2005 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/443
Browse
9 results
Search Results
Item VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems: Physical conditions in the neutral gas(2005-06-01) Srianand, R.; Petitjean, Patrick; Ledoux, C´edric; et al.We study the physical conditions in damped Lyman-α systems (DLAs), using a sample of 33 systems toward 26 QSOs acquired for a recently completed survey of H2 by Ledoux et al. (2003). We use the column densities of H2 in different rotational levels, together with those of C i, C i ∗, C i ∗∗, C ii ∗ and singly ionized atomic species to discuss the kinetic temperature, the density of hydrogen and the electronic density in the gas together with the ambient UV radiation field. Detailed comparisons are made between the observed properties in DLAs, the interstellar medium (ISM) of the Galaxy, the large and small Magellanic clouds (LMC and SMC).The mean kinetic temperature of the gas corresponding to DLA subcomponents in which H2 absorption line is detected, derived from the ortho-to-para ratio (153±78 K), is higher than that measured in the ISM (77±17 K) and the Magellanic clouds (82±21 K). Typical pressure in these components (corresponding to T = 100−300 K and nH = 10−200 cm−3), measured using C i fine-structure excitation, are higher than what is measured along ISM sightlines. This is consistent with the corresponding higher values for N(H2,J=2)/N(H2,J=0) seen in DLAs. From the column densities of the high-J rotational levels, we derive that the typical radiation field in the H2 bearing components is of the order of or slightly higher than the mean UV field in the Galactic ISM. Determination of electron density in the gas with H2 and C i show the ionization rate is similar to that of a cold neutral medium (CNM) in a moderate radiation field. This, together with the fact that we see H2 in 13-20% of the DLAs, can be used to conclude that DLAs at z > 1.9 could contribute as much as 50% star formation rate density seen in Lyman break galaxies (LBGs).C ii ∗ absorption line is detected in all the components where H2 absorption line is seen. The excitation of C ii in these systems is consistent with the physical parameters derived from the excitation of H2 and C i. We detect C ii ∗ in about 50% of the DLAs and therefore in a considerable fraction of DLAs that do not show H2. In part of the later systems, physical conditions could be similar to that in the CNM gas of the Galaxy. However, the absence of C i absorption line and the presence of Al iii absorption lines with a profile similar to the profiles of singly ionized species suggest an appreciable contribution from warm (WNM) and/or partially ionized gas. The absence of H2, for the level of metallicity and dust depletion seen in these systems, are consistent with low densities (i.e nH 6 1 cm−3) for a radiation field similar to the mean Galactic UV field.Item Physical conditions in the ISM towards HD185418(2005-06-01) Shaw, Gargi; Srianand, R.We have developed a complete model of the hydrogen molecule as part of the spectral simulation code Cloudy. Our goal is to apply this to spectra of high-redshift star-forming regions where H2 absorption is seen, but where few other details are known, to understand its implication for star formation. The microphysics of H2 is intricate, and it is important to validate these numerical simulations in better-understood environments. This paper studies a well-defined line-of-sight through the Galactic interstellar medium (ISM) as a test of the microphysics and methods we use. We present a self-consistent calculation of the observed absorption-line spectrum to derive the physical conditions in the ISM towards HD185418, a ine-of-sight with many observables. We deduce density, temperature, local radiation field, cosmic ray ionization rate, chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density, similar abundances, and require a cosmic ray flux enhanced over the Galactic background value, consistent with enhancements redicted by MHD simulations.Item Outflowing material in the CSS quasar 3C48: evidence of jet-cloud interaction?(2005-05-01) Gupta, Neeraj; Srianand, R.; Saikia, D. J.We report the detection of a zabs=0.3654 associated absorption-line system in the UV spectrum of the CSS quasar 3C48. The absorbing material is blue shifted with respect to the quasar emission-line redshift, zem =0.3700, suggesting an outflow velocity of ∼1000 km s−1. We detect absorption lines over a range of ionization states from Lyβ, Lyγ, C iv, N iv, S vi to O vi and possibly O iv and Ne viii. The kinematical properties of the absorption-line system are similar to the blue-shifted emission line gas seen in [Oiii] λ5007 (Chatzichristou, Vanderriest & Jaffe 1999), which is believed to have interacted with the radio jet. We study the properties of the absorbing material using CLOUDY and find that photoionization models with Solar abundance ratios (with overall metallicity in the range 0.16Z/Z⊙61.3) are enough to explain the observed column densities of all the species except Ne viii , detection of which requires confirmation. Since the cooling and recombination time for the gas is ∼105 yr, the consistency with the photoionization models suggests that any possible