arXiv subjects

R. Srianand

Publications and source records attributed to R. Srianand.

At least 145 records · Page 8Linked to original sources

arXiv2001

HD Molecular Lines in an Absorption System at Redshift z = 2.3377

We have analyzed the spectrum of the quasar PKS 1232+082 obtained by Petitjean et al. (2000). HD molecular lines are identified in an absorption system at the redshift z=2.3377. The column density of HD molecules in the system is estimated, N(HD)=(1-4)x10^{14} cm^{-2}. The temperature of excitation of the first rotational level J=1 relative to the ground state J=0 is T_ex=70+-7 K. This is, to our knowledge, the first detection of HD molecules at high redshift.

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arXiv2001

Semi analytic approach to understanding the distribution of neutral hydrogen in the universe: Comparison of simulations with observations

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 (Ω_m=0.4, Ω_Λ=0.6 and h=0.65), we obtain, using statistics obtained from the transmitted flux, constraints on (i) the combination f=(Ω_B h^2)^2/J_{-12}, where Ω_B is the baryonic density parameter and J_{-12} is the total photoionisation rate in units of 10^{-12} s^{-1}, (ii) temperature T_0 corresponding to the mean density and (iii) the slope γof the effective equation of state of the IGM at a mean redshift z \simeq 2.5. We find that 0.8 <(T_0/10^4 K)< 2.5 and 1.3<γ<2.3. while the constraint obtained on f is 0.020^2 1.2, the lower bound on Ω_B h^2 is in violation of the BBN value.

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arXiv2001

A near-solar metallicity damped Lyman-alpha system toward the BAL quasar Tol 1037-2703

We report the detection of a Broad Absorption Line (BAL) outflow in the spectrum of Tol 1037-2703 with three main BALs at 36000, 25300 and 22300~\kms outflow velocities. Although the overall flow is dominated by high ionization lines like and C IV, the gas of highest velocity shows absorption from Mg I, Mg II and Fe II. Covering factor arguments suggest that the absorbing complexes are physically associated with the QSO and have transverse dimensions smaller than that of the UV continuum emitting region (r<0.1 pc). We show that the C IV absorption at \zabs=2.082 has a covering factor $\sim$ 0.86 and the absorption profile has varied over the last four years. The detection of absorption from excited fine structure levels of C II and Si II in narrow components embedded in the C IV trough reveals large density inhomogeneities. IR pumping is the most likely excitation process. The \zabs=2.139 system is a damped system with log N(H I) ~ 19.7. The weakness of the metal lines together with the high quality of the data make the metallicity measurements particularly reliable. The absolute metallicity is close to solar with [Zn/H]=-0.26. The $α$-chain elements have metallicities consistently solar (respectively +0.05, -0.02, -0.03 and -0.15 for [Mg/H], [Si/H], [P/H] and [S/H]) and iron peak elements are depleted by a factor of about two ([Fe/Zn], [Cr/Zn], [Mn/Zn] and [Ni/Zn] are equal to -0.39, -0.27, -0.49, -0.30). Lines from C I are detected but H$_2$ is absent with a molecular fraction less than 8$\times10^{-6}$. High metallicity and low nitrogen abundance, [N/Zn]=-1.40, favor the idea that metals have been released by massive stars during a starburst of less than 0.5 Gyr of age. (abridged)

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arXiv2000

The microwave background temperature at the redshift of 2.33771

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, $T_{\rm CMBR,0}$ = 2.726$\pm$ 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 {\sl 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 < T_{\rm CMBR} < 14 K at z = 2.33771 when 9.1 K is expected in the Hot Big Bang cosmology.

