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

TL;DR

This paper investigates calibration errors in measuring the fine structure constant's variability over cosmic time, revealing significant local wavelength calibration issues that impact the reliability of such measurements.

Contribution

It demonstrates the presence of local calibration errors in cosmic absorption line measurements, highlighting the need for careful analysis when estimating alpha's variability.

Findings

Some lines are aligned within 20 m/s, others show deviations up to 200 m/s.

Deviations correspond to delta alpha/alpha > 1E-5, indicating potential calibration errors.

Calibration irregularities may be linked to CCD manufacturing issues.

Abstract

Laser Comb Wavelength calibration shows that the ThAr one is locally unreliable 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 deviation corresponds to delta alpha/alpha ~ 7E-6 for a FeII-MgII pair. Comparison of line shifts among the 5 FeII lines, with almost identical sensitivity to fine structure constant changes, offers a clean way to directly test the presence of possible local wavelength calibration errors of whatever origin. We analyzed 5 absorption 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 reveal large deviations reaching 200 m/s or higher and corresponding to a delta alpha/alpha > 1E-5 level. The origin of these deviations is not clearly identified but could be related to the adaptation of wavelength calibration to CCD manufacturing irregularities. These results suggest that to draw conclusions from delta alpha/alpha analysis based on one or only few lines must be done with extreme care.