R. Srianand

The Very Large Telescope Ultraviolet and Visible Echelle Spectrograph survey for molecular hydrogen in high-redshift damped Lyman α systems

We have searched for molecular hydrogen in damped Lyman-alpha (DLA) and sub-DLA systems at z>1.8 using UVES at the VLT. 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, H2 is detected in 13 to 20 percent of the newly-surveyed systems. We report new detections of molecular hydrogen at z=2.087 and 2.595 toward, respectively, Q 1444+014 and Q 0405-443, and also reanalyse the system at z=3.025 toward Q 0347-383. We find that there is a correlation between metallicity and depletion factor in both our sample and also the global population of DLA systems (60 systems in total). The DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and 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 is generally small in DLA systems and similar to what is observed in the Magellanic Clouds. From 58 to 75 percent of the DLA systems have log f 1000 K) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.

Limits on the time variation of the electromagnetic fine-structure constant in the low energy limit from absorption lines in the spectra of distant quasars.

We present the results of a detailed many-multiplet analysis performed on a new sample of Mg ii systems observed in high quality quasar spectra obtained using the Very Large Telescope. The weighted mean value of the variation in alpha derived from our analysis over the redshift range 0.4</=z</=2.3 is Deltaalpha/alpha=(-0.06+/-0.06)x10(-5). The median redshift of our sample (z approximately 1.55) corresponds to a look-back time of 9.7 Gyr in the most favored cosmological model today. This gives a 3sigma limit, -2.5 x 10(-16)</=(Deltaalpha/alphaDeltat)</=+1.2 x 10(-16) yr(-1), for the time variation of alpha, that forms the strongest constraint obtained based on high redshift quasar absorption line systems.

Probing the cosmological variation of the fine-structure constant: Results based on VLT-UVES sample ?

Development of fundamental physics relies on the constancy of various fundamental quantities such as the fine- structure constant. Detecting or constraining the possible time variations of these fundamental physical quantities is an im- portant step toward a complete understanding of basic physics. High-quality absorption lines seen in the spectra of distant QSOs allow one to probe time variations of several of these quantities. Here we present the results from a detailed many- multiplet analysis, to detect the possible variation of the fine-structure constant, performed using high signal-to-noise ratio, (∼70 per pixel), high spectral resolution (R ≥ 45 000) observations of 23 Mg  systems detected toward 18 QSOs in the redshift range 0.4 ≤ z ≤ 2.3 obtained using UVES at the VLT. We validate our procedure and define the selection criteria that will avoid possible systematics using a detailed analysis of a simulated data set. The spectra of Mg  doublets and Fe  multiplets are generated considering variations in α and specifications identical to that of our UVES spectra. We show that our Voigt profile fitting code recovers the variation in α very accurately when we use single component systems and multiple-component systems that are not heavily blended. Spurious detections are frequently seen when we use heavily blended systems or systems with very weak lines. Thus we avoided these system while analysing the UVES data. To make the analysis transparent and accessible to the community for critical scrutiny, all the steps involved in the analysis are presented in detail. The weighted mean value of the variation in α obtained from our analysis over the redshift range 0.4 ≤ z ≤ 2. 3i s∆α/α = (−0.06 ± 0.06) × 10 −5 . The median redshift of our sample is 1.55 and corresponds to a look-back time of 9.7 Gyr in the most favored cosmological model today. The 3σ upper limit on the time variation of α is −2.5 × 10 −16 yr −1 ≤ (∆α/α∆t) ≤ +1.2 × 10 −16 yr −1 . To our knowledge this is the strongest constraint from quasar absorption line studies to till date.