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arXiv2000

Molecular hydrogen and the nature of damped Lyman-alpha systems

We report results from our mini-survey for molecular hydrogen in eight high redshift damped Lyman-alpha (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 sample the only system where H2 was previously detected and studied at high-spectral resolution. Altogether our sample consists of eleven absorbers with 1.85 3000 K) implying a low formation rate of H2 onto dust grains. Therefore, most of the DLA systems arise in warm and diffuse neutral gas. (Abridged)

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arXiv2000

Semi analytic approach to understanding the distribution of neutral hydrogen in the universe

Analytic derivations of the correlation function and the column density distribution for neutral hydrogen in the 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. Our approach is completely analytic, which allows us to explore a wide region of the parameter space for our model. The analytic results have been compared with observations, whenever possible. Two kinds of correlation functions are defined: along the line-of-sight (LOS) and 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, γare of the same order, which means that we cannot constrain both the parameters simultaneously. However, it is possible to constrain γand its evolution using the observed LOS correlation function at different epochs, provided one knows the background cosmology. We suggest that the constraints on the evolution of γobtained 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 find that the excess probability, over random, of finding two neutral hydrogen overdense regions separated by an angle θ, 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 \simeq 2.31 using the column density distribution. However, one cannot rule higher values of γat higher redshifts.

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arXiv2000

Structure of the MgII and damped Lyman-alpha systems along the line of sight to APM 08279+5255

A study of the absorption systems toward the gravitationally lensed quasar APM 08279+5255 is presented. Most of the MgII systems in the redshift range 1.2-2.07, although saturated, show large residuals at the bottom of the lines. The most likely interpretation is that individual clouds within MgII halos do cover only one of the two brightest QSO images. This reveals that MgII halos are made of a collection of clouds of radius smaller than about 1 kpc. Two strong MgII absorbers at z = 1.062 and 1.181 are studied in detail. This is the first time that the NaI3303 doublet is detected in such high redshift systems. Together with the detection of the MgI2852 transition, this strongly constrains the physical characteristics of the gas. The N(NaI)/N(MgI) ratio is found to be larger than unity, implying that the gas is cool and neutral. The column densities of NaI, CaII, MgI, TiII, MnII and FeII observed at z = 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 and the relative depletion of Fe, Ti, Mn and Ca are consistent with the presence of dust at z = 1 (AV = 0.5 mag). Altogether it is found that the HI column density at z = 1 is of the order of 1 to 5 10**21 cm-2, the corresponding metallicity is in the range 1--0.3 Zsol. The objects associated with these two systems could both contribute to the lens together with another possible strong system at z = 1.1727 and the strong Lyman-alpha system at z = 2.974. The probable damped Ly-alpha system at z = 2.974 has 19.8 < log N(HI) < 20.3. The transverse dimension of the absorber is larger than 200 pc. Abundances relative to solar are -2.31, -2.26, -2.10, -2.35 and -2.37 for, respectively, Fe, Al, Si, C and O (for log N(HI) = 20.3). (Abridged)

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arXiv2000

Physical conditions in broad and associated narrow absorption-line systems toward APM 08279+5255

Results of a careful analysis of the absorption systems with zabs = zem seen toward the bright, z_em ~ 3.91, gravitationally lensed quasar APM 08279+5255 are presented. Two of the narrow-line systems, at z_abs = 3.8931 and z_abs = 3.9135, show absorptions from singly ionized species with weak or no NV and O V absorptions at the same redshift. Absorption due to fine structure transitions of C II and Si II (excitation energies corresponding to, respectively, 156$μ$m and 34$μ$m) are detected at z_abs = 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 z_abs = 3.9135 suggests that the gas responsible for this system is farther away from the IR source. Abundances are ~ 0.01 and 1$Z_{\odot}$ at z_abs = 3.913 and 3.8931 and aluminum could be over-abundant with respect to silicon and carbon by at least a factor of two and five. All this suggests that whereas the \zabs = 3.8931 system is probably located within 200 pc from the QSO and ejected at a velocity larger than 1000 kms^{-1}, the \zabs = 3.9135 system is farther away and part of the host-galaxy. (abridged)

