2007 (IPP)

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3D Automated Classification Scheme for the TAUVEX data pipeline
(2007-11-28) Bora, Archana; Gupta, Ranjan; Singh, Harinder P.; et al.
In order to develop a pipeline for automated classi cation of stars to be observed by the TAUVEX ultraviolet space Telescope, we employ an arti cial neural network (ANN) technique for classifying stars by using synthetic spectra in the UV region from 1250 A to 3220 A as the training set and International Ultraviolet Explorer (IUE) low resolution spectra as the test set. Both the data sets have been pre-processed to mimic the observations of the TAUVEX ultraviolet imager. We have successfully classi ed 229 stars from the IUE low resolution catalog to within 3-4 spectral sub-class using two di erent simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE spectra of 277 spectral types. Further, we have also been able to obtain the colour excess (i.e. E(B-V) in magnitude units) or the interstellar reddening for those IUE spectra which have known reddening to an accuracy of better than 0.1 magnitudes. It has been shown that even with the limitation of data from just photometric bands, ANNs have not only classi ed the stars, but also provided satisfactory estimates for interstellar extinction. The ANN based classi cation scheme has been successfully tested on the simulated TAUVEX data pipeline. It is expected that the same technique can be employed for data validation in the ultraviolet from the virtual observatories. Finally, the interstellar extinction estimated by applying the ANNs on the TAUVEX data base would provide an extensive extinction map for our galaxy and which could in turn be modeled for the dust distribution in the galaxy.
Astronomical Data Management
(2007-01-01) Norris, Ray; Andernach, Heinz; Eichhorn, Guenther; et al.
We present a summary of the major contributions to the Special Session on Data Management held at the IAU General Assembly in Prague in 2006. While recent years have seen enormous improvements in access to astronomical data, and the Virtual Observatory aims to provide astronomers with seamless access to on-line resources, more attention needs to be paid to ensuring the quality and completeness of those resources. For example, data produced by telescopes are not always made available to the astronomical community, and new instruments are sometimes designed and built with insuﬃcient planning for data management, while older but valuable legacy data often remain undigitised. Data and results published in journals do not always appear in the data centres, and astronomers in developing countries sometimes have inadequate access to on-line resources. To address these issues, an “Astronomers Data Manifesto” has been formulated with the aim of initiating a discussion that will lead to the development of a “code of best practice” in astronomical data management.
Axisymmetric black hole accretion in the Kerr metric as an autonomous dynamical system
(2007-02-07) Goswami, Sanghamitra; Khan, Saba Nashreen; Ray, Arnab K.; et al.
In a stationary, general relativistic, axisymmetric, inviscid and rotational accretion ﬂow, described within the Kerr geometric framework, transonicity has been examined by setting up the governing equations of the ﬂow as a ﬁrst-order autonomous dynamical system. The consequent linearised analysis of the critical points of the ﬂow leads to a comprehensive mathematical prescription for classifying these points, showing that the only possibilities are saddle points and centre-type points for all ranges of values of the ﬁxed ﬂow parameters. The spin parameter of the black hole inﬂuences the multitransonic character of the ﬂow, as well as some of its speciﬁc critical properties. The special case of a ﬂow in the space-time of a non-rotating black hole, characterised by the Schwarzschild metric, has also been studied for comparison and the conclusions are compatible with what has been seen for the Kerr geometric case.
C iv absorption in damped and sub-damped Lyman-alpha systems: Correlations with metallicity and implications for galactic winds at z~2-3
(2007-07-27) Fox, Andrew J.; Ledoux, C.; Petitjean, Patrick; et al.
We present a study of Civ absorption in a sample of 63 damped Lyman-α (DLA) systems and 11 sub-DLAs in the redshift range 1.75
CMB anisotropy power spectrum using linear combinations of WMAP maps
(2007-06-25) Saha, Rajib; Souradeep, Tarun; Jain, Pankaj; et al.
In recent years the goal of estimating diﬀerent cosmological parameters precisely has set new challenges in the eﬀort to accurately measure the angular power spectrum of CMB. This has required removal of foreground contamination as well as detector noise bias with reliability and precision. Recently, a novel model-independent method for the estimation of CMB angular power spectrum solely from multi-frequency observations has been proposed and implemented on the ﬁrst yearWMAP data by Saha et al. 2006. All previous estimates of power spectrum of CMB are based upon foreground templates using data sets from diﬀerent experiments. However our methodology demonstrates that CMB angular spectrum can be reliably estimated with precision from a self contained analysis of the WMAP data. In this work we provide a detailed description of this method. We also study and identify the biases present in our power spectrum estimate. We apply our methodoly to extract the power spectrum from the WMAP 1 year and 3 year data.
