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We present a high-resolution (R=140,000) spectrum of the bright quasarHE0001-2340 (z=2.26), obtained with ESPRESSO at the Very Large Telescope. Weanalyse three systems at z=0.45, z=1.65, and z=2.19 using multiple-componentVoigt-profile fitting. We also compare our spectrum with those obtained withVLT/UVES, covering a total period of 17 years. We disentangle turbulent andthermal broadening in many components spread over about 400 km/s in the z~2.19sub-DLA system. We derive an average temperature of 16000+/-1300 K, i.e., abouttwice the canonical value of the warm neutral medium in the Galacticinterstellar medium. A comparison with other high-z, low-metallicity absorbersreveals an anti-correlation between gas temperature and total HI columndensity. Although requiring confirmation, this could be the first observationalevidence of a thermal decrease with galacto-centric distance, i.e., we may bewitnessing a thermal transition between the circum-galactic medium and thecooler ISM. We revisit the Mg isotopic ratios at z=0.45 and z=1.65 andconstrain them to be xi = (26Mg+25Mg)/24Mg <0.6 and <1.4 in these two systems,respectively. These values are consistent with the standard Solar ratio, i.e.,we do not confirm strong enhancement of heavy isotopes previously inferred fromUVES data. Finally, we confirm the partial coverage of the quasar emission-lineregion by a FeI-bearing cloud in the z=0.45 system and present evidence forvelocity sub-structure of the gas that has Doppler parameters of the order ofonly ~0.3 km/s. This work demonstrates the uniqueness of high-fidelity,high-resolution optical spectrographs on large telescopes as tools toinvestigate the thermal state of the gas in and around galaxies as well as itsspatial and velocity structure on small scales, and to constrain the associatedstellar nucleosynthetic history. [abridged]

astronomy and astrophysics

Sharpening quasar absorption lines with ESPRESSO: Temperature of warm  gas at $z\sim2$, constraints on the Mg isotopic ratio, and structure of cold  gas at $z\sim0.5$