arXiv subjects

Raghunathan Srianand

Publications and source records attributed to Raghunathan Srianand.

At least 109 records · Page 6Linked to original sources

arXiv2009

Constraining fundamental constants of physics with quasar absorption line systems

We summarize the attempts by our group and others to derive constraints on variations of fundamental constants over cosmic time using quasar absorption lines. Most upper limits reside in the range 0.5-1.5x10-5 at the 3sigma level over a redshift range of approximately 0.5-2.5 for the fine-structure constant, alpha, the proton-to-electron mass ratio, mu, and a combination of the proton gyromagnetic factor and the two previous constants, gp(alpha^2/mu)^nu, for only one claimed variation of alpha. It is therefore very important to perform new measurements to improve the sensitivity of the numerous methods to at least <0.1x10-5 which should be possible in the next few years. Future instrumentations on ELTs in the optical and/or ALMA, EVLA and SKA pathfinders in the radio will undoutedly boost this field by allowing to reach much better signal-to-noise ratios at higher spectral resolution and to perform measurements on molecules in the ISM of high redshift galaxies.

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arXiv2009

Models of high redshift luminosity functions and galactic outflows: The dependence on halo mass function

The form of the halo mass function is a basic ingredient in any semi-analytical galaxy formation model. We study the existing forms of the mass functions in the literature and compare their predictions for semi-analytical galaxy formation models. Two methods are used in the literature to compute the net formation rate of halos, one by simply taking the derivative of the halo mass function and the other using the prescription due to Sasaki (1994). For the Press-Schechter (PS) mass function, we compare various model predictions, using these two methods. However, as the Sasaki formalism cannot be easily generalized for other mass functions, we use the derivative while comparing model predictions of different mass functions. We show that the reionization history and UV luminosity function of Lyman break galaxies (LBGs) predicted by the PS mass function differs from those using any other existing mass function, like Sheth-Tormen (ST) mass function.In particular the reionization efficiency of molecular cooled halos has to be substantially reduced when one uses the ST and other mass functions obtained from the simulation instead of the PS mass function. Using $χ^2$-minimization, we find that the observed UV luminosity functions of LBGs at $3.0\le z\le 7.4$ are better reproduced by models using the ST mass function compared to models that use the PS mass function. On the other hand, the volume filling factor of the metals expelled from the galaxies through supernovae driven outflows differs very little between models with different mass functions. It depends on the way we treat merging outflows. We also show that the porosity weighted average quantities related to the outflow are not very sensitive to the differences in the halo mass function.

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arXiv2008

Constrained semi-analytical models of Galactic outflows

We present semi-analytic models of galactic outflows, constrained by available observations on high redshift star formation and reionization. Galactic outflows are modeled in a manner akin to models of stellar wind blown bubbles. Large scale outflows can generically escape from low mass halos (M<10^9 M_sun) for a wide range of model parameters but not from high mass halos (M> 10^{11} M_sun). The gas phase metallicity of the outflow and within the galaxy are computed. Ionization states of different metal species are calculated and used to examine the detectability of metal lines from the outflows. The global influence of galactic outflows is also investigated. Models with only atomic cooled halos significantly fill the IGM at z~3 with metals (with -2.5>[Z/Z_sun]>-3.7), the actual extent depending on the efficiency of winds, the IMF, the fractional mass that goes through star formation and the reionization history of the universe. In these models, a large fraction of outflows at z~3 are supersonic, hot (T> 10^5 K) and have low density, making metal lines difficult to detect. They may also result in significant perturbations in the IGM gas on scales probed by the Lyman-alpha forest. On the contrary, models including molecular cooled halos with a normal mode of star formation can potentially volume fill the universe at z> 8 without drastic dynamic effects on the IGM, thereby setting up a possible metallicity floor (-4.0<[Z/Z_sun]<-3.6). Interestingly, molecular cooled halos with a ``top-heavy'' mode of star formation are not very successful in establishing the metallicity floor because of the additional radiative feedback, that they induce. (Abridged)

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arXiv2007

In response to the comments by Murphy et al. (arxiv:0708.3677)

In their comment, Murphy et al. (arxiv:0708.3677) criticize the fitting procedure we used in two previous papers [Srianand et al. 2004(Paper I) and Chand et al. 2004 (Paper II)] and conclude that the above papers offers no stringent test to previous evidence for varying fine structure constant. We think this is a hasty conclusion as (a) our procedure is robust as shown in Paper II; (b) the data used by Murphy et al., in particular the error array, are different from ours and there are differences in the fitting procedure; (c) despite these differences, 70 per cent of their individual measurements are consistent with that quoted in Paper II. Our reanalysis using VPFIT with identical initial guess parameters leads to Δα/α= (0.01+/-0.15)x 10^{-5}$ for 21 systems (excluding two systems that deviate at more than 3$σ$ level) with very little scatter (χ^2_ν~1).