The VLT‐UVES survey for molecular hydrogen in high‐redshift damped Lyman α systems: physical conditions in the neutral gas

We study the physical conditions in damped Lyman α systems (DLAs), using a sample of 33 systems towards 26 quasi-stellar objects (QSOs) acquired for a recently completed survey of H2 by Ledoux, Petitjean & Srianand. 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 ultraviolet (UV) radiation field. Detailed comparisons are made between the observed properties in DLAs, the interstellar medium (ISM) of the Galaxy and the Large and Small Magellanic Clouds (LMCs and SMCs). The mean kinetic temperature of the gas corresponding to DLA subcomponents in which an H2 absorption line is detected, derived from the ortho-to-para ratio (OPR) (153 ± 78 K), is higher than that measured in the ISM (77 ± 17 K) and in Magellanic Clouds (82 ± 21 K). Typical pressure in these components (corresponding to T = 100‐300 K and n H = 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 shows that 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 per cent of the DLAs, can be used to conclude that DLAs at z > 1.9 could contribute as much as 50 per cent star formation rate (SFR) 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 .W edetect C II ∗ in about 50 per cent of the DLAs and, therefore, in a considerable fraction of DLAs that do not show H2 .I npart 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 a warm neutral medium (WNM) and/or partially ionized gas. The absence of H2, for the level of metallicity and dust depletion seen in these systems, is consistent with low densities (i.e. n H 1c m −3 ) for a radiation field similar to the mean Galactic UV field. Ke yw ords: galaxies: haloes ‐ galaxies: ISM ‐ quasars: absorption lines ‐ cosmology:

First detection of CO in a high-redshift damped Lyman-α system

We present the first detection of carbon monoxide (CO) in a damped Lyman-α system (DLA) at zabs = 2.41837 toward SDSS J143912.04+111740.5. We also detected H2 and HD molecules. The measured total column densities (in log units) of H i ,H 2 ,a nd CO are 20.10± 0.10, 19.38± 0.10, and 13.89± 0.02, respectively. The molecular fraction, f = 2N(H2)/(N(H i)+2N(H2)) = 0.27 +0.10 −0.08 ,i s the highest among all known DLAs. The abundances relative to solar of S, Zn, Si, and Fe are −0.03 ± 0.12, +0.16 ± 0.11, −0.86 ± 0.11, and −1.32 ± 0.11, respectively, indicating a high metal enrichment and a depletion pattern onto dust-grains similar to the cold ISM of our Galaxy. The measured N(CO)/N(H2) = 3 × 10 −6 is much less than the conventional CO/H2 ratio used to convert the CO emission into gaseous mass but is consistent with what is measured along translucent sightlines in the Galaxy. The CO rotational excitation temperatures are higher than those measured in our Galactic ISM for similar kinetic temperature and density. Using the C i fine structure absorption lines, we show that this is a consequence of the excitation being dominated by radiative pumping by the cosmic microwave background radiation (CMBR). From the CO excitation temperatures, we derive TCMBR = 9.15 ± 0.72 K, while 9.315 ± 0.007 K is expected from the hot big-bang theory. This is the most precise high-redshift measurement of TCMBR and the first confirmation of the theory using molecular transitions at high redshift.

The evolution of the cosmic microwave background temperature - Measurements of TCMB at high redshift from carbon monoxide excitation

A milestone of modern cosmology was the prediction and serendipitous discovery of the cosmic microwave background (CMB), the radiation leftover after decoupling from matter in the early evolutionary stages of the Universe. A prediction of the standard hot Big-Bang model is the linear increase with redshift of the black-body temperature of the CMB (TCMB). This radiation excites the rotational levels of some interstellar molecules, including carbon monoxide (CO), which can serve as cosmic thermometers. Using three new and two previously reported CO absorption-line systems detected in quasar spectra during a systematic survey carried out using VLT/UVES, we constrain the evolution of TCMB to z ∼ 3. Combining our precise measurements with previous constraints, we obtain TCMB(z) = (2.725 ± 0.002) × (1 + z) 1−β K with β = −0.007 ± 0.027, a more than two-fold improvement in precision. The measurements are consistent with the standard (i.e. adiabatic, β = 0) Big-Bang model and provide a strong constraint on the effective equation of state of decaying dark energy (i.e. weff = −0.996 ± 0.025).