CMB power spectrum estimation with non-circular beam and incomplete sky coverage
(2007-02-05) Mitra, Sanjit; Sengupta, Anand; Souradeep, Tarun; et al.
Over the last decade, measurements of the CMB anisotropy has spearheaded the remarkable transition of cosmology into a precision science. However, addressing the systematic eﬀects in the increasingly sensitive, high resolution, ‘full’ sky measurements from diﬀerent CMB experiments pose a stiﬀ challenge. The analysis techniques must not only be computationally fast to contend with the huge size of the data, but, the higher sensitivity also limits the simplifying assumptions which can then be invoked to achieve the desired speed without compromising the ﬁnal precision goals. While maximum likelihood is desirable, the enormous computational cost makes the suboptimal method of power spectrum estimation using Pseudo-Cl unavoidable for high resolution data. The debiasing of the Pseudo-Cl needs account for non-circular beams, together with non-uniform sky coverage. We provide an analytic framework for correcting the power spectrum for the eﬀect of beam noncircularity and non-uniform sky coverage (including incomplete/masked sky maps). The approach is perturbative in the distortion of the beam from non-circularity allowing for rapid computations when the beam is mildly non-circular. When non-circular beam eﬀect is important, we advocate that it is computationally advantageous to employ ‘soft’ azimuthally apodized masks whose spherical harmonic transform die down fast with m.
Cross-correlation search for periodic gravitational waves
(2007-12-10) Dhurandhar, Sanjeev; Mukhopadhyay, Himan; Badri, Krishnan; et al.
In this paper we study the use of cross-correlations between multiple gravitational wave (GW) data streams for detecting long-lived periodic signals. Cross orrelation searches between data from multiple detectors have traditionally been used to search for stochastic GW signals, but recently they have also been used in directed searches for periodic GWs. Here we further adapt the cross-correlation statistic for periodic GW searches by taking into account both the non-stationarity and the long term-phase coherence of the signal. We study the statistical properties and sensitivity of this search, its relation to existing periodic wave searches, and describe the precise way in which the cross- correlation statistic interpolates between semi-coherent and fully-coherent methods. Depending on the maximum duration over we wish to preserve phase coherence, the cross-correlation statistic can be tuned to go from a standard cross-correlation statistic using data from distinct detectors, to the semi-coherent time-frequency methods with increasing coherent time baselines, and all the way to a full coherent search. This leads to a uni ed framework for studying periodic wave searches and can be used to make informed trade-o s between computational cost, sensitivity, and robustness against signal uncertainties.
Detecting gravitional waves from inspiraling binaries with a network of detectors: coherent strategies by correlated detectors
(2007-02-03) Tagoshi, H.; Mukhopadhyay, Himan; Dhurandhar, Sanjeev; et al.
We discuss the coherent search strategy to detect gravitational waves from inspiraling compact binaries by a network of correlated laser interferometric detectors. From the maximum likelihood ratio statistic, we obtain a coherent statistic which is slightly diﬀerent from and generally better than what we obtained in our previous work. In the special case when the cross spectrum of two detectors normalized by the power spectrum density is constant, the new statistic agrees with the old one. The quantitative diﬀerence of the detection probability for a given false alarm rate is also evaluated in a simple case.
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.
Excitation mechanisms in newly discovered H2-bearing Damped Lyman-alpha clouds: systems with low molecular fractions
(2007-07-30) Noterdaeme, P.; Ledoux, C.; Petitjean, Patrick; et al.
