Probing 3-D matter distribution at z~2 with QSO multiple lines of sight

We investigate the 3-D matter distribution at z~2 with high resolution (R ~ 40000) spectra of QSO pairs and groups obtained with the UVES spectrograph at ESO VLT. Our sample is unique for the number density of objects and the variety of separations, between 0.5 and 7 proper Mpc. We compute the real space cross-correlation function of the Lyman-alpha forest transmitted fluxes. There is a significant clustering signal up to ~2 proper Mpc, which is still present when absorption lines with high column density (log N > 13.8) are excluded.Comment: Poster paper presented at the IAU Colloquium #199 on "Probing Galaxies through Quasar Absorption Lines" held in Shanghai, China from March 14th to 18th, 200

Tomography of the intergalactic medium with Ly-alpha forests in close QSO pairs

We study the three-dimensional distribution of non virialised matter at z~2 using high resolution spectra of QSO pairs and simulated spectra drawn from cosmological hydrodynamical simulations. We have collected the largest sample of QSO pairs ever observed with UVES at the ESO-VLT, with angular separations between ~1 and 14 arcmin. The observed correlation functions of the transmitted flux in the HI Lyman alpha forest along and transverse to the lines of sight are in good agreement implying that the distortions in redshift space due to peculiar velocities are small. The clustering signal is significant up to velocity separations of ~200 km/s, or about 3 h^{-1} comoving Mpc. The regions at lower overdensity (rho/ < 6.5) are still clustered but on smaller scales (Delta v < 100 km/s). The observed and simulated correlation functions are compatible at the 3 sigma level. A better concordance is obtained when only the low overdensity regions are selected for the analysis or when the effective optical depth of the simulated spectra is increased artificially, suggesting a deficiency of strong lines in the simulated spectra. We found that also a lower value of the power-law index of the temperature-density relation for the Lyman alpha forest gas improves the agreement between observed and simulated results. If confirmed, this would be consistent with other observations favouring a late HeII reionization epoch (at z~3). We remark the detection of a significant clustering signal in the cross correlation coefficient at a transverse velocity separation Delta v_{\perp} ~500 km/s whose origin needs further investigation.Comment: Accepted for publication in MNRAS, revised version matching the accepted on

Fast, large volume, GPU enabled simulations for the Ly-alpha forest: power spectrum forecasts for baryon acoustic oscillation experiments

High redshift measurements of the baryonic acoustic oscillation scale (BAO) from large Ly-alpha forest surveys represent the next frontier of dark energy studies. As part of this effort, efficient simulations of the BAO signature from the Ly-alpha forest will be required. We construct a model for producing fast, large volume simulations of the Ly-alpha forest for this purpose. Utilising a calibrated semi-analytic approach, we are able to run very large simulations in 1 Gpc^3 volumes which fully resolve the Jeans scale in less than a day on a desktop PC using a GPU enabled version of our code. The Ly-alpha forest spectra extracted from our semi-analytical simulations are in excellent agreement with those obtained from a fully hydrodynamical reference simulation. Furthermore, we find our simulated data are in broad agreement with observational measurements of the flux probability distribution and 1D flux power spectrum. We are able to correctly recover the input BAO scale from the 3D Ly-alpha flux power spectrum measured from our simulated data, and estimate that a BOSS-like 10^4 deg^2 survey with ~15 background sources per square degree and a signal-to-noise of ~5 per pixel should achieve a measurement of the BAO scale to within ~1.4 per cent. We also use our simulations to provide simple power-law expressions for estimating the fractional error on the BAO scale on varying the signal-to-noise and the number density of background sources. The speed and flexibility of our approach is well suited for exploring parameter space and the impact of observational and astrophysical systematics on the recovery of the BAO signature from forthcoming large scale spectroscopic surveys.Comment: 16 pages, 11 figures, accepted to MNRA

Compact strain-sensitive flexible photonic crystals for sensors

A promising fabrication route to produce absorbing flexible photonic crystals is presented, which exploits self-assembly during the shear processing of multi-shelled polymer spheres. When absorbing material is incorporated in the interstitial space surrounding high-refractive-index spheres, a dramatic enhancement in the transmission edge on the short-wavelength side of the band gap is observed. This effect originates from the shifting optical field spatial distribution as the incident wavelength is tuned around the band gap, and results in a contrast up to 100 times better than similar but nonabsorbing photonic crystals. An order-of-magnitude improvement in strain sensitivity is shown, suggesting the use of these thin films in photonic sensors

Cosmography beyond standard candles and rulers

We perform a cosmographic analysis using several cosmological observables such as the luminosity distance moduli, the volume distance, the angular diameter distance and the Hubble parameter. These quantities are determined using different data sets: Supernovae type Ia and Gamma Ray Bursts, the Baryonic Acoustic Oscillations, the cosmic microwave background power spectrum and the Hubble parameter as measured from surveys of galaxies. This data set allows to put constraints on the cosmographic expansion with unprecedented precision. We also present forecasts for the coefficients of the kinematic expansion using future but realistic data sets: constraints on the coefficients of the expansions are likely to improve by a factor ten with the upcoming large scale structure probes. Finally, we derive the set of the cosmographic parameters for several cosmological models (including $\Lambda$CDM) and compare them with our best fit set. While distance measurements are unable to discriminate among these models, we show that the inclusion of the Hubble data set leads to strong constraints on the lowest order coefficients and in particular it is incompatible with $\Lambda$CDM at 3-$\sigma$ confidence level. We discuss the reliability of this determination and suggest further observations which might be of crucial importance for the viability of cosmographic tests in the next future.Comment: 15 pages, 2 figures, 2 tables, Accepted for publication in PR

