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Optical quasar spectra can be used to trace variations of the fine-structureconstant alpha. Controversial results that have been published in last yearssuggest that in addition to to wavelength calibration problems systematicerrors might arise because of insufficient spectral resolution. The aim of thiswork is to estimate the impact of incorrect line decompositions in fittingprocedures due to asymmetric line profiles. Methods are developed todistinguish between different sources of line position shifts and thus tominimize error sources in future work. To simulate asymmetric line profiles,two different methods were used. At first the profile was created as anunresolved blend of narrow lines and then, the profile was created using amacroscopic velocity field of the absorbing medium. The simulated spectra wereanalysed with standard methods to search for apparent shifts of line positionsthat would mimic a variation of fundamental constants. Differences betweenposition shifts due to an incorrect line decomposition and a real variation ofconstants were probed using methods that have been newly developed or adaptedfor this kind of analysis. The results were then applied to real data. Apparentrelative velocity shifts of several hundred meters per second are found in theanalysis of simulated spectra with asymmetric line profiles. It was found thateach system has to be analysed in detail to distinguish between differentsources of line position shifts. A set of 16 FeII systems in seven quasarspectra was analysed. With the methods developed, the mean alpha variation thatappeared in these systems was reduced from the originalDalpha/alpha=(2.1+/-2.0)x10^-5 to Dalpha/alpha=(0.1+/-0.8)x10^-5. We thusconclude that incorrect line decompositions can be partly responsible for theconflicting results published so far.

astronomy and astrophysics

The precision of line position measurements of unresolved quasar  absorption lines and its influence on the search for variations of  fundamental constants