Aims.We probe the physical conditions in high-redshift damped Lyman-α systems (DLAs) using the observed molecular fraction and the rotational excitation of molecular hydrogen. Methods. We search for Lyman- and Werner-band absorption lines of molecular hydrogen in the VLT/UVES spectra of background QSOs at the redshift of known DLAs. Results. We report two new detections of molecular hydrogen in the systems at zabs = 2.402 and 1.989 toward, respectively, HE 0027−1836 and HE 2318−1107, discovered in the course of the Hamburg-ESO DLA survey. We also present a detailed analysis of our recent H2 detection toward Q2343+125. All three systems have low molecular fractions, log f ≤ −4, with f = 2N(H2)/(2N(H2) + N(H)). Only one such H2 system was known previously. Two of them (toward Q2343+125 and HE 2318−1107) have high-metallicities, [X/H] > −1, whereas the DLA toward HE 0027−1836 is the system with the lowest metallicity ([Zn/H] = −1.63) among known H2-bearing DLAs. The depletion patterns for Si, S, Ti, Cr, Mn, Fe and Ni in the three systems are found to be very similar to what is observed in diﬀuse gas of the Galactic halo. Molecular hydrogen absorption from rotational levels up to J = 5 is observed in a single well-deﬁned component toward HE 0027−1836. We show that the width (Doppler parameter) of the H2 lines increases with increasing J and that the kinetic energy derived from the Doppler parameter is linearly dependent on the relative energy of the rotational levels. There is however no velocity shift between lines from diﬀerent rotational levels. The excitation temperature is found to be 90 K for J = 0 to J = 2 and ∼500 K for higher J levels. Single isothermal PDR models fail to reproduce the observed rotational excitations. A two-component model is needed: one component of low density (∼50 cm−3 ) with weak illumination (χ = 1) to explain the J ≤ 2 rotational levels and another of high density (∼500 cm−3 ) with strong illumination (χ = 30) for J ≥ 3 levels. However, the juxtaposition of these two PDR components may be ad-hoc and the multicomponent structure could result either from turbulent dissipation or C-shocks.
Giant Ringlike Radio Structures Around Galaxy Cluster Abell 3376
(2007-11-09) Bagchi, Joydeep; Durret, Florence; Paul, Surajit; et al.
In the current paradigm of cold dark matter cosmology, large-scale structures are assembling through hierarchical clustering of matter. In this process, an important role is played by megaparsec (Mpc)-scale cosmic shock waves, arising in gravity-driven supersonic ﬂows of intergalactic matter onto dark matter-dominated collapsing structures such as pancakes, ﬁlaments and clusters of galaxies. Here we report Very Large Array telescope observations of giant (∼ 2 Mpc ×1.6 Mpc), ring-shaped non-thermal radio emitting structures, found at the outskirts of the rich cluster of galaxies Abell 3376. These structures may trace the elusive shock waves of cosmological large scale matter ﬂows, which are energetic enough to power them. These radio sources may also be the acceleration sites where magnetic shocks are possibly boosting 1cosmic-ray particles with energies of up to 1018 to 1019 electron volts.
Gravitational wave radiometry: Mapping a stochastic gravitational wave background
(2007-08-20) Mitra, Sanjit; Dhurandhar, Sanjeev; Souradeep, Tarun; et al.
The problem of the detection and mapping of a stochastic gravitational wave background (SGWB), either cosmological or astrophysical, bears a strong semblance to the analysis of the cosmic microwave background (CMB) anisotropy and polarization, which too is a stochastic eld, statistically described in terms of its correlation properties. An astrophysical gravitational wave background (AGWB) will likely arise from an incoherent superposition of unmodelled and/or unresolved sources and cosmological gravitational wave backgrounds (CGWB) are also predicted in certain scenarios. The basic statistic we use is the cross-correlation between the data from a pair of detectors. In order to `point' the pair of detectors at di erent locations one must suitably delay the signal by the amount it takes for the gravitational waves (GW) to travel to both detectors corresponding to a source direction. Then the raw (observed) sky map of the SGWB is the signal convolved with a beam response function that varies with location in the sky. We rst present a thorough analytic understanding of the structure of the beam response function using an analytic approach employing the stationary phase approximation. The true sky map is obtained by numerically deconvolving the beam function in the integral (convolution) equation. We adopt the maximum likelihood framework to estimate the true sky map using the conjugate gradient method that has been successfully used in the broadly similar, well-studied CMB map making problem. We numerically implement and demonstrate the method on signal generated by simulated (unpolarized) SGWB for the GW radiometer consisting of the LIGO pair of detectors at Hanford and Livingston. We include `realistic' additive Gaussian noise in each data stream based on the LIGO-I noise power spectral density. The extension of the method to multiple baselines and polarized GWB is outlined. In the near future the network of GW detectors, including the Advanced LIGO and Virgo detectors that will be sensitive to sources within a thousand times larger spatial volume, could provide promising data sets for GW radiometry.
Hot Halos around High Redshift Protogalaxies: Observations of O VI and N V Absorption in Damped Lyman Alpha systems
(2007-01-15) Fox, Andrew J.; Petitjean, Patrick; Ledoux, C´edric; et al.