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arXiv2007

C IV absorption in damped and sub-damped Lyman-alpha systems: correlations with metallicity and implications for galactic winds at z~2-3

We present a study of C IV absorption in a sample of 63 damped Lyman-alpha (DLA) systems and 11 sub-DLAs in the redshift range 1.75<z_abs<3.61, using a dataset of high-resolution (6.6 km/s FWHM), high signal-to-noise VLT/UVES spectra. Narrow and broad C IV absorption line components indicate the presence of both warm, photoionized and hot, collisionally ionized gas. We report new correlations between the metallicity (measured in the neutral-phase) and each of the C IV column density, the C IV total line width, and the maximum C IV velocity. We explore the effect on these correlations of the sub-DLAs, the proximate DLAs (defined as those within 5 000 km/s of the quasar), the saturated absorbers, and the metal line used to measure the metallicity, and we find the correlations to be robust. There is no evidence for any difference between the measured properties of DLA C IV and sub-DLA C IV. In 25 DLAs and 4 sub-DLAs, covering 2.5 dex in [Z/H], we directly observe C IV moving above the escape speed, where v_esc is derived from the total line width of the neutral gas profiles. These high-velocity C IV clouds, unbound from the central potential well, can be interpreted as highly ionized outflowing winds, which are predicted by numerical simulations of galaxy feedback. The distribution of C IV column density in DLAs and sub-DLAs is similar to the distribution in Lyman Break galaxies, where winds are directly observed, supporting the idea that supernova feedback creates the ionized gas in DLAs. The unbound C IV absorbers show a median mass flow rate of ~22(r/40 kpc) solar masses per year, where r is the characteristic C IV radius. Their kinetic energy fluxes are large enough that a star formation rate (SFR) of ~2 solar masses per year is required to power them.

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arXiv2007

Multiphase Plasma in Sub-Damped Lyman Alpha Systems: A Hidden Metal Reservoir

We present a VLT/UVES spectrum of a proximate sub-damped Lyman-alpha (sub-DLA) system at z=2.65618 toward the quasar Q0331-4505 (z_qso=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 log N(H I)=19.82+/-0.05. The absorber is at a velocity of 1820+/-250 km/s 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.

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arXiv2007

Probing the star formation history using the redshift evolution of luminosity functions

We present a self-consistent, semi-analytical LCDM model of star formation and reionization. For the cosmological parameters favored by the WMAP data, our models consistently reproduce the optical depth to reionization, redshift of reionization and the observed luminosity functions (LF) and hence the star formation rate (SFR) density at 3 8. Molecular cooled halos, which may be important for reionizing the universe, are not detectable in the present deep field observations. However, their presence and contribution to reionization can be inferred indirectly from the redshift evolution of the LF at 6<z< 12.Accurately measuring the LF at high z can be used to understand the nature of star formation in the dark ages and probe the history of reionization. (Abridged)

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arXiv2007

Hot Halos around High Redshift Protogalaxies: Observations of O VI and N V Absorption in Damped Lyman Alpha systems

(ABRIDGED) We present a study of the highly ionized gas (plasma) associated with damped Lyman-alpha (DLA) systems at z=2.1-3.1. We search for O VI absorption and corresponding Si IV, C IV, and N V in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering O VI at S/N>10. We report twelve DLAs (nine intervening and three at <5000 km/s from the QSO redshift) with detections of O VI absorption. There are no clear O VI non-detections, so the incidence of O VI in DLAs is between 34% (12/35) and 100%. 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 O VI components), and a warm, photoionized phase (seen just in narrow C IV and Si IV components). 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 (O VI-bearing) and warm (C IV-bearing) phases in the range log N(Hot H II) >19.5 to >21.1, and log N(Warm 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. If the temperature in the O VI phase is ~10^6 K and so f(O VI)=O VI/O<<0.2 the plasma can make a significant contribution to the metal budget at high redshift.

