2000 (IPP)
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Item Structure of the Mg II and damped Lyman- systems along the line of sight to APM 08279+5255(2000-05-24) Petitjean, Patrick; Aracil, Bastien; Srianand, R.; et al.A study of the absorption systems toward the gravitationally lensed quasar APM 08279+5255 is pre-sented.Most of the Mg ii systems in the redshift range z ∼ 1.2– 2.07, although saturated, show large residuals at the bot- tom of the lines. The most likely interpretation is that individual clouds within Mg ii halos do cover only one of the two brightest QSO images. The separation between the two lines of sight decreases from 1.7 to 0.7 h−1 75 kpc (qo = 0.5, zlens = 1) between z = 1.22 and z = 2.07. This reveals that Mg ii halos are made of a collection of clouds of radius smaller than about 1 h−1 75 kpc. Two strong Mg ii absorbers at zabs = 1.062 and 1.181 are studied in detail. This is the first time that the Na iλ3303 doublet is detected in such high redshift systems. To- gether with the detection of the Mg iλ2852 transition, this strongly constrains the physical characteristics of the gas. The N(Na i)/N(Mg i) ratio is found to be larger than unity, implying that the gas is cool and neutral. The Doppler parameters measured in individual and well de- tached components is probably as small as 1 km s−1. The column densities of Na i, Ca ii, Mg i, Ti ii, Mn ii and Fe ii observed at zabs = 1.1801 are very close to that observed along the line of sight towards 23 Ori in our Galaxy. The shape of the QSO continuum is consistent with attenu- ation by dust at z ∼ 1 (AV ∼ 0.5 mag). Altogether it is found that the H i column density at z = 1 is of the order of 1 to 5 1021 cm−2, the corresponding metallicity is in the range 1–0.3 Z⊙, the overall dust-to-metal ratio is about half that in our Galaxy and the relative deple- tion of iron, titanium, manganese and calcium is similar to what is observed in cool gas in the disk of our Galaxy. The objects associated with these two systems could both con-tribute to the lens together with another possible strong system at zabs = 1.1727 and the strong Lyman-α system at zabs = 2.974. The probable damped Lyman-α system at zabs = 2.974 has 19.8 < log N(H i) < 20.3. The transverse dimension of the absorber is larger than 200 h−1 75 pc. Column densities of Al ii, Fe ii, Si ii, C ii and O i indicate abundances rela- tive to solar of −2.31, −2.26, −2.10, −2.35 and −2.37 for, respectively, Fe, Al, Si, C and O (for log N(H i) = 20.3). These surprizingly similar values indicate that the amount of dust in the cloud is very small as are any deviations from relative solar abundances. It seems likely that the upper limits found for the zinc metallicity of several damped Lyman-α systems at z > 3 in previous surveys is indica- tive of a true cosmological evolution of the metallicity in individual systems.Item Semi analytic approach to understanding the distribution of neutral hydrogen in the universe : Comparison of simulations with observations(2000-08-25) Choudhury, T. Roy; Srianand, R.; Padmanabhan, T.Following Bi & Davidsen (1997), we perform one dimensional semi analytic simulations along the lines of sight to model the intergalactic medium (IGM). Since this procedure is computationally efficient in probing the parameter space – and reasonably accurate – we use it to recover the values of various parameters related to the IGM (for a fixed background cosmology) by comparing the model predictions with different observations. For the currently favoured LCDM model (Ωm = 0.4, ΩΛ = 0.6 and h = 0.65), we obtain, using statistics obtained from the transmitted flux, constraints on (i) the combination f = (ΩBh2 )2 /J−12, where ΩB is the baryonic density parameter and J−12 is the total photoionisation rate in units of 10−12 s−1 , (ii) temperature T0 corresponding to the mean density and (iii) the slope γ of the effective equation of state of the IGM at a mean redshift z ≃ 2.5. We find that 0.8 < (T0/104 K) < 2.5 and 1.3 < γ < 2.3. while the constraint obtained on f is 0.0202 < f < 0.0322 . A reliable lower bound on J−12 can be used to put a lower bound on ΩBh2 , which can be compared with similar constraints obtained from Big Bang Nucleosynthesis (BBN) and CMBR studies. We find that if J−12 > 1.2, the lower bound on ΩBh2 is in violation of the BBN value.Item Semi analytic approach to understanding the distribution of neutral hydrogen in the universe(2000-10-28) Choudhury, T. Roy; Padmanabhan, T.; Srianand, R.Analytic derivations of the correlation function and the column density distribution for neutral hydrogen in the intergalactic medium (IGM) are presented, assuming that the non-linear baryonic mass density distribution in the IGM is lognormal. This ansatz was used earlier by Bi & Davidsen (1997) to perform 1D simulations of lines-of-sight and analyse the properties of absorption systems. We have taken a completely ana- lytic approach, which allows us to explore a wide region of the parameter space for our model. The analytic results have been compared with observations to constrain var- ious cosmological and IGM parameters, whenever possible. Two kinds of correlation functions are defined : (i) along the line-of-sight (LOS) and (ii) across the transverse direction. We find that the effects on the LOS correlation due to change in cosmology and the slope of the equation of state of the IGM, γ are of the same order, which means that we cannot constrain both the parameters simultaneously. However, it is possible to constrain γ and its evolution using the observed LOS correlation func- tion at different epochs provided one knows the background cosmology. We suggest that the constraints on the evolution of γ obtained using the LOS correlation can be used as an independent tool to probe the reionisation history of the universe. From the transverse correlation function, we obtain the excess probability, over random, of finding two neutral hydrogen overdense regions separated by an angle θ. We find that this excess probability is always less than 1 per cent for redshifts greater than 2. Our models also reproduce the observed column density distribution for