Valentina D'Odorico

Cosmic dynamics in the era of Extremely Large Telescopes

The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universes expansion rate. A measurement of this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterising the cosmological redshift drift. We consider the Lyman alpha forest in the redshift range 2 < z < 5 and other absorption lines in the spectra of high redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Ly alpha forest as a function of signal-to-noise and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ~20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.

Early stages of nitrogen enrichment in galaxies: Clues from measurements in damped Lyman α systems ,

We present 4 new measurements of nitrogen abundances and one upper limit in damped Ly α absorbers (DLAs) obtained by means of high resolution (FWHM � 7k m s −1 ) UVES/VLT spectra. In addition to these measurements we have compiled data from all DLAs with measurements of nitrogen and α-capture elements (O, S or Si) available in the literature, including all HIRES/Keck and UVES/VLT data for a total of 32 systems, i.e. the largest sample investigated so far. We find that (N/α) ratios are distributed in two groups: 75% of the DLAs show a mean value of (N/α) = −0.87 with a scatter of 0.17 dex, while the remaining 25% shows ratios clustered at (N/α) = −1.45 with an even lower dispersion of 0.05 dex. The high (N/α) �− 0.9 plateau is consistent with the one observed in metal-poor H  regions of blue compact dwarf (BCD) galaxies ((N/α) = −0.73 ± 0.13), while the (N/α) �− 1.5 values are the lowest ever observed in any astrophysical site. These low (N/α) ratios are real and are not due to ionization effects. They provide crucial evidence against the primary production of N by massive stars as being responsible for the plateau at −0.9/−0.7 dex observed in DLAs and BCD galaxies. The transition between the low-N ((N/α) �− 1.5) and high-N ((N/α) �− 0.9) DLAs occurs at a nitrogen abundance of (N/H) �− 2.8, suggesting that the separation may result from some peculiarity of the nitrogen enrichment history. The (N/α) �− 1.5 values and their low dispersion are consistent with a modest production of primary N in massive stars; however, due to the limited sample, specially for the low-N DLAs, we cannot exclude a primary origin in intermediate mass stars as responsible for the low N abundances observed. Nitrogen is an important element when attempting to under- stand the chemical evolution of galaxies. The synthesis of N in the CNO cycle during hydrogen burning is reasonably well understood, but the characteristics of the stars that produce this element (range of masses, stage of evolution, etc.) are not completely clear. N is mostly a secondary element, produced in the CNO cycle from seed C and O nuclei created in ear- lier generation of stars. Primary N is produced when freshly synthesized C in the helium-burning shell penetrates into the hydrogen-burning shell where it will be converted in primary N by means of the CNO cycle. In evolution models of intermedi- ate mass stars, thermal pulses occurring during the asymptotic giant branch AGB phase are responsible for the transport of

ESPRESSO: the Echelle spectrograph for rocky exoplanets and stable spectroscopic observations

ESPRESSO, the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, will combine the efficiency of modern echelle spectrograph design with extreme radial-velocity precision. It will be installed on ESOs VLT in order to achieve a gain of two magnitudes with respect to its predecessor HARPS, and the instrumental radialvelocity precision will be improved to reach cm/s level. Thanks to its characteristics and the ability of combining incoherently the light of 4 large telescopes, ESPRESSO will offer new possibilities in various fields of astronomy. The main scientific objectives will be the search and characterization of rocky exoplanets in the habitable zone of quiet, nearby G to M-dwarfs, and the analysis of the variability of fundamental physical constants. We will present the ambitious scientific objectives, the capabilities of ESPRESSO, and the technical solutions of this challenging project.

