2007 (IPP)
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Item Probing the star formation history using the redshift evolution of luminosity fuctions(2007-03-30) Samui, Saumyadip; Srianand, R.; Subramanian, KandaswamyWe present a self-consistent, semi-analytical ΛCDM model of star formation and reionization. For the cosmological parameters favored by the WMAP data, our models consistently reproduce the electron scattering optical depth to reionization, redshift of reionization and the observed luminosity functions (LF) and hence the star formation rate (SFR) density at 3 ≤ z ≤ 6 for a reasonable range of model parameters. While simple photoionization feedback produces the correct shape of LF at z = 6, for z = 3 we need additional feedback that suppresses star formation activities in halos with 1010 . (M/M⊙) . 1011. Models with prolonged continuous star formation activities are preferred over those with short bursts as they are consistent with the existence of a Balmer break in considerable fraction of observed galaxies even at z ∼ 6. The halo number density evolution from the standard ΛCDMstructure formation model that fits LF up to z = 6 is consistent with the upper limits on z ≃ 7 LF and source counts at 8 ≤ z ≤ 12 obtained fromthe Hubble Ultra Deep Field (HUDF) observations without requiring any dramatic change in the nature of star formation. However, to reproduce the observed LF at 6 ≤ z ≤ 10, obtained from the near-IR observations around strong lensing clusters, we need a strong evolution in the initial mass function, reddening correction and the mode of star formation at z & 8. We show that low mass molecular cooled halos, which may be important for reionizing the universe, are not detectable in the present deep field observations even if a considerable fraction of its baryonic mass goes through a star burst phase. However, their presence and contribution to reionization can be inferred indirectly from the redshift evolution of the luminosity function in the redshift range 6 ≤ z ≤ 12. In our model calculations, the contribution of low mass halos to global SFR density prior to reionization reveals itself in the form of second peak at z ≥ 6. However this peak will not be visible in the observed SFR density as a function of z as most of these galaxies have luminosity below the detection threshold of various ongoing deep field surveys. Accurately measuring the LF at high redshifts can be used to understand the nature of star formation in the dark ages and probe the history of reionization.Item Physical conditions in the neutral interstellar medium at z=2.43 toward Q2348-011(2007-03-29) Noterdaeme, P.; Srianand, R.; Petitjean, Patrick; et al.Aims. We aim at deriving the physical conditions in the neutral gas associated with damped Lyman-α systems using observation and analysis of H2 and C absorptions. Methods. We obtained a high-resolution VLT-UVES spectrum of the quasar Q2348−011 over a wavelength range that covers most of the prominent metal and molecular absorption lines from the log N(H ) = 20.50±0.10 damped Lyman-α system at zabs = 2.4263. We detected H2 in this system and measured column densities of H2, C , C ∗, C∗∗ , Si , P , S , Fe , and Ni . From the column density ratios and, in particular, the relative populations of H2 rotational and C fine-structure levels, we derived the physical conditions in the gas (relative abundances, dust-depletion, particle density, kinetic temperature, and ionising flux) and discuss physical conditions in the neutral phase. Results. Molecular hydrogen was detected in seven components in the first four rotational levels (J = 0-3) of the vibrational ground state. Absorption lines of H2 J = 4 (resp. J = 5) rotational levels are detected in six (resp. two) of these components. This leads to a total molecular fraction of log f ≃ −1.69+0.37 −0.58. Fourteen components are needed to reproduce the metal-line profiles. The overall metallicity is found to be −0.80, −0.62, −1.17±0.10 for, respectively, [Si/H], [S/H] and [Fe/H]. We confirm the earlier findings that there is a correlation between log N(Fe )/N(S ) and log N(Si )/N(S ) from different components indicative of a dust-depletion pattern. Surprisingly, however, the depletion of metals onto dust in the H2 components is not large in this system: [Fe/S] = −0.8 to −0.1. The gas in H2-bearing components is found to be cold but still hotter than similar gas in our Galaxy (T > 130 K, instead of typically 80 K) and dense (n ∼ 100 − 200 cm−3 ). There is an anti-correlation (R = −0.97) between the logarithm of the photo-absorption rate, log β0, and log N(H2)/N(C ) derived for each H2 component. We show that this is mostly due to shielding effects and imply that the photo-absorption rate β0 is a good indicator of the physical conditions in the gas. We find that the gas is immersed in an intense UV field, about one order of magnitude higher than in the solar vicinity. These results suggest that the gas in H2-bearing DLAs is clumpy, and star-formation occurs in the associated objectItem Nitrogen and Oxygen abundances in the neutral gas at high redshift(2007-12-17) Petitjean, Patrick; Ledoux, C.; Srianand, R.Aims. We study the Oxygen and Nitrogen abundances in the interstellar medium of high-redshift galaxies. Methods. We use high resolution and high signal-to-noise ratio spectra of Damped Lyman-α (DLA) systems detected along the line-of-sight to quasars to derive robust abundance measurements from unsaturated metal absorption lines. Results. We present results for a sample of 16 high-redshift DLAs and strong sub-DLAs (log N(H i) > 19.5, 2.4 < zabs <3.6) including 13 new measurements. We find that the Oxygen to Iron abundance ratio is pretty much constant with [O/Fe] ∼ +0.32±0.10 for −2.5 < [O/H] < −1.0 with a small scatter around this value. The Oxygen abundance follows quite well the Silicon abundance within ∼0.2 dex although the Silicon abundance could be slightly smaller for [O/H] < −2. The distribution of the [N/O] abundance ratio, measured from components that are detected in both species, is somehow double peaked: five systems have [N/O] > −1 and nine systems have [N/O] < −1.15. In the diagram [N/O] versus [O/H], a loose plateau is possibly present at [N/O] ∼ −0.9 that is below the so-called primary plateau as seen in local metal-poor dwarf galaxies ([N/O] in the range −0.57 to −0.74). No system is seen above this primary plateau whereas the majority of the systems lie well below with a large scatter. All this suggests a picture in which DLAs undergo successive star-bursts. During such an episode, the [N/O] ratio decreases sharply because of the rapid release of Oxygen by massive stars whereas inbetween two bursts, Nitrogen is released by low and intermediate-mass stars with a delay and the [N/O] ratio increases.Item Evidence for overdensity around z_em > 4 quasars from the proximity effect(2007-02-14) Guimaraes, R.; Srianand, R.; Petitjean, Patrick; et al.We study the density field around zem > 4 quasars using high quality medium spectral resolution ESI-Keck spectra (R ∼ 4300, SNR > 25) of 45 high-redshift quasars selected from a total of 95 spectra. This large sample considerably increases the statistics compared to previous studies. The redshift evolution of the mean photo-ionization rate and the median optical depth of the intergalactic medium (IGM) are derived statistically from the observed transmitted flux and the pixel optical depth probability distribution function respectively. This is used to study the so-called proximity effect, that is, the observed decrease of the median optical depth of the IGM in the vicinity of the quasar caused by enhanced photo-ionization rate due to photons emitted by the quasar. We show that the proximity effect is correlated with the luminosity of the quasars, as expected. By comparing the observed decrease of the median optical depth with the theoretical expectation we find that the optical depth does not decrease as rapidly as expected when approaching the quasar if the gas in its vicinity is part of the standard IGM. We interpret this effect as revealing gaseous overdensities on scales as large as ∼15h−1 Mpc. The mean overdensity is of the order of two and five within, respectively, 10 and 3h−1 Mpc. If true, this would indicate that high redshift quasars are located in the center of overdense regions that could evolve with time into massive clusters of galaxies. The overdensity is correlated with luminosity: brighter quasars show higher overdensities.