2003 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/626
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Item VLT-UVEs survey for molecular hydrogen in high-redshift damped lyman-alfa system(2011-07-05) Ledoux, C.; Petitjean, Patrick; Srianand, R.We have searched for molecular hydrogen in damped Lyman-α (DLA) and sub- DLA systems at high redshift (zabs > 1.8) using UVES at the VLT down to a detection limit of typically N(H2) = 2×1014 cm−2. Out of the 33 systems in our sample, 8 have firm and 2 have tentative detections of associated H2 absorption lines. Considering that 3 detections were already known from past searches, molecular hydrogen is detected in 13 to 20 percent of the newly-surveyed systems.We report new detections of molecular hydrogen at zabs = 2.087 and 2.595 toward, respectively, Q1444+014 and Q0405−443, and also reanalyse the system at zabs = 3.025 toward Q0347−383. In all of the systems, we measure metallicities relative to Solar, [X/H] (with either X=Zn, or S, or Si), and depletion factors of Fe, [X/Fe], supposedly onto dust grains, and compare the characteristics of our sample with those of the global population of DLA systems (60 systems in total). We find that there is a correlation between metallicity and depletion factor in both our sample and also the global population of DLA systems. Although H2 molecules are detected in systems with [Zn/Fe] as small as 0.3, the DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. In particular, H2 is detected in the five systems having the largest depletion factors. Moreover, the individual components where H2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H2- detected components even has [Zn/Fe] > 1.4. These are the largest depletion factors ever seen in DLA systems. All this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H2 molecular fraction, f = 2N(H2)/[2N(H2) + N(Hi)], is generally small in DLA systems (typically log f < −1) and similar to what is observed in the Magellanic Clouds. There is no correlation between the observed amount of H2 and the Hi column density. In fact, two systems where H2 is detected have logN(Hi) < 20.3 and, therefore, are sub-DLA systems. From 58 to 75 percent of the DLA systems have log f < −6. This can be explained if the formation rate of H2 onto dust grains is reduced in those systems, probably because the gas is warm (T > 1000 K) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.Item Statistical isotropy of the cosmic microwave background(2011-07-05) Souradeep, Tarun; Hajian, AmirThe breakdown of statistical homogeneity and isotropy of cosmic perturbations is a generic feature of ultra large scale structure of the cosmos, in particular, of non trivial cosmic topol- ogy. The statistical isotropy (SI) of the Cosmic Microwave Background temperature fluctuations (CMB anisotropy) is sensitive to this breakdown on the largest scales comparable to, and even be- yond the cosmic horizon. We propose a set of measures, κℓ (ℓ = 1, 2, 3, . . .) which for non-zero values indicate and quantify statistical isotropy violations in a CMB map. We numerically compute the predicted κℓ spectra for CMB anisotropy in flat torus universe models. Characteristic signature of different models in the κℓ spectrum are noted.Item Measuring statistical isotropy of the CMB anisotropy(2003-07-05) Ajian, Amir; Souradeep, TarunThe statistical expectation values of the temperature fluctuations of cosmic microwave background (CMB) are assumed to be preserved under rotations of the sky. This assumption of statistical isotropy (SI) of the CMB anisotropy should be observationally verified since detection of violation of SI could have profound implications for cosmology. We propose a set of measures, κℓ (ℓ = 1, 2, 3, . . .) for detecting violation of statistical isotropy in an observed CMB anisotropy sky map indicated by non zero κℓ. We define an estimator for the κℓ spectrum and ana- lytically compute its cosmic bias and cosmic variance. The results match those obtained by measuring κℓ using simulated sky maps. Non-zero (bias corrected) κℓ larger than the SI cosmic variance will imply violation of SI. The SI mea- sure proposed in this paper is an appropriate statistics to investigate preliminary indication of SI violation in the recently released WMAP data.