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arXiv2006

Multiwavelength investigation of a near-solar metallicity sub-DLA at z =1.3647 towards PKS 0237-233

We searched for 21-cm absorption associated with the z_abs = 1.3647 absorption system toward PKS 0237-233 using the GMRT. A high quality UVES spectrum shows that C I and C I* are detected at this redshift together with C II*, Mg I, Mg II, Si II, Al II, Fe II and Mn II. The complex profiles, spread over ~300 km/s, are fitted with 21 Voigt profile components. None of these components are detected in 21-cm absorption down to a detection limit of τ(3σ)\le 3x10^{-3} (or N(HI)/T_S <10^{17} cm ^-2 K^-1). We derive log N(HI)<19.30$\pm$0.30 using the Lyman alpha absorption line detected in the IUE spectrum of the quasar. Mg II, Si II and Al II column densities are consistent with near solar metallicity and we measure [O/H]>-0.33. Using photoionization models constrained by the fine-structure excitations of C I and C II, and the 21-cm optical depth, we show that the C I absorption arises predominantly either in WIM or WNM in ionization and thermal equilibrium with the meta-galactic UV background dominated by QSOs and star forming galaxies. The estimated thermal pressure of the gas is of the same order of magnitude over different velocity ranges through the absorption profile (2.6\le log [P/k cm^{-3} K]\le 4.0). The gas-phase metallicity corrected for ionization is Z>0.5 Z_\odot with a signature of Fe co-production elements being under abundant compared to α-process elements by ~0.5 dex. At z>1.9, C I absorption is usually associated with H_2 absorption arising from cold gas in DLAs. This system and the z=2.139 toward Tol 1037-270 are the only two systems known which show that C I absorption can also be detected in warm gas provided the metallicity is high enough. Interestingly, both the systems are part of unusual concentrations of absorption lines.

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arXiv2006

ELT requirements for future observations of the Intergalactic Medium

We summarise the science cases for an ELT that were presented in the parallel session on the intergalactic medium, and the open discussion that followed the formal presentations. Observations of the IGM with an ELT provides tremendous potential for dramatic improvements in current programmes in a very wide variety of subjects. These range from fundamental physics (expansion of the Universe, nature of the dark matter, variation of physical constants), cosmology (geometry of the Universe, large-scale structure), reionisation (ionisation state of the IGM at high redshift>6, to more traditional astronomy, such as the interactions between galaxies and the IGM (metal enrichment, galactic winds and other forms of feedback), and the study of the interstellar medium in high redshift galaxies through molecules. The requirements on ELTs and their instruments for fulfilling this potential are discussed.

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arXiv2006

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

We present a detailed analysis of a very high resolution (R\approx 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 HARPS spectrum, of very high wavelength calibration accuracy (better than 1 mÅ), is used to search for possible systematic inaccuracies in the wavelength calibration of the UV Echelle Spectrograph (UVES) mounted on the ESO Very Large Telescope (VLT). We have carried out cross-correlation analysis between the Th-Ar lamp spectra obtained with HARPS and UVES. The shift between the two spectra has a dispersion around zero of σ\simeq 1 mÅ. This is well within the wavelength calibration accuracy of UVES (i.e σ\simeq 4 mÅ). We show that the uncertainties in the wavelength calibration induce an error of about, Δα/α\le 10^{-6}, in the determination of the variation of the fine-structure constant. Thus, the results of non-evolving Δα/αreported 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-αsystem toward HE 0515-4414 reveals more components compared to the UVES spectrum. Using the Voigt profile decomposition that simultaneously fits the high resolution HARPS data and the higher signal-to-noise ratio UVES data, we obtain, Δα/α=(0.05\pm0.24)x10^{-5} at z_{abs}=1.1508. This result is consistent with the earlier measurement for this system using the UVES spectrum alone.

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arXiv2005

A multiwavelength investigation of the temperature of the cold neutral medium

We present measurements of the H I spin temperatures (T_s) of the Cold Neutral Medium (CNM) towards radio sources that are closely aligned with stars for which published H_2 ortho-para temperatures (T_{01}) 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 H_2 absorption. We then find that T_{01} and T_s match within observational uncertainties for lines-of-sight with H_2 column density above 10^{15.8} cm^{-2}, but deviate from each other below this threshold. This is consistent with the expectation that in the CNM T_s tracks the kinetic temperature due to collisions and that T_{01} is driven towards the kinetic temperature by proton exchange reactions.

