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-Pérot 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.
The spectrograph ESPRESSO recently obtained a limit on the variation of the fine-structure constant, \(\alpha\), through measurements along the line of sight of a bright quasar with a precision of ...\(1.36\) ppm at \(1\sigma\) level. This imposes new constraints on cosmological models with a varying \(\alpha\). We assume such a model where the electromagnetic sector is coupled to a scalar field dark energy responsible for the current acceleration of the Universe. We parametrise the variation of \(\alpha\) with two extra parameters, one defining the cosmological evolution of the quintessence component and the other fixing the coupling with the electromagnetic field. The objective of this work is to constrain these parameters with both astrophysical and local probes. We also carried out a comparative analysis of how each data probe may constrain our parametrisation. We performed a Bayesian analysis by comparing the predictions of the model with observations. The astrophysical datasets are composed of quasar spectra measurements, including the latest ESPRESSO data point, as well as Planck observations of the cosmic microwave background. We combined these with local results from atomic clocks and the MICROSCOPE experiment. The constraints placed on the quintessence parameter are consistent with a null variation of the field, and are therefore compatible with a \(\Lambda\)CDM cosmology. The constraints on the coupling to the electromagnetic sector are dominated by the E\"otv\"os parameter local bound. More precise measurements with ESPRESSO will be extremely important to study the cosmological evolution of \(\alpha\) as it probes an interval of redshift not accessible to other types of observations. However, for this particular model, current available data favour a null variation of \(\alpha\) resulting mostly from the strong MICROSCOPE limits.
We present the Lyman-\(\alpha\) flux power spectrum measurements of the XQ-100 sample of quasar spectra obtained in the context of the European Southern Observatory Large Programme "Quasars and their ...absorption lines: a legacy survey of the high redshift universe with VLT/XSHOOTER". Using \(100\) quasar spectra with medium resolution and signal-to-noise ratio we measure the power spectrum over a range of redshifts \(z = 3 - 4.2\) and over a range of scales \(k = 0.003 - 0.06\,\mathrm{s\,km^{-1}}\). The results agree well with the measurements of the one-dimensional power spectrum found in the literature. The data analysis used in this paper is based on the Fourier transform and has been tested on synthetic data. Systematic and statistical uncertainties of our measurements are estimated, with a total error (statistical and systematic) comparable to the one of the BOSS data in the overlapping range of scales, and smaller by more than \(50\%\) for higher redshift bins (\(z>3.6\)) and small scales (\(k > 0.01\,\mathrm{s\,km^{-1}}\)). The XQ-100 data set has the unique feature of having signal-to-noise ratios and resolution intermediate between the two data sets that are typically used to perform cosmological studies, i.e. BOSS and high-resolution spectra (e.g. UVES/VLT or HIRES). More importantly, the measured flux power spectra span the high redshift regime which is usually more constraining for structure formation models.
We present new measurements of the free-streaming of warm dark matter (WDM) from Lyman-\(\alpha\) flux-power spectra. We use data from the medium resolution, intermediate redshift XQ-100 sample ...observed with the X-shooter spectrograph (\(z=3 - 4.2\)) and the high-resolution, high-redshift sample used in Viel et al. (2013) obtained with the HIRES/MIKE spectrographs (\(z=4.2 - 5.4\)). Based on further improved modelling of the dependence of the Lyman-\(\alpha\) flux-power spectrum on the free-streaming of dark matter, cosmological parameters, as well as the thermal history of the intergalactic medium (IGM) with hydrodynamical simulations, we obtain the following limits, expressed as the equivalent mass of thermal relic WDM particles. The XQ-100 flux power spectrum alone gives a lower limit of 1.4 keV, the re-analysis of the HIRES/MIKE sample gives 4.1 keV while the combined analysis gives our best and significantly strengthened lower limit of 5.3 keV (all 2\(\sigma\) C.L.). The further improvement in the joint analysis is partly due to the fact that the two data sets have different degeneracies between astrophysical and cosmological parameters that are broken when the data sets are combined, and more importantly on chosen priors on the thermal evolution. These results all assume that the temperature evolution of the IGM can be modelled as a power law in redshift. Allowing for a non-smooth evolution of the temperature of the IGM with sudden temperature changes of up to 5000K reduces the lower limit for the combined analysis to 3.5 keV. A WDM with smaller thermal relic masses would require, however, a sudden temperature jump of \(5000\,K\) or more in the narrow redshift interval \(z=4.6-4.8\), in disagreement with observations of the thermal history based on high-resolution resolution Lyman-\(\alpha\) forest data and expectations for photo-heating and cooling in the low density IGM at these redshifts.
