Fundamental physics with Espresso: Towards an accurate wavelength calibration for a precision test of the fine-structure constant

TL;DR

This paper evaluates the wavelength calibration accuracy of the Espresso spectrograph, comparing it to laser frequency comb calibration, and demonstrates its potential for precise measurements of the fine-structure constant variation.

Contribution

It provides a detailed assessment of Espresso's wavelength calibration systematics and compares its default solution to an independent laser frequency comb calibration.

Findings

Wavelength discrepancies up to 24 m/s were found.

Espresso's calibration accuracy exceeds previous spectrographs.

Constraints on fine-structure constant variation at 10^{-6} level are feasible.

Abstract

Observations of metal absorption systems in the spectra of distant quasars allow to constrain a possible variation of the fine-structure constant throughout the history of the Universe. Such a test poses utmost demands on the wavelength accuracy and previous studies were limited by systematics in the spectrograph wavelength calibration. A substantial advance in the field is therefore expected from the new ultra-stable high-resolution spectrograph Espresso, recently installed at the VLT. In preparation of the fundamental physics related part of the Espresso GTO program, we present a thorough assessment of the Espresso wavelength accuracy and identify possible systematics at each of the different steps involved in the wavelength calibration process. Most importantly, we compare the default wavelength solution, based on the combination of Thorium-Argon arc lamp spectra and a Fabry-P\'erot interferometer, to the fully independent calibration obtained from a laser frequency comb. We find wavelength-dependent discrepancies of up to 24m/s. This substantially exceeds the photon noise and highlights the presence of different sources of systematics, which we characterize in detail as part of this study. Nevertheless, our study demonstrates the outstanding accuracy of Espresso with respect to previously used spectrographs and we show that constraints of a relative change of the fine-structure constant at the $10^{-6}$ level can be obtained with Espresso without being limited by wavelength calibration systematics.