Browsing by Author "Srianand, R."

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21-cm absorbers at intermediate redshifts
(2009-02-01) Gupta, N.; Srianand, R.; Petitjean, Patrick; et al.
Damped Lyman-α systems (DLAs) seen in the spectra of high-z QSOs allow us to probe the physical conditions in protogalaxies. Our understanding of physical conditions in DLAs at high-z is primarily based on the absorption lines of H2 molecules and ﬁne-structure transitions. Another important way of probing the thermal state of interstellar medium in these systems is by studying the 21-cm absorption in the spectra of background quasars. Here we report the main results of our GMRT survey to search for 21-cm absorption in a representative and unbiased sample of 35 DLA candidates at 1.10≤z≤1.45. Our sample of DLA candidates is drawn from the strong Mg ii systems in SDSS DR5 and has resulted in discovery of 9 new 21-cm absorbers. Prior to our survey only one 21-cm absorber was known in the redshift range: 0.7≤z≤2. This survey has allowed us to investigate the dependence of detectability of 21-cm absorption on the properties of UV absorption lines detected in SDSS spectra and estimate the number per unit redshift of 21-cm absorbers. Our GMRT survey provides a representative sample of systems that can be used in combination with various follow-up observations: (1) for investigating the physical conditions in the absorbing gas using spin temperature (TS) measurements, (2) for investigating the eﬀect of metallicity and dust content on the detectability of 21-cm absorption, (3) for studying the morphology of the absorbing gas and (4) for probing the time evolution of various fundamental constants. Results from the ﬁrst phase of our survey are presented in Gupta et al. (2007). Detailed description of the entire sample and results from the survey are presented in Gupta et al. (2009).
Analysis of low z absorbers in the QSO spectra
(2015-02-07) Srianand, R.
Analysis of Ly α absorption lines in the vicinity of QSOS
(2015-02-07) Srianand, R.; Khare, Pushpa
Collimated flow driven by radiative pressure from the nucleus of quasar Q 1511 + 091
(2011-07-06) Srianand, R.; Petitjean, Patrick; Ledoux, C.; et al.
High velocity outﬂows from quasars are revealed by the absorption signatures they produce in the spectrum of the quasar. Clues on the nature and origin of these ﬂows are important for our understanding of the dynamics of gas in the central regions of the Active Galactic Nucleus (AGNs) but also of the metal enrichment of the intergalactic space. Line radiation pressure has often been suggested to be an important process in driving these outﬂows, however no convincing evidence has been given so far. Here we report observation of a highly structured ﬂow, toward Q 1511+091, where the velocity separations between distinct components are similar to O vi, N v and C iv doublet splittings with some of the proﬁles matching perfectly. This strongly favors the idea that the absorbing clumps originate at similar physical location and are driven by radiative acceleration due to resonance lines. The complex absorption can be understood if the ﬂow is highly collimated so that the diﬀerent optically thick clouds are aligned and cover the same region of the background source. One component shows saturated H i Lyman series lines together with absorptions from excited levels from C ii and Si ii but covers only 40% of the source of continuum. The fact that clouds cover only part of the small continuum source implies that the ﬂow is located very close to it.
Complete sample of 21-cm absorbers at z ~ 1.3: Giant Metrewave Radio Telescope Survey Using Mg II Systems
(2009-04-01) Gupta, N.; Srianand, R.; Petitjean, Patrick; et al.