Metals in the IGM approaching the re-ionization epoch: results from X-shooter at the VLT

We present the results of observations taken with the X-shooter spectrograph devoted to the study of quasars at z ∼ 6. This paper focuses on the properties of metals at high redshift traced, in particular, by the C IV doublet absorption systems. Six objects were observed with resolutions �27 and 34 km s −1 in the visual, and 37.5 and 53.5 km s −1 in the near-infrared. We detected 102 C IV lines in the range: 4.35 <z< 6.2 of which 27 are above z ∼ 5. Thanks to the characteristics of resolution and spectral coverage of X-shooter, we could also detect 25 Si IV doublets associated with the C IV at z 5. The column density distribution function of the C IV line sample is observed to evolve in redshift for z 5.3, with respect to the normalization defined by low-redshift (1.5 <z <4) C IV lines. This behaviour is reflected in the redshift evolution of the C IV cosmic mass density, � C IV, of lines with column density in the range 13.4 < log N(C IV) < 15, which is consistent with a drop of a factor of ∼ 2f or z 5.3. Considering only the stronger C IV lines (13.8 < log N(C IV) < 15), � C IV gently rises by a factor of ∼10 between z � 6.2 and z � 1.5 with a possible flattening towards z ∼ 0. The increase is well fitted by a power law: � C IV = (2 ± 1) × 10 −8 [(1 + z)/4] −3.1±0.1 .A n insight into the properties of the C IV absorbers and their evolution with redshift is obtained by comparing the observed column densities of associated C IV ,S iIV and C II absorptions with the output of a set of CLOUDY photoionization models. As already claimed by cosmological simulations, we find that C IV is a good tracer of the metallicity in the low-density intergalactic medium (IGM) gas at z ∼ 5−6 while at z ∼ 3 it arises in gas with overdensity δ ∼ 100.

Simulating cosmic metal enrichment by the first galaxies

We study cosmic metal enrichment via adaptive mesh refinement hydrodynamical simulations in a (10 Mpc h-1)3 volume following the Population III (PopIII)-PopII transition and for different PopIII initial mass function (IMFs). We have analysed the joint evolution of metal enrichment on galactic and intergalactic scales at z = 6 and z = 4. Galaxies account for ≲9 per cent of the baryonic mass; the remaining gas resides in the diffuse phases: (a) voids, i.e. regions with extremely low density (Δ ≤ 1), (b) the true intergalactic medium (IGM, 1 10^{4.5} K state. Due to these physical conditions, C {IV} absorption line experiments can probe only ≃2 per cent of the total carbon present in the IGM/CGM; however, metal absorption line spectra are very effective tools to study reionization. Finally, the PopIII star formation history is almost insensitive to the chosen PopIII IMF. PopIII stars are preferentially formed in truly pristine (Z = 0) gas pockets, well outside polluted regions created by previous star formation episodes.

New constraints on the free-streaming of warm dark matter from intermediate and small scale Lyman-alpha forest data

We present new measurements of the free-streaming of warm dark matter (WDM) from Lyman-α 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-α 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 2σ 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 5000K 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-α forest data and expectations for photo-heating and cooling in the low density IGM at these redshifts.

The rise of the C iv mass density at z < 2.5

The cosmic evolution of the metal content of the intergalactic medium puts stringent constraints on the properties of galactic outflows and on the nature of UV background. In this paper, we present a new measure of the redshift evolution of the mass density of CIV, Ω CIV , in the interval 1.5 ≲ z ≲ 4 based on a sample of more than 1500 C IV lines with column densities 10 12 ≲ N(CIV) ≲ 10 15 cm -2 . This sample more than doubles the absorption redshift path covered in the range z < 2.5 by previous samples. The result shows a significant increase of Ω CIV towards the lower redshifts at variance with the previously pictured constant behaviour.

The evolution of neutral gas in damped Lyman α systems from the XQ-100 survey

We present a sample of 38 intervening Damped Lyman

Tomography of the intergalactic medium with Lyα forests in close QSO pairs

\alpha

The ESO UVES advanced data products quasar sample – III. Evidence of bimodality in the [N/α] distribution

(DLA) systems identified towards 100