Bounds on the fine structure constant variability from FeII absorption lines in QSO spectra

TL;DR

This paper introduces the SIDAM method for measuring variations in the fine structure constant using FeII absorption lines in QSO spectra, achieving high accuracy and highlighting calibration challenges in previous datasets.

Contribution

The paper presents the SIDAM method and demonstrates its high precision in measuring alpha variability, while analyzing calibration issues in existing data and emphasizing the need for advanced spectrographs.

Findings

SIDAM achieves ~1.8 ppm accuracy at z=1.15 and 5.66 ppm at z=1.84

Calibration errors in previous datasets can be around 10 ppm

Photo-ionization Doppler shifts can affect FeII line measurements

Abstract

The Single Ion Differential alpha Measurement (SIDAM) method for measuring fine stucture variations (daa)and its figures of merit are illustrated together with the results produced by means of FeII absorption lines of QSO intervening systems. The method provides daa ~= -0.12(+/- 1.79) ppm (parts-per-million) at zabs = 1.15 towards HE 0515--4414 and daa = 5.66(+/-2.67) ppm at zabs= 1.84 towards Q 1101--264, which are so far the most accurate measurements for single systems. SIDAM analysis for 3 systems from the Chand et al. (2004) sample provides inconsistent results which we interpret as due to calibration errors of the Chand et al. data at the level of about 10 ppm. In one system evidence for photo-ionization Doppler shift between MgII and FeII lines is found. This evidence has important bearings on the Many Multiplet method where the signal for daa variability is carried mainly by systems involving MgII absorbers. Some correlations are also found in the Murphy et al. sample which suggest larger errors than previously reported. Thus, we consider unlikely that both the Chand et al. and Murphy et al. datasets could provide an estimate of daa with an accuracy at the level of 1 ppm. A new spectrograph like the ESPRESSO project will be crucial to make progress in the astronomical determination of daa.