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arXiv1999

High resolution study of associated C IV absorption systems in NGC 5548

We present the results of a careful analysis of associated absorption systems toward NGC 5548. Most of the well resolved narrow components in the associated system, defined by the Lyman alpha, C IV and N V profiles, show velocity separation similar (to within 10~\kms) to the C IV doublet splitting. We estimate the chance probability of occurrence of such pairs with velocity separation equal to C IV doublet splitting to be $6\times10^{-3}$. Thus it is more likely that most of the narrow components are line-locked with C IV doublet splitting. This will mean that the radiative acceleration plays an important role in the kinematics of the absorbing clouds. We build grids of photoionization models and estimate the radiative acceleration due to all possible bound-bound transitions. We show that the clouds producing absorption have densities less than $10^9 cm^{-3}$, and are in the outer regions of the broad emission line region (BLR). We note that the clouds which are line-locked cannot produce appreciable optical depths of O VII and O VIII, and hence cannot be responsible for the observed ionized edges, in the soft X-ray. We discuss the implications of the presence of optically thin clouds in the outer regions of the BLR to the models of broad emission lines.

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arXiv1999

The zabs=zem Absorption Line Systems Toward QSO J2233-606 in the Hubble Deep Field South: NeVIII770,780 absorption and partial coverage

Results of a careful analysis of the highly ionized absorption systems, observed over the redshift range 2.198--2.2215 in the zem=2.24 HDFS-QSO J2233-606, are presented. Strong OVI and NeVIII absorptions are detected. Most of the lines show signature of partial coverage which varies from species to species. This can be understood if the clouds cover the continuum emission region completely and only a fraction of the broad emission line region. Using photo-ionization models we analyze in more detail the component at zabs = 2.198. Absolute abundances are close to solar but the [N/C] abundance ratio is larger than solar. This result, which is consistent with the analysis of high-z QSO broad emission-lines, confirms the physical association of the absorbing gas with the AGN. The observed column densities of NIV, NV and NeVIII favor a two-zone model for the absorbing region where NeVIII is predominantly produced in the highly ionized zone. It is most likely that in QSO J2233-606, the region producing the NeVIII absorption can not be a warm absorber. One of the Lyalpha absorption lines at zabs = 2.2215 has a flat bottom typical of saturated lines and non-zero residual intensity in the core, consistent with partial coverage. There is no metal-line from this Lyalpha cloud detectable in the spectrum which suggests either large chemical inhomogeneities in the gas or that the gas is very highly ionized. If the latter is true the cloud could have a total hydrogen column density consistent with that of X-ray absorbers. It is therefore of first importance to check whether or not there is an X-ray warm-absorber in front of this QSO.

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arXiv1999

Probing the BLR in AGNs using time variability of associated absorption line

It is know that most of the clouds producing associated absorption in the spectra of AGNs and quasars do not completely cover the background source (continuum + broad emission line region, BLR). We note that the covering factor derived for the absorption is the fraction of photons occulted by the absorbing clouds, and is not necessarily the same as the fractional area covered. We show that the variability in absorption lines can be produced by the changes in the covering factor caused by the variation in the continuum and the finite light travel time across the BLR. We discuss how such a variability can be distinguished from the variability caused by other effects and how one can use the variability in the covering factor to probe the BLR.

