Calibration issues in estimating variability of the fine structure constant (alpha) with cosmic time

摘要

Laser Comb Wavelength calibration shows that the ThAr one is locallyunreliable with possible deviations of up to 100 m/s within one order range,while delivering an overall 1 m/s accuracy (Wilken et al 2009). Such deviationcorresponds to delta alpha/alpha ~ 7E-6 for a FeII-MgII pair. Comparison ofline shifts among the 5 FeII lines, with almost identical sensitivity to finestructure constant changes, offers a clean way to directly test the presence ofpossible local wavelength calibration errors of whatever origin. We analyzed 5absorption systems, with zabs ranging from 1.15 to 2.19 towards 3 bright QSOs.The results show that while some lines are aligned within 20 m/s, others reveallarge deviations reaching 200 m/s or higher and corresponding to a deltaalpha/alpha > 1E-5 level. The origin of these deviations is not clearlyidentified but could be related to the adaptation of wavelength calibration toCCD manufacturing irregularities. These results suggest that to drawconclusions from delta alpha/alpha analysis based on one or only few lines mustbe done with extreme care.