neutral hydrogen and the shape of the distribution depends on γ. Our calculations suggest that one can rule out γ > 1.6 for z ≃ 2.31 using the column density distribution. However, one cannot rule higher values of γ at higher redshifts.Item Physical conditions in broad and associated narrow absorption-line systems toward APM 08279+5255(2000-03-21) Srianand, R.; Petitjean, PatrickResults of a careful analysis of the absorption systems with zabs≃zem seen toward the bright, zem ∼3.91, gravitationally lensed quasar APM 08279+5255 are pre- sented. Two of the narrow-line systems, at zabs = 3.8931 and zabs = 3.9135, show absorptions from singly ionized species with weak or no N v and O vi absorptions at the same redshift. Absorption due to fine structure transitions of C ii and Si ii (excitation energies cor- responding to, respectively, 156µm and 34µm) are de- tected at zabs = 3.8931. Excitation by IR radiation is favored as the column density ratios are consistent with the shape of APM 08279+5255 IR spectrum. The low- ionization state of the system favors a picture where the cloud is closer to the IR source than to the UV source, supporting the idea that the extension of the IR source is larger than ∼200 pc. The absence of fine structure lines at zabs = 3.9135 suggests that the gas responsible for this system is farther away from the IR source. Abundances are ∼0.01 and 1 Z⊙ at zabs = 3.913 and 3.8931 and alu- minum could be over-abundant with respect to silicon and carbon by at least a factor of two and five. All this sug- gests that whereas the zabs= 3.8931 system is probably located within 200 pc from the QSO and ejected at a ve- locity larger than 1000 km s−1, the zabs= 3.9135 system is farther away and part of the host-galaxy. Several narrow-line systems have strong absorption lines due to C iv, O vi and N v and very low neutral hy- drogen optical depths. This probably implies metallicities Z ≥ Z⊙ although firm conclusion cannot be drawn as the exact value depends strongly on the shape of the ionizing spectrum. The C iv broad absorption has a complex structure with mini-BALs (width ≤ 1000 km s−1) and narrow compo-nents superposed on a continuous absorption of smaller optical depth. The continuous absorption is much stronger in O vi indicating that the corresponding gas-component is of higher ionization than the other components in the flow and that absorption structures in the BAL-flow are mainly due to density inhomogeneities. There is a ten- dency for mini-BALs to have different covering factors for different species. It is shown that a few of the absorbing clouds do not cover all the three QSO images, especially we conclude that the zabs = 3.712 system covers only im- age C. Finally we identify narrow components within the BAL- flow with velocity separations within 5 km s−1 of the O vi, N v and Si iv doublet splittings suggesting that line driven radiative acceleration is an important process to explain the out-flow.Item H2 molecules and the nature of damped Lyman-alpha systems(2000-12-14) Petitjean, Patrick; Srianand, R.; Ledoux, C.We report results from our mini-survey for molecular hydrogen in eight high-redshift damped Lyman- α (DLA) systems using the ESO Ultra-violet and Visible Spectrograph on the VLT. In addition, we investigate two systems using ESO public data. We include in the sam- ple the only system where H2 was previously detected and studied at high-spectral resolution. Altogether our sample consists of eleven absorbers with 1.85 < zabs < 3.4. We confirm the presence of H2 in the zabs = 2.3377, metal-poor ([Si/H] = −1.20), system toward PKS 1232+082. The derived molecular fraction, f = 2N(H2)/(2N(H2)+N(H i)) = 4×10−4, is two orders of magnitude less than what has been claimed previ- ously from low-resolution data. The physical conditions within the cloud can be constrained directly from ob- servation. The kinetic temperature and particle density are in the ranges, respectively, 100 < T < 300 K and 30 < nH < 50 cm−3. In addition, UV pumping is of the same order of magnitude than in our Galaxy. The upper limits on the molecular fraction derived in nine of the systems are in the range 1.2×10−7−1.6×10−5. There is no evidence in this sample for any correlation be- tween H2 abundance and relative heavy element depletion into dust grains. This should be investigated using a larger sample however. The molecular abundance in a few DLA systems (and in particular in the two systems where H2 is detected) is consistent with what is seen in the Magel- lanic clouds. But most of the DLA measurements are well below these values. This is probably partly due to small amounts of dust and/or high UV flux. We argue however that the lack of molecules is a direct consequence of high kinetic temperature (T > 3000 K) implying a low forma- tion rate of H2 onto dust grains. Therefore, most of the DLA systems arise in warm and diffuse neutral gas.Item Cosmic microwave background temperature at a redshift of 2.33771(2000-12-18) Srianand, R.; Petitjean, Patrick; Ledoux, C.The Cosmic Microwave Background radiation is a fundamental prediction of Hot Big Bang cosmology. The temperature of its black-body spectrum has been measured at the present time, TCMBR,0 = 2.726±0.010 K, and is predicted to have been higher in the past. At earlier time, the temperature can be measured, in principle, using the excitation of atomic fine structure levels by the radiation field. All previous measurements however give only upper limits as they assume that no other significant source of excitation is present. Here we report the detection of absorption from the first and second fine-structure levels of neutral carbon atoms in an isolated remote cloud at a redshift of 2.33771. In addition, the unusual detection of molecular hydrogen in several rotational levels and the presence of ionized carbon in its excited fine structure level make the absorption system unique to constrain, directly from observation, the different excitation processes at play. It is shown for the first time that the cosmic radiation was warmer in the past. We find 6.0 < TCMBR < 14 K at z = 2.33771 when 9.1 K is expected in the Hot Big Bang cosmology.