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arXiv1998

On the kinematics of damped Lyman-alpha systems

We report on high spectral resolution observations of five damped Ly-alpha absorbers. Line velocity profiles and heavy element abundances are discussed. Nitrogen is found to have abundances less than silicon in the systems toward Q 0347-383, Q 0913+072, and Q 1213+093. The absorber toward Q 0913+072 is the most metal-deficient damped system known, with [Fe/H] < -3.2. The simple kinematical structure of the metal absorptions makes this system ideal to discuss the [O/Si] and [N/O] ratios. We find [O/H] ~ -2.7 and -2.7 < [Si/H] < -2.2. By combining these data with information gathered in the literature, we study the kinematics of the low and high ionization phases in a sample of 26 damped Ly-alpha systems in the redshift range 1.17-4.38. We note a strong correlation between the velocity broadenings of the SiII,1808 and FeII,1608 lines whatever the line optical depth, implying that the physical conditions are quite homogeneous in the sample. Statistically this shows that large variations of abundance ratios and thus large variations of depletion into dust grains are unlikely. The velocity broadening of the absorption lines, Delta V, is correlated with the asymmetry of the lines for Delta V < 150 km/s. The broader the line the more asymmetric it is, as expected in case rotation dominates the line broadening. However this correlation does not hold for larger Delta V suggesting that evidence for rotational motions is restricted to velocity broadenings Delta V < 150 km/s. The systems with Delta V > 200 km/s are peculiar with kinematics consistent with random motions. They show sub-systems as those expected if the objects are in the process of merging (abridged).

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arXiv1998

Molecules in the $z_{\rm abs}$~=~2.8112 damped system toward PKS~0528--250

We present a detailed analysis of a high resolution spectrum of the damped Ly$α$ system at $z_{\rm abs}$~=~2.8112toward PKS~0528-250. The absorption redshift is slightly larger than the emission redshift of the quasar. We estimate the column density of H$_2$ molecules $N$(H$_2$)~$\sim$~6$\times$10$^{16}$~cm$^{-2}$ and the fractional abundance of H$_2$, $f$~=~5.4$\times$10$^{-5}$. The excitation temperature derived for different transitions suggests that the kinetic temperature of the cloud is $\sim$200~K and the density $n$~$\sim$~1000~cm$^{-3}$. The cloud therefore has a dimension of $\sim$1~pc along the line of sight. Since it obscures the broad-line emission region, its transverse dimension should be larger than 10~pc.\par We obtain upper limits on the column densities of C~{\sc i} ($<$~10$^{12.7}$~cm$^{-2}$) and CO ($<$~10$^{13.2}$~cm$^{-2}$; $N$(CO)/$N$(H~{\sc i})~$<$~7$\times$10$^{-9}$).We suggest that the ratio $N$(H$_2$)/$N$(C~{\sc i}) is a useful indicator of the physical conditions in the absorber. Simple photo-ionization models assuming solar relative abundances show that radiation fields with spectra similar to typical AGNs or starbursts are unable to reproduce all the constraints and in particular the surprisingly small $N$(C~{\sc i})/ $N$(H$_2$) and $N$(Mg~{\sc i})/$N$(H$_2$) ratios. In view of the models we explored, the most likely ionizing spectrum is a composite of a UV- "big bump" possibly produced by a local starburst and a power-law spectrum from the QSO that provides the X-rays. Dust is needed to explain the production of molecules in the cloud. The amount of dust is broadly consistent with the [Cr/Zn] abundance determination.

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arXiv1998

Do the central engines of quasars evolve by accretion ?

According to a currently popular paradigm, nuclear activity in quasars is sustained via accretion of material onto super-massive black holes located at the quasar nuclei. A useful tracer of the gravitational field in the vicinity of such central black holes is available in the form of extremely dense gas clouds within the broad emission-line region (BLR) on the scale of $\sim 1~$parsec. Likewise, the radio sizes of the lobe-dominated radio sources are believed to provide a useful statistical indicator of their ages. Using two homogeneously observed (and processed) sets of lobe-dominated radio-loud quasars, taken from literature, we show that a positive correlation exists between the radio sizes of the quasars and the widths of their broad $Hβ$ emission lines, and this correlation is found to be significantly stronger than the other well known correlations involving radio size. This statistical correlation is shown to be consistent with the largest (and, hence, very possibly the oldest) radio sources harboring typically an order-of-magnitude more massive central engines, as compared to the physically smaller and, hence, probably much younger radio sources. This inference is basically in accord with the "accreting central engine" picture for the radio-loud quasars.