Aims. We present a study of the highly ionized gas (plasma) associated with damped Lyman-α (DLA) systems at z=2.1–3.1. Methods. We search for Ovi absorption and corresponding Si iv, Civ, and Nv in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering Ovi at S/N>10. We then use optical depth proﬁle comparisons and ionization modelling to investigate the properties, phase structure, and origin of the plasma. Results. We report twelve DLAs (nine intervening and three at <5 000 kms −1 from the QSO redshift) with detections of Ovi absorption. There are no clear Ovi non-detections, so the incidence of Ovi in DLAs is between 34% (12/35) and 100%. Among these 12 DLAs, Civ and Si iv are seen whenever data is available, and Nv is detected in 3 cases. Analysis of the line widths together with photoionization modelling suggests that two phases of DLA plasma exist: a hot, collisionally ionized phase (seen in broad Ovi components), and a warm, photoionized phase (seen just in narrow Civ and Si iv components). The presence of inﬂows and/or outﬂows is indicated by individual Ovi and Civ components displaced from the neutral gas (either blueshifted or redshifted) by up to 400 kms −1 . We ﬁnd tentative evidence (98% conﬁdence) for correlations between the DLA metallicity (measured in the neutral gas) and high-ion column density, and between the DLA metallicity and high-ion line width, as would be expected if supernova-driven galactic outﬂows rather than accretion produced the high ions. Using conservative ionization corrections, we ﬁnd lower limits to the total hydrogen column densities in the hot (Ovi-bearing) and warm (Civ-bearing) phases in the range logNHot H ii > 19.5 to > 21.1, and logNWarm H ii > 19.4 to > 20.9. On average, the hot and warm phases thus contain > ∼40% and > ∼20% of the baryonic mass of the neutral phase in DLAs, respectively. Conclusions. If the temperature in the Ovi phase is ≈ 106 K and so fOvi = Ovi/O ≪ 0.2, the plasma can make a signiﬁcant contribution to the metal budget at high redshift. Additional searches for Ovi in Lyman Limit Systems (QSO absorbers with 17.0 < NHi < 20.3) will be necessary to determine the total quantity of baryons and metals hidden in hot halos at z ≈ 2.
Influence of the jet opening angle on the derived kinematical parameters of blazar jets having uniform and stratified bulk motion
(2007-07-26) Krishna, Gopal; Dhurde, Samir; Sircar, Pronoy; et al.
We present analytical modelling of conical relativistic jets, in order to evaluate the role of the jet opening angle on certain key parameters that are inferred from VLBI radio observations of blazar nuclear jets. The key parameters evaluated are the orientation angle (i.e., the viewing angle) of the jet and the apparent speed and Doppler factor of the radio knots on parsec scales. Quantitative comparisons are made of the inﬂuence of the jet opening angle on the above parameters of the radio knots, as would be estimated for two widely discussed variants of relativistic nuclear jets, namely, those having uniform bulk speed and those in which the bulk Lorentz factor of the ﬂow decreases with distance from the jet axis (a ‘spine–sheath’ ﬂow). Our analysis shows that for both types of jet velocity distributions the expectation value of the jet orientation angle at ﬁrst falls dramatically with increases in the (central) jet Lorentz factor, but it levels oﬀ at a fraction of the opening angle for extremely relativistic jets. We also ﬁnd that the eﬀective values of the apparent speeds and Doppler factors of the knots always decline substantially with increasing jet opening angle, but that this eﬀect is strongest for ultra-relativistic jets with uniform bulk speed. We suggest that the paucity of highly superluminal parsec-scale radio components in TeV blazars can be understood if their jets are highly relativistic and, being intrinsically weaker, somewhat less well collimated, in comparison to the jets in other blazars.
Lenticular Galaxy Formation - Possible Luminosity Dependence
(2007-03-27) Barway, Sudhanshu; Kembhavi, A.K.; Wadadekar, Yogesh; et al.
We investigate the correlation between the bulge eﬀective radius (re) and disk scale length (rd), in the near-infrared K band for lenticular galaxies in the ﬁeld and in clusters. We ﬁnd markedly diﬀerent relations between the two parameters as a function of luminosity. Lenticulars with total absolute magnitude fainter than MT = −24.5 show a positive correlation, in line with predictions of secular formation processes for the pseudo bulges of late-type disk galaxies. But brighter lenticulars with MT < −24.5 show an anti-correlation, indicating that they formed through a diﬀerent mechanism. The available data are insuﬃcient to reliably determine the eﬀect of galaxy environment on this correlation.