interaction of absorbing material with the jet must have taken place before ∼105 yr. The abundance ratio of nitrogen to carbon is close to Solar values, unlike in the case of most quasars, especially at high-redshifts, which have super-Solar values.We observed 3C48 with the Giant Metrewave Radio Telescope (GMRT) to search for redshifted 21cm H i absorption. However, we did not detect any significant feature in our spectra and estimate the 3σ upper limit to the optical depth to be in the range 0.001 to 0.003. However, due to the diffuse nature of the radio source, optical depths as high as 0.1 towards individual knots or compact components cannot be ruled out.Item New constraint on the time dependence of the proton-to-electron mass ratio. Analysis of the Q 0347-383 and Q 0405-443 spectra(2005-07-01) Ivanchik, A.; Srianand, R.; Varshalovich, D.A new limit on the possible cosmological variation of the proton-to-electron mass ratio µ = mp/me is estimated by measuring wavelengths of H2 lines of Lyman and Werner bands from two absorption systems at zabs = 2.5947 and 3.0249 in the spectra of quasars Q 0405−443 and Q 0347−383, respectively. Data are of the highest spectral resolution (R = 53000) and S/N ratio (30÷70) for this kind of study. We search for any correlation between zi, the redshift of observed lines, determined using laboratory wavelengths as references, and Ki, the sensitivity coefficient of the lines to a change of µ, that could be interpreted as a variation of µ over the corresponding cosmological time. We use two sets of laboratory wavelengths, the first one, Set (A) (Abgrall et al. 1993), based on experimental determination of energy levels and the second one, Set (P) (Philip et al. 2004), based on new laboratory measurements of some individual rest-wavelengths. We find ∆µ/µ = (3.05±0.75)×10−5 for Set (A), and ∆µ/µ = (1.65±0.74)×10−5 for Set (P). The second determination is the most stringent limit on the variation of µ over the last 12 Gyrs ever obtained. The correlation found using Set (A) seems to show that some amount of systematic error is hidden in the determination of energy levels of the H2 molecule.Item Multiwavelength investigation of the temperature of the cold neutral medium(2005-11-01) Roy, Nirupam; Chengalur, Jayaram; Srianand, R.We present measurements of the H i spin temperatures (Ts) of the Cold Neutral Medium (CNM) towards radio sources that are closely aligned with stars for which published H2 ortho-para temperatures (T01) are available from UV observations. Our sample consists of 18 radio sources close to 16 nearby stars. The transverse separation of the lines of sight of corresponding the UV and radio observations varies from 0.1 to 12.0 pc at the distance of the star. The ultraviolet (UV) measurements do not have velocity information, so we use the velocities of low ionization species (e.g Na i/K i/C i) observed towards these same stars to make a plausible identification of the CNM corresponding to the H2 absorption. We then find that T01 and Ts match within observational uncertainties for lines-of-sight with H2 column density above 1015.8 cm−2, but deviate from each other below this threshold. This is consistent with the expectation that in the CNM Ts tracks the kinetic temperature due to collisions and that T01 is driven towards the kinetic temperature by proton exchange reactions.Item Molecular hydrogen in the diffuse interstellar medium at high redshift(2005-06-01) Srianand, R.; Shaw, Gargi; Ferland, G. J.The physical conditions within damped Lyα systems (DLAs) can reveal the star formation history, determine the chemical composition of the associated ISM, and hence document the first steps in the formation of present day galaxies. Here we present calculations that self-consistently determine the gas ionization, level populations (atomic fine-structure levels and rotational levels of H2), grain physics, and chemistry. We show that for a low-density gas (nH6 0.1 cm−3) the meta-galactic UV background due to quasars is sufficient to maintain H2 column densities below the detection limit (i.e N(H2)6 1014 cm−2) irrespective of the metallicity and dust content in the gas. Such a gas will have a 21 cm spin temperature in excess of 7000 K and very low C i and C ii ∗ column densities for H i column densities typically observed 50 per cent in DLAs.We show that the observed properties of the ∼ 15 per cent of the DLAs that do show detectable H2 absorption cannot be reproduced with only the quasar dominated meta-galactic UV radiation field. Gas with higher densities (nH> 10 cm−3), a moder- ate radiation field (flux density one to ten times that of the background radiation of the Galactic ISM), the observed range of metallicity and dust-to-gas ratio reproduce all the observed properties of the DLAs that show H2 absorption lines. This favors the presence of ongoing star formation in DLAs with H2. The absence of detectable H2 and C i absorption in a large fraction of DLAs can be explained if they originate either in a low-density gas or in a high-density gas with a large ambient radiation field. The absence of 21 cm absorption and C ii ∗ absorption will be consistent with the first possibility. The presence of 21 cm absorption and strong C ii ∗ without H2 and C i absorption