Wavelength Accuracy of the Keck HIRES Spectrograph and Measuring Changes in the Fine Structure Constant

Details Wavelength Accuracy of the Keck HIRES Spectrograph and Measuring Changes in the Fine Structure Constant Wavelength Accuracy of the Keck HIRES Spectrograph and Measuring Changes in the Fine Structure Constant

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009aas...21360905w&link_type=abstract

American Astronomical Society, AAS Meeting #213, #609.05

Astronomy and Astrophysics

Astronomy

Scientific paper

We report on an attempt to accurately wavelength calibrate four nights of data taken with the Keck HIRES spectrograph on QSO PHL-957. In order to determine whether the fine structure constant was different in the past we measured the redshifts of various Ni, Fe, Si, 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 find wavelength offsets of 0.05-0.07 Angstroms depending on wavelength, with drifts of more than 0.01 Angstrom over a single night, and drifts of over 0.02 Angstroms over several nights. These offsets correspond to an absolute redshift of uncertainty of about Delta z 3 × 10-5 (Delta v ≈ 3000 m/sec), with daily drifts of around Delta z 5 × 10-6, and multiday drifts of Delta z 10-5. 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 it very difficult to use Keck HIRES data to constrain the change in the fine structure constant (alpha). Using our calibrated data, we applied both our own 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.

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