Wavelength Accuracy of the Keck HIRES Spectrograph and Measuring Changes
  in the Fine Structure Constant
  We report on an attempt to accurately wavelength calibrate four nights of
data taken with the Keck HIRES spectrograph on QSO PHL957, for the purpose of
determining whether the fine structure constant was different in the past.
Using new software and techniques, we measured the redshifts of various Ni II,
Fe II, Si II, etc. lines in a damped Ly-alpha system at z=2.309. Roughly half
the data was taken through the Keck iodine cell which contains thousands of
well calibrated iodine lines. Using these iodine exposures to calibrate the
normal Th-Ar Keck data pipeline output we found absolute wavelength offsets of
500 m/s to 1000 m/s with drifts of more than 500 m/s over a single night, and
drifts of nearly 2000 m/s over several nights. These offsets correspond to an
absolute redshift of uncertainty of about Delta z=10^{-5} (Delta lambda= 0.02
Ang), with daily drifts of around Delta z=5x10^{-6} (Delta lambda =0.01 Ang),
and multiday drifts of nearly Delta z=2x10^{-5} (0.04 Ang). The causes of the
wavelength offsets are not known, but since claimed shifts in the fine
structure constant would result in velocity shifts of less than 100 m/s, this
level of systematic uncertainty makes may make it difficult to use Keck HIRES
data to constrain the change in the fine structure constant. Using our
calibrated data, we applied both our own fitting software and standard fitting
software to measure (Delta alpha)/alpha, but discovered that we could obtain
results ranging from significant detection of either sign, to strong null
limits, depending upon which sets of lines and which fitting method was used.
We thus speculate that the discrepant results on (Delta alpha)/alpha reported
in the literature may be due to random fluctuations coming from under-estimated
systematic errors in wavelength calibration and fitting procedure.