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arXiv1996

Lyman Alpha Forest towards B2 1225+317

We present observations of the Lyman alpha forest towards B2 1225+317 taken at a resolution of 18 km/s. A clean sample of Lyman alpha forest lines is extracted after a careful profile fitting analysis and removal of absorption lines of heavy elements. The sample is analyzed for statistical properties. Eighty percent of the column densities are < 10^{14} cm^{-2}. A single power law is inconsistent with the column density distribution and a steepening/break in the distribution is indicated. The average velocity dispersion parameter is 29.4 km/s. We find 3 sigma evidence for a correlation between column density and the velocity dispersion parameter. The correlation, however, is mainly due to narrow lines and weakens to 1.2 sigma if lines with velocity dispersion parameter smaller than 20 km/s are excluded. An excess of line pairs with velocity separation $\le$ 100 km/s over the expected number is found.

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arXiv1996

Radiation Induced Void in the Spectrum of TOL 1038-2712

Detection of a large void ($\sim $7 Mpc) is reported between the redshifts 2.16286 and 2.20748 in the Ly~$α$ forest of TOL 1038-2712. This void is centered near a foreground QSO TOL 1037-2704 which is at a distance $\sim$4.4 Mpc away from the void. The estimated probability for the void to occure by chance in front of the foreground QSO is few times $10^{-3}$. Various implications of the void being produced by excess ionization due to foreground QSO are discussed.

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arXiv1995

Dynamical History of Ly αclouds

The clustering properties of Ly $α$ lines are analysed using an intermediate resolution ($\sim 1Å$) spectra of 67 QSOs compiled from the literature. The pair velocity correlation function indicates a weak excess in the velocity intervals up to ${\rm Δv\;=\;600\; km\;s^{-1}}$. The z-integrated probability distribution of interline spacings also confirms the excess in the low velocity intervals. The dependence of pair velocity correlation on redshift and equivalent width are investigated. The cross-correlation properties of Ly $α$ clouds with metal line redshifts are analysed. We do not find any tendency of Ly $α$ lines to cluster around metal line systems. Depedence of the cross-correlation on redshift and equivalent widths of Ly $α$ clouds are investigated. Various implications of the results are discussed.

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arXiv1995

Analysis of Low z Absorbers in the QSO Spectra

We present the results of reanalysis of low z Mg II absorption line sample compiled by SS92. The conditions on cloud parameters to produce N(Fe II)>N(Mg II) are obtained using photoionization models and curve of growth. Properties of Mg II absorbers with W(Fe II)/W(MgII) (definded as R) >and< 0.5 are analysed separately. Contrary to the whole Mg II sample, the clouds with R<0.5 show a steep increase in number density (N(z)) with z. These systems also show clear increase in W(Mg II) and doublet ratio(dr) of Mg II with z. However there is no correlation between W(Mg II) and dr. In the case of R>0.5 clouds W(Mg II) and dr are not correlated with z. However there is a clear anticorrelation between dr and W(Mg II). We find a clear decrease in the ratio of W(Fe II) and W(Mg II) with z. The N(z) as well as average N(Fe II) of Fe II lines selected absorbers are not evolving with z. The available data of LLS, are not showing any dependence of (tau_{LLS}) on z. We collected the LLS information for 53 QSO sight lines, for which details of Mg II absorption are available. There are 4 Mg II absorption systems which are not LLS at z which are lower than the mean z of the sample (z=1.1). In the higher z, where one expect to see 2.5\pm1.4 such absorbers, we do not find any nonLLS Mg II absorbers. Individual systems with tau_{LLS}<3.0 are analysed. Our results imply some of the absorbers at z=0.6 have reached metallicity roughly around solar value. The required ionization parameters for these systems are less than 0.001 in most cases. Our results together with results obtained for high z absorbers confirm that mean ionization state of metal rich absorbing clouds falls with z.

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