will suggest the second alternative. The N(Al ii)/N(Al iii) ratio can be used to understand the physical properties when only C ii ∗ absorption is present. We find nH in components that show C ii ∗ (without H2) is less than that typically inferred from the components with H2 absorption. We also calculate the column density of various atoms in the excited fine-structure levels. The expected column densities of O i ∗, O i ∗∗, and Si ii ∗ in a high-density cold gas is in the range of 1011−1012 cm−2 for log N(H i)> 20 and the observed range of metallicities. It will be possible to confirm whether DLAs that do not show H2 originate predominantly in a high-density gas by detecting these lines in very high S/N ratio spectra.Item Long-term optical photometric monitoring of the quasar SDSS J153259.96 - 003944.1(2005-02-01) Stalin, C. S.; Srianand, R.We report optical Cousins R and I band monitoring observations of the high redshift (z = 4.67) QSO SDSS J153259.96−003944.1 that does not show detectable emission lines in its optical spectrum. We show this object varies with a maximum amplitude of ∼0.4 mag during a year and three months of monitoring. Combined with two other epochs of photometric data available in the literature, we show the object has gradually faded by ∼0.9 mag during the period June 1998 − April 2001. A linear least squares fit to all available observations gives a slope of ∼0.35 mag/yr which translates to ∼1.9 mag/yr in the rest frame of the quasar. Such a variability is higher than that typically seen in QSOs but consistent with that of BL Lacs, suggesting that the optical continuum is Doppler boosted. Alternatively, within photometric errors, the observed lightcurve is also consistent with the object going through a microlensing event. Photoionization model calculations show the mass of the Broad Line Region to be few tens of M⊙ similar to that of low luminosity Seyfert galaxies, but ∼2 orders of magnitude less than that of luminous quasars. Further frequent photometric/spectroscopic monitoring is needed to support or refute the different alternatives discussed here on the nature of SDSS J153259.96−003944.1Item Kinematics and star formation activity in the z=2.03954 damped Lyman-alpha system towards PKS 0458-020(2005-12-01) Heinmuller, Janine; Srianand, R.; Petitjean, Patrick; et al.We present UVES observations of the log N(H ) = 21.7 damped Lyman-α system at zabs = 2.03954 towards the quasar PKS 0458−020. H Lyman-α emission is detected in the center of the damped Lyman-α absorption trough. Metallicities are derived for Mg , Si , P , Cr , Mn , Fe and Zn and are found to be −1.21 ± 0.12, −1.28 ± 0.20, −1.54 ± 0.11, −1.66 ± 0.10, −2.05 ± 0.11, −1.87 ± 0.11, −1.22 ± 0.10, respectively, relative to solar. The depletion factor is therefore of the order of [Zn/Fe] = 0.65. We observe metal absorption lines to be blueshifted compared to the Lyman-α emission up to a maximum of ∼100 and 200 km s−1 for low and high-ionization species respectively. This can be interpreted either as the consequence of rotation in a large (∼7 kpc) disk or as the imprint of a galactic wind. The star formation rate (SFR) derived from the Lyman-α emission, 1.6M⊙yr −1 , is compared with that estimated from the observed C ∗ absorption. No molecular hydrogen is detected in our data, yielding a molecular fraction log f < −6.52. This absence of H2 can be explained as the consequence of a high ambient UV flux which is one order of magnitude larger than the radiation field in the ISM of our Galaxy and originates in the observed emitting region.Item Density structure around quasars from optical depth statistics(2005-05-01) Rollinde, Emmanuel; Srianand, R.; Chand, Hum; et al.We present a method for studying the proximity effect and the density structure around redshift z=2-3 quasars. It is based on the probability distribution of Lyman-α pixel optical depths and its evolution with redshift. We validate the method using mock spectra obtained from hydrodynamical simulations, and then apply it to a sample of 12 bright quasars at redshifts 2-3 observed with UVES at the VLT-UT2 Kueyen ESO telescope. These quasars do not show signatures of associated absorption and have a mean monochromatic luminosity of 5.4 × 1031 h−2 erg s−1 Hz−1 at the Lyman limit. The observed distribution of optical depth within 10 h−1Mpc from the QSO is statistically different from that measured in the general intergalacticmedium at the same redshift. Such a change will result from the combined effects of the increase in photoionisation rate above the mean UV-background due to the extra ionizing photons from the quasar radiation (proximity effect), and the higher density of the IGM if the quasars reside in overdense regions (as expected from biased galaxy formation). The first factor decreases the optical depth whereas the second one increases the optical depth, but our measurement cannot distinguish a high background from a low overdensity. An overdensity of the order of a few is required if we use the amplitude of the UV-background inferred from the mean Lyman-α opacity. If no overdensity is present, then we require the UV-background to be higher, and consistent with the existing measurements based on standard analysis of the proximity effect.