then this velocity is
adopted and the radial velocity flag is set to `RVOK(20)'.
- If the error from the
ELODIE template comparison is larger than 20 km s-1 and the relative
difference between the two reported radial velocities is less than 40 km
s-1, then we take an average of the two techniques, and the radial
velocity flag is set to `RVOK(40) '.
- If the error in the reported ELODIE velocity is
larger than 20 km s-1, and the difference of between the
two estimates is between 40 and 100 km s-1, we take an average
of the two and the radial velocity flag is set to `RVOK(100)'.
- If none of the above conditions are satisfied (which happens only
mainly for quite low S/N spectra, or for hot/cool stars without adequate
templates), then we obtain an independent estimate of the radial velocity based
on our own IDL routines. The calculation of the radial velocity is carried out
by determining wavelength shifts for several strong absorption line features
(Ca II K, Ca II H, Hδ, Ca I, Hγ, Hβ, Na I, Hα, the Ca~II
triplet). After ignoring the calculated velocity above +500 km s-1
-500 km s-1 from the individual lines (which are very often
obtain a 3σ clipped average of the remaining radial velocities. If this
computed average falls between -500 km s-1 and
+500 km s-1, we take
the calculated radial velocity as the adopted radial velocity and set
velocity flag to `RVCALOK'.
It should be noted that many of the techniques used for atmospheric parameter
estimation in the SSPP work well even when the velocity determination for a
given star has errors of up to 100 km s-1 or more.
Hence, we choose not to
ignore spectra with high velocity errors, but rather simply indicate caution
with the appropriate radial velocity flag.
If none of the above methods yield an acceptable estimate of radial
if the reported velocity is apparently spurious (greater than 1000
or less than -1,000 km s-1), we simply ignored the
spectrum of the star in
our subsequent analysis, and set the radial velocity flag to `RVNOTOK'.
Checks on Radial Velocities − Zero Points and Scatter
To check on the accuracy of the SSPP radial velocities, we compared with the
sample of over 150 high-resolution spectra of SDSS-I/SEGUE stars that have been
obtained in order to calibrate and validate the stellar atmospheric parameters
obtained by the SSPP.
After rejecting problematic spectra (e.g., low S/N high-resolution spectra,
or stars that appear to be spectroscopic binaries at high spectral resolution),
137 stars remain to compare with the radial velocity results obtained for the
medium-resolution SDSS spectra with the SSPP. A consistent offset of
about -6.6 km s-1 (with a standard deviation of 5.2 km
s-1) is obtained from a Gaussian fit to the
residuals; this offset appears constant over the color range 0.1 &le: g-r ≤
An additional comparison with the radial velocity distribution of likely
member stars in the Galactic globular clusters M~15 and M~13 reveals similar
offsets (-6.8 km s-1 and -8.6 km s-1, respectively; see
Lee et al.
2007 for a more detailed analysis).
The origin of this velocity offset has not
yet been identified, but we expect that it may be tied to the wavelength
solutions obtained for the individual fibers. However, in order to account for
its presence, we apply an empirical +7.3 km s-1 shift
(the mean of the
offsets from analysis of the high-resolution and the globular-cluster data), to
each radial velocity obtained by the SSPP. For the time being, we conclude that
the zero-point uncertainties in the corrected radial velocities determined by
the SSPP (and the SDSS spectroscopic reduction pipeline it depends on)
should be close to zero, with scatter on the order of 5 km s-1.
Note that the scatter in the determination of radial velocities,
based on the average displacements of
the `quality assurance' stars with multiple measurements, varies from 3.5 km
s-1 (for brighter stars) to 20 km s-1
(for fainter stars).