A translucent interstellar cloud at z = 2.69 - CO, H2, and HD in the line-of-sight to SDSS J123714.60 + 064759.5

We present the analysis of a sub-damped Lyman-α system with neutral hydrogen column density, log N(H 0 ) (cm -2 ) = 20.0 ± 0.15 at z abs = 2.69 toward SDSSJ123714.60 + 064759.5 (z em = 2.78). Using the VLT/UVES and X-shooter spectrographs, we detect H 2 , HD, and CO molecules in absorption with log N(H 2 , HD, CO) (cm -2 ) = 19.21 +0.13 -0.12 , 14.48 ± 0.05 and 14.17 ± 0.09 respectively. The overall metallicity of the system is super-solar ([Zn/H] = +0.34 relative to solar) and iron is highly depleted ([Fe/Zn] = -1.39), revealing metal-rich and dusty gas. Three H 2 velocity components spanning ∼125 km s -1 are detected. The strongest H 2 component, at z abs = 2.68955, with logN(H 2 ) = 19.20, does not coincide with the centre of the HI absorption. This implies that the molecular fraction in this component, ƒ H2 = 2N(H 2 )/(2N(H 2 ) +N(H 0 )), is higher than the mean molecular fraction fH 2 = ¼ in the system. We also found the Cl 0 associated with this H 2 component to have N(Cl 0 )/N(Cl+) > 0.4, which points in the same direction. Cl 0 is tied to H 2 by charge exchange reactions, this means that the molecular fraction in this component is not far from unity. The kinetic temperature derived from the J = 0 and 1 rotational levels of H 2 is T = 108 +84 -33 K and the particle density derived from the C 0 ground-state fine structure level populations is n H0 ∼ 50-60 cm -3 . We derive an electronic density -3 for a UV field similar to the Galactic one and show that the carbon-to-sulphur ratio in the cloud is close to the solar ratio. The size of the molecular cloud is probably smaller than 1 pc. Both the CO/H 2 = 10 -5 and CO/C 0 ∼ 1 ratios for ƒ H 2 > 0.24 indicate that the cloud classifies as translucent, i.e., a regime where carbon is found both in atomic and molecular form. The corresponding extinction, A V = 0.14, albeit lower than the definition of a translucent sightline (based on extinction properties), is high for the observed H0 column density. This means that intervening clouds with similar local properties but with higher column densities (i.e. larger physical extent) could be missed by current magnitude-limited QSO surveys. The excitation of CO is dominated by radiative interaction with the cosmic microwave background radiation (CMBR) and we derive T ex (CO) = 10.5 +0.8 -0.6 K when T CMBR (z = 2.69) = 10.05 K is expected. We measure N(HD) / 2N(H 2 ) = 10 -5 . This is about 10 times higher than what is measured in the Galactic ISM for ƒ H 2 = ¼ but similar to what is measured in the Galactic ISM for higher molecular fractions. The astration factor of deuterium with respect to the primordial D/H ratio is only about 3. This can be the consequence of accretion ofunprocessed gas from the intergalactic medium onto the associated galaxy. In the future, it will be possible to search efficiently for molecular-rich DLAs/sub-DLAs with X-shooter, but detailed studies of the physical state of the gas will still need UVES observations.

Diffuse molecular gas at high redshift ⋆ Detection of CO molecules and the 2175 Å dust feature at z= 1.64

We present the detection of carbon monoxide molecules (CO) at z = 1.6408 towards the quasar SDSS J160457.50+220300.5 using the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph. We detected CO absorption in at least two components in the first six A−X bands and one d−X(5−0) interband system. This is the second detection of its kind along a quasar line of sight. The CO absorption profiles are well modelled by assuming rotational excitation of CO in the range 6 < Tex < 16 K, which is consistent with or higher than the temperature of the cosmic microwave background radiation at this redshift. We derive a total CO column density of N(CO) = 4 × 10 14 cm −2 . The measured column densities of S i ,M gi ,Z nii ,F eii ,a nd Siii indicate a dust depletion pattern typical of cold gas in the Galactic disc. The background quasar spectrum is significantly reddened (u − K ∼ 4.5 mag) and exhibits a pronounced 2175 A dust absorption feature at the redshift of the CO absorber. Using a control sample of ∼500 quasars, we find that the chance probability that this feature is spurious is ∼0.3%. We show that the spectral energy distribution (SED) of the quasar is well fitted by a QSO composite spectrum reddened with a Large Magellanic Cloud supershell extinction law at the redshift of the absorber. It is noticeable that this quasar is absent from the colour-selected SDSS quasar sample. This demonstrates that our current view of the Universe may be biased against dusty sightlines. These direct observations of carbonaceous molecules and dust open up the possibility of studying the physical conditions and chemistry of diffuse molecular gas in high redshift galaxies.