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arXiv2005

Molecular hydrogen in the diffuse interstellar medium at high redshift

Physical conditions within 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 H_2), grain physics, and chemistry. We show that for a low-density gas (n< 0.1 cm^-3) the meta-galactic UV background due to quasars is sufficient to maintain H_2 column densities below the detection limit (i.e N(H_2)< 10^14 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 in DLAs. We show that the observed properties of the ~15% per cent of the DLAs that do show detectable H_2 absorption cannot be reproduced with only the quasar dominated meta-galactic UV radiation field. Gas with higher densities (n>10 cm^-3), a moderate 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 H_2 absorption lines. This favors the presence of ongoing star formation in DLAs with H_2. The absence of detectable H_2 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 H_2 and C I absorption will suggest the second alternative. (Abridged)

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arXiv2005

The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems: Physical conditions in the neutral gas

We study the physical conditions in DLAs, using a sample of 33 systems toward 26 QSOs acquired for a recently completed survey of H_2 by Ledoux et al. (2003). We use the column densities of H_2 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, LMC and SMC. The mean kinetic temperature of the gas corresponding to DLA subcomponents in which H_2 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 n = 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(H_2,J=2)/N(H_2,J=0) seen in DLAs. From the column densities of the high-J rotational levels, we derive that the typical radiation field in the H_2 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 H_2 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 H_2 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). (Abridged)

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arXiv2005

Can Cosmic Rays Heat the Intergalactic Medium?

Supernova explosions in the early star forming galaxies will accelerate cosmic rays (CRs). CRs are typically confined in the collapsed objects for a short period before escaping into the intergalactic medium (IGM). Galactic outflows can facilitate this escape by advecting CRs into the IGM. An outflow that results in a termination shock can also generate more CRs. We show that the CR protons from the above processes can significantly affect the thermal history of the IGM. Within plausible range of parameters, cosmic ray heating can compensate for adiabatic cooling and explain the measured IGM temperature at redshifts z between 2 to 4, even with early reionization.

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arXiv2004

Probing the time-variation of the fine-structure constant: Results based on Si IV doublets from a UVES sample

We report a new constraint on the variation of the fine-structure constant based on the analysis of 15 Si IV doublets selected from a ESO-UVES sample. We find Δα/α= (+0.15+/-0.43) x 10^-5 over a redshift range of 1.59< z < 2.92 which is consistent with no variation in α. This result represents a factor of three improvement on the constraint on Δα/αbased on Si IV doublets compared to the published results in the literature. The alkali doublet method used here avoids the implicit assumptions used in the many-multiplet method that chemical and ionization inhomogeneities are negligible and isotopic abundances are close to the terrestrial value.

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arXiv2004

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

Most of the successful physical theories rely on the constancy of few fundamental quantities (such as the speed of light, $c$, the fine-structure constant, α, the proton to electron mass ratio, μ, etc), and constraining the possible time variations of these fundamental quantities is an important step toward a complete physical theory. Time variation of αcan be accurately probed using absorption lines seen in the spectra of distant quasars. Here, 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 αderived from our analysis over the redshift range 0.4<z<2.3 is Δα/α= (-0.06+/-0.06) x 10^{-5}. The median redshift of our sample (z=1.55) corresponds to a look-back time of 9.7 Gyr in the most favored cosmological model today. This gives a 3σlimit, -2.5 x 10^{-16} yr^-1 <(Δα/αΔt) <+1.2x10^{-16} yr^-1, for the time variation of α, that forms the strongest constraint obtained based on high redshift quasar absorption line systems.

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arXiv2003

Outflowing material in the zem = 4.92 BAL QSO SDSS~J160501.21-011220.0

We present the analysis of two BALs seen in the spectrum of the QSO SDSS J160501.21-011220.0. Our UVES spectrum shows two well detached BALs at z(abs)= 4.685 and 4.855. The system at z(abs)= 4.855 covers the background source completely suggesting that the gas is located outside the BLR. On the contrary the system at z(abs)= 4.685 has a covering factor of ~0.9. The observed H I absorption line together with the limits on C II and Si II absorptions suggest that 16<log N(H I) (cm^{-2})<~17 in z(abs) = 4.855 absorption system. Comparison with models show that the observed column densities of N V, Si IV and C IV in this system require that N is underabundant by more than a factor 3 compared to Si if the ionizing radiation is similar to a typical QSO spectrum. This is contrary to what is usually derived for the emission line gas in QSOs. We show that the relative suppression in the N V column density can be explained for Solar abundance ratios or abundance ratios typical of Starburst abundances if an ionizing spectrum devoid of X-rays is used instead. Thus, if the composition of BAL is like that of BLR then it is most likely that the cloud sees a spectrum devoid of X-rays similar to what we observe from this QSO. This is consistent with the fact that none of our models have high Compton optical depth to remove X-rays from the QSO. Similar arguments lead to the conclusion that the system at z(abs)= 4.685 as well is not Compton thick. The estimated black hole mass is 8 10^8 solar mass suggesting the accretion onto the seed black hole must have started as early as z = 11.

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