Effect of Primordial Black Holes on the Cosmic Microwave Background and Cosmological Parameter Estimates

We investigate the effect of non-evaporating primordial black holes (PBHs) on the ionization and thermal history of the universe. X-rays emitted by gas accretion onto PBHs modify the cosmic recombination history, producing measurable effects on the spectrum and anisotropies of the Cosmic Microwave Background (CMB). Using the third-year WMAP data and FIRAS data we improve existing upper limits on the abundance of PBHs with masses >0.1 Msun by several orders of magnitude. Fitting WMAP3 data with cosmological models that do not allow for non-standard recombination histories, as produced by PBHs or other early energy sources, may lead to an underestimate of the best-fit values of the amplitude of linear density fluctuations (sigma_8) and the scalar spectral index (n_s). Cosmological parameter estimates are affected because models with PBHs allow for larger values of the Thomson scattering optical depth, whose correlation with other parameters may not be correctly taken into account when PBHs are ignored. Values of tau_e=0.2, n_s=1 and sigma_8=0.9 are allowed at 95% CF. This result that may relieve recent tension between WMAP3 data and clusters data on the value of sigma_8. PBHs may increase the primordial molecular hydrogen abundance by up to two orders of magnitude, this promoting cooling and star formation. The suppression of galaxy formation due to X-ray heating is negligible for models consistent with the CMB data. Thus, the formation rate of the first galaxies and stars would be enhanced by a population of PBHs.Comment: 17 pages (Apj style), 9 figures, submitted to Ap

Tracing the Warm Hot Intergalactic Medium in the local Universe

We present a simple method for tracing the spatial distribution and predicting the physical properties of the Warm-Hot Intergalactic Medium (WHIM), from the map of galaxy light in the local universe. Under the assumption that biasing is local and monotonic we map the ~ 2 Mpc/h smoothed density field of galaxy light into the mass density field from which we infer the spatial distribution of the WHIM in the local supercluster. Taking into account the scatter in the WHIM density-temperature and density-metallicity relation, extracted from the z=0 outputs of high-resolution and large box size hydro-dynamical cosmological simulations, we are able to quantify the probability of detecting WHIM signatures in the form of absorption features in the X-ray spectra, along arbitrary directions in the sky. To illustrate the usefulness of this semi-analytical method we focus on the WHIM properties in the Virgo Cluster region.Comment: 16 pages 11 Figures. Discussion clarified, alternative methods proposed. Results unchanged. MNRAS in pres

Neutrinos in Non-linear Structure Formation - The Effect on Halo Properties

We use N-body simulations to find the effect of neutrino masses on halo properties, and investigate how the density profiles of both the neutrino and the dark matter components change as a function of the neutrino mass. We compare our neutrino density profiles with results from the N-one-body method and find good agreement. We also show and explain why the Tremaine-Gunn bound for the neutrinos is not saturated. Finally we study how the halo mass function changes as a function of the neutrino mass and compare our results with the Sheth-Tormen semi-analytic formulae. Our results are important for surveys which aim at probing cosmological parameters using clusters, as well as future experiments aiming at measuring the cosmic neutrino background directly.Comment: 20 pages, 8 figure

Fluctuations in 21cm Emission After Reionization

The fluctuations in the emission of redshifted 21cm photons from neutral inter-galactic hydrogen will provide an unprecedented probe of the reionization era. Conventional wisdom assumes that this 21cm signal disappears as soon as reionization is complete, when little atomic hydrogen is left through most of the volume of the inter-galactic medium (IGM). However observations of damped Ly-alpha absorbers indicate that the fraction of hydrogen in its neutral form is significant by mass at all redshifts. Here we use a physically-motivated model to show that residual neutral gas, confined to dense regions in the IGM with a high recombination rate, will generate a significant post-reionization 21cm signal. We show that the power-spectrum of fluctuations in this signal will be detectable by the first generation of low-frequency observatories at a signal-to-noise that is comparable to that achievable in observations of the reionization era. The statistics of 21cm fluctuations will therefore probe not only the pre-reionization IGM, but rather the entire process of HII region overlap, as well as the appearance of the diffuse ionized IGM.Comment: 10 pages, 5 figures, submitted to MNRA

Use of satellite imagery to predict the distribution of bluetongue vector C. imicola in Corsica

Due to climatic changes and human activities, many ecosystems are acquiring the capacity to shelter major pathogens. Because environmental parameters impact insects bionomics, vector and vector-borne disease distributions have significantly changed during the past years. Our aim is to study the conditions required for the establishment of a new vector in a new ecosystem, taking as example Culicoides imicola, one of the main vectors of bluetongue, in Corsica. The favorable environment for C. imicola is described using remote sensing and Geographical Information Systems (GIS). Parameters likely to influence vector distribution were extracted from a SPOT image (10 meters pixel) and from a digital elevation model (DEM). The normalized difference vegetation index was calculated and a supervised classification was carried out. Slope, altitude, sunshine, and orientation of study sites were extracted from the DEM. Two types of epidemiological data were used to model the distribution of favorable sites for C. imicola. Data on sheep disease cases (farm descriptors and health status) highlighted the impact of environmental variables such as latitude and some types of vegetation. Another model was established using entomological data collected during one-night catches in a hundred sites in June 2005. The neighborhood of farms and trapping sites was compared to build logistic models on bluetongue and vector occurrence. Data, methods, results, applications and limits of the two approaches are discussed here. (Texte intégral