Variation of the fine-structure constant : Very high resolution spectrum of QSO HE 0515-4414

Aims, We present a detailed analysis of a very high resolution (R ≈ 112 000) spectrum of the quasar HE 0515-4414 obtained using the High Accuracy Radial velocity Planet Searcher (HARPS) mounted on the ESO 3.6 m telescope at the La Silla observatory. The main aim is to use a HARPS spectrum of very high wavelength calibration accuracy (better than 1 mA), to constrain the variation of α ≡ e 2 /hc and investigate any possible systematic inaccuracies in the wavelength calibration of the UV Echelle Spectrograph (UVES) mounted on the ESO Very Large Telescope (VLT). Methods. A cross-correlation analysis between the Th-Ar lamp spectra obtained with HARPS and UVES is carried out to detect any possible shift between the two spectra. Absolute wavelength calibration accuracies, and how that translates into the uncertainties in Δα/α are computed using Gaussian fits for both lamp spectra. The value of Δα/α at z abs = 1.1508 is obtained using the many multiplet method and simultaneous Voigt profile fits of HARPS and UVES spectra. Results. We find the shift between the HARPS and UVES spectra has a mean around zero with a dispersion of σ ≃ 1 mA. This is shown to be well within the wavelength calibration accuracy of UVES (i.e. σ ≃ 4 mA). We show that the uncertainties in the wavelength calibration induce an error of about Δα/α ≤ 10 -6 in determining the variation of the fine-structure constant. Thus, the results of non-evolving Δα/α reported in the literature based on UVES/VLT data should not be heavily influenced by problems related to wavelength calibration uncertainties. Our higher resolution spectrum of the z abs = 1.1508 Damped Lyman-a system toward HE 0515-4414 reveals more components compared to the UVES spectrum. Using only Fe II lines of the z abs = 1.1508 system, we obtain Δα/α = (0.05 ± 0.24) x 10 -5 . This result is consistent with the earlier measurement for this system using the UVES spectrum alone.

Detection of molecular hydrogen in a near Solar-metallicity damped Lyman-α system at

We report the detection of H 2 , C I , C I * , C I * * and Cl 1 lines in a near Solar-metallicity ([Zn/H] = -0.13) damped Lyman-a (DLA) system at z a b s = 1.962 observed on the line of sight to the quasar Q0551-366. The iron-peak elements, X = Fe, Cr and Mn are depleted compared to zinc, [X/Zn] ∼ -0.8, probably because they are tied up onto dust grains. Among the three detected H 2 -bearing clouds, spanning 55 km s - 1 in velocity space, we derive a total molecular hydrogen column density N(H 2 ) = 2.6 × 10 1 7 cm - 2 and a mean molecular fraction f = 2N(H 2 )/(2N(H 2 ) + N(H I )) = 1.7 x 10 - 3 . The depletion of heavy elements (S, Si, Mg, Mn, Cr, Fe, Ni and Ti) in the central component is similar to that observed in the diffuse neutral gas of the Galactic halo. This depletion is approximately the same in the six C i-detected components independently of the presence or absence of H 2 . The gas clouds in which H 2 is detected always have large densities, n H > 30 cm - 3 , and low temperatures, T 0 1 < 100 K. This shows that presence of dust, high particle density and/or low temperature is required for molecules to be present. The photo-dissociation rate derived in the components where H 2 is detected suggests the existence of a local UV radiation field similar in strength to the one in the Galaxy. Star formation therefore probably occurs near these H 2 -bearing clouds.