Multifrequency study of giant radio sources I. Low-frequency Gaint Metrewave Radio Telescope observations of selected sources
(2007-09-27) Konar, C.; Jamrozy, M.; Saikia, D. J.; et al.
We present low-frequency observations with the Giant Metrewave Radio Telescope (GMRT) of a sample of giant radio sources (GRSs), and high-frequency observations of three of these sources with the Very Large Array (VLA). From multifrequency observations of the lobes we estimate the magnetic ﬁeld strengths using three diﬀerent approaches, and show that these diﬀer at most by a factor of ∼3. For these large radio sources the inverse-Compton losses usually dominate over synchrotron losses when estimates of the classical minimum energy magnetic ﬁeld are used, consistent with earlier studies. However, this is often not true if the magnetic ﬁelds are close to the values estimated using the formalism of Beck & Krause. We also examine the spectral indices of the cores and any evidence of recurrent activity in these sources. We probe the environment using the symmetry parameters of these sources and suggest that their environments are often asymmetric on scales of ∼1 Mpc, consistent with earlier studies.
Multifrequency study of giant radio sources II. Spectral ageing analysis of the lobes of selected sources
(2007-12-02) Jamrozy, M.; Konar, C.; Machalski, J.; et al.
Multifrequency observations with the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA) are used to determine the spectral breaks in consecutive strips along the lobes of a sample of selected giant radio sources (GRSs) in order to estimate their spectral ages. The maximum spectral ages estimated for the detected radio emission in the lobes of our sources range from ∼6 to 36 Myr with a median value of ∼20 Myr using the classical equipartition ﬁelds. Using the magnetic ﬁeld estimates from the Beck & Krause formalism the spectral ages range from ∼5 to 38 Myr with a median value of ∼22 Myr. These ages are signiﬁcantly older than smaller sources. In all but one source (J1313+6937) the spectral age gradually increases with distance from the hotspot regions, conﬁrming that acceleration of the particles mainly occurs in the hotspots. Most of the GRSs do not exhibit zero spectral ages in the hotspots, as is the case in earlier studies of smaller sources. This is likely to be largely due to contamination by more extended emission due to relatively modest resolutions. The injection spectral indices range from ∼0.55 to 0.88 with a median value of ∼0.6. We discuss these values in the light of theoretical expectations, and show that the injection spectral index appears to be correlated with luminosity and/or redshift as well as with linear size.
Multiphase plasma in sub-damped Lyman-Alpha systems : A hidden metal reservoir
(2007-08-22) Fox, Andrew J.; Petitjean, Patrick; Ledoux, C.; et al.
We present a VLT/UVES spectrum of a proximate sub-damped Lyman-α (sub-DLA) system at zabs = 2.65618 toward the quasar Q0331-4505 (zqso = 2.6785±0.0030). Absorption lines of O I, Si II,Si III, Si IV, C II, C III, C IV, Fe II, Al II, and O VI are seen in the sub-DLA, which has a neutral hydrogen column density logNHI = 19.82 ± 0.05. The absorber is at a velocity of 1 820 ± 250 kms−1 from the quasar; however, its low metallicity [O/H]=−1.64 ± 0.07, lack of partial coverage, lack of temporal variations between observations taken in 2003 and 2006, and non-detection of N V imply the absorber is not a genuine intrinsic system. By measuring the O VI column density and assuming equal metallicities in the neutral and ionized gas, we determine the column density of hot ionized hydrogen in this sub-DLA, and in two other sub-DLAs with O VI drawn from the literature. Coupling this with determinations of the typical amount of warm ionized hydrogen in sub-DLAs, we conﬁrm that sub-DLAs are a more important metal reservoir than DLAs, in total comprising at least 6–22% of the metal budget at z ≈ 2.5.
On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei
(2007-10-27) Shaw, Gargi; Stancil, P. C.; Ferland, G. J.; et al.
The spatial distribution of the cosmic-ray flux is important in understanding the Interstellar Medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate (ζ=χCRζstandard) in diffuse clouds compared to the standard value, ζstandard (=2.5x10-17 s-1), measured toward dense molecular clouds. In an earlier work we reported an enhancement χCR =20 towards HD185418. McCall et al. have reported χCR =48 towards ζ Persei based on the observed abundance of H3+ while Le Petit et al. found χCR ≈ 10 to be consistent with their models for this same sight line. Here we revisit ζ Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of χCR deduced from the H3 + column density, N(H3 +), in the diffuse region of the sightline depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with χCR ≈ 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.
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 object