We present the results of a systematic Giant Metrewave Radio Telescope (GMRT) survey of 21-cm absorption in a representative and unbiased sample of 35 strong Mg ii systems in the redshift range: zabs∼1.10−1.45, 33 of which have Wr ≥1˚ A. The survey using ∼400 hrs of telescope time has resulted in 9 new 21-cm detections and stringent 21-cm optical depth upper limits (median 3σ optical depth per 10 kms−1 of 0.017) for the remaining 26 systems. This is by far the largest number of 21-cm detections from any single survey of intervening absorbers. Prior to our survey no intervening 21-cm system was known in the above redshift range and only one system was known in the redshift range 0.7 ≤ z ≤ 1.5. We discuss the relation between the detectability of 21-cm absorption and various properties of UV absorption lines. We show that if Mg ii systems are selected with the following criteria, Mg ii doublet ratio ≤1.3 and Wr(Mg i)/Wr(Mg ii) ≥ 0.3, then a detection rate of 21-cm absorption up to 90% can be achieved. We estimate n21, the number per unit redshift of 21-cm absorbers with Wr(Mg ii) > Wo and integrated optical depth T21 > To and show that n21 decreases with increasing redshift. In particular, for Wo = 1.0 ˚ A and To > 0.3 km s−1, n21 falls by a factor 4 from < z > = 0.5 to < z > = 1.3. The evolution seems to be stronger for stronger Mg ii systems. Using a subsample of systems for which high frequency VLBA images are available, we show that the eﬀect is not related to the structure of the background radio sources and is most probably due to the evolution of the cold neutral medium ﬁlling factor in Mg ii systems. We ﬁnd no correlation between the velocity spread of the 21-cm absorption feature and Wr(Mg ii) at z ∼ 1.3
Constrained semi-analytical models of Galactic outflows
(2008-01) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.
We present semi-analytic models of galactic outﬂows that are constrained by available ob- servations 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 . 109 M ) for a wide range of model parameters while this is not the case in high mass halos (M & 1011 M ). The ﬂow generically accelerates within the halo virial radius, then starts to decelerate, and traverses well into the intergalactic medium (IGM), before freezing to the Hubble ﬂow. The acceleration phase can result in shell fragmentation due to the Rayleigh-Taylor instability, although the ﬁnal outﬂow radius is not signiﬁcantly altered. The gas phase metallicity of the outﬂow and within the galaxy are computed assuming uniform instantaneous mixing. 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 using porosity weighted averages and probability density functions of various physical quantities. Models with only atomic cooled halos signiﬁcantly ﬁll the IGM at z 3 with metals (with 2:5 & [Z=Z ] & 3:7), the actual extent depending on the efﬁciency of winds, the initial mass function (IMF) and the fractional mass that goes through star formation. The reionization history has a signiﬁcant effect on the volume ﬁlling factor, due to radiative feedback. In these models, a large fraction of outﬂows at z 3 are supersonic, hot (T 105K) 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- 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 ] 3:6). The order unity ﬂuctuations at z 8 that becomes the mildly non-linear ﬂuctuations traced by Lyman- forest at z < 4 will then have this metallicity. 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.
Cosmic microwave background temperature at a redshift of 2.33771
(2000-12-18) Srianand, R.; Petitjean, Patrick; Ledoux, C.
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, TCMBR,0 = 2.726±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 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 < TCMBR < 14 K at z = 2.33771 when 9.1 K is expected in the Hot Big Bang cosmology.
Cosmic ray driven outflows from high redshift galaxies
(2010-01-10) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.
We study winds in high redshift galaxies driven by a relativistic cosmic ray (proton) component in addition to the hot thermal gas component. Cosmic rays (CRs) are likely to be efﬁciently generated in supernova(SNe)shocks inside galaxies. We obtain solutions of such CR driven free winds in a gravitational potential of the Navarro-Frenk-White (NFW) form, relevant to galaxies. Cosmic rays naturally provide the extra energy and/or momentum input to the system, needed for a transonic wind solution in a gas with adiabatic index = 5=3.We show that cosmic rays can effectively drive winds even when the thermal energy of the gas is lost due to radiative cooling. These wind solutions predict an asymptotic wind speed closely related to the circular velocity of the galaxy. Furthermore, the mass outﬂow rate per unit star formation rate ( w) is predicted to be ~ 0:2 0:5 for massive galaxies, with masses M ~ 1011 1012M .We show w to be inversely proportional to the square of the circular velocity. Magnetic ﬁelds at the G levels are also required in these galaxies to have a signiﬁcant mass loss. A large w for small mass galaxies implies that cosmic ray driven outﬂows could provide a strong negative feedback to the star formation in dwarf galaxies. Further, our results will also have important implications to the metal enrichment of the intergalactic medium. These conclusions are applicable to the class of free wind models where the source region is conﬁned to be within the sonic point.
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.
Detecting cold gas at intermediate redshifts: GMRT survey using MgII systems
(2006-11-28) Gupta, Neeraj; Srianand, R.; Petitjean, Patrick; et al.
Intervening H i 21-cm absorption systems at z ≥ 1.0 are very rare and only 4 conﬁrmed detections have been reported in the literature. Despite their scarcity, they provide interesting and unique insights into the physical conditions in the interstellar medium of high-z galaxies. Moreover, they can provide independent constraints on the variation of fundamental constants. We report 3 new detections based on our ongoing Giant Metrewave Radio Telescope (GMRT) survey for 21-cm absorbers at 1.10 ≤ zabs ≤ 1.45 from candidate damped Lyman-α systems. The 21-cm lines are narrow for the zabs= 1.3710 system towards SDSS J0108−0037 and zabs= 1.1726 system toward SDSS J2358−1020. Based on line full-width at half maximum, the kinetic temperatures are ≤ 5200 K and ≤ 800 K, respectively. The 21-cm absorption proﬁle of the third system, zabs=1.1908 system towards SDSS J0804+3012, is shallow, broad and complex, extending up to 100 km s−ᶥ . The centroids of the 21-cm lines are found to be shifted with respect to the corresponding centroids of the metal lines derived from SDSS spectra. This may mean that the 21-cm absorption is not associated with the strongest metal line component.
Detection of 21-cm, H2 and Deuterium absorption at z>3 along the line-of-sight to J1337+3152
(2010-02-24) Srianand, R.; Gupta, Neeraj; Petitjean, Patrick; et al.
We report the detection of 21-cm and molecular hydrogen absorption lines in the same damped Lyman-α system (with log N(H i)=21.36±0.10) at zabs=3.17447 towards SDSS J133724.69+315254.55 (zem ∼3.174). We estimate the spin temperature of the gas to be, TS = 600+222−159 K, intermediate between the expected values for cold and warm neutral media. This suggests that the H i absorption originates from a mixture of diﬀerent phases. The total molecular fraction is low, fH2=10−7, and H2 rotational level populations are not in equilibrium. The average abundance of the α- elements is, [S/H]=−1.45 ± 0.22. Nitrogen and iron are found underabundant with respect to α-elements by ∼1.0 dex and ∼0.5 dex respectively. Using photoionization models we conclude that the gas, of mean density, nH ∼2 cm−3, is located more than 270 kpc away from the QSO. While the position of 21-cm absorption line coincides with the H2 velocity proﬁle, its centroid is shifted by ∼2.7±1.0 km s−1 with respect to the redshift measured from the H2 lines. However, the position of the strongest metal absorption component matches the position of the 21-cm absorption line within 0.5 km s−1. From this, we constrain the variation of the combination of fundamental constants x = α2Gp/µ, ∆x/x = −(1.7±1.7)×10−6. This system is unique as we can at the same time have an independent constrain on µ using H2 lines. However, as the H2 column density is low, only Werner band absorption lines are seen and, unfortunately, the range of sensitivity coeﬃcients is too narrow to provide a stringent constraint: ∆µ/µ ≤ 4.0 × 10−4. The Ultraviolet and Visual Echelle Spectrograph (UVES) spectrum reveals another DLA at zabs= 3.16768 with log N(H i) = 20.41±0.15 and low metallicity, [Si/H] = −2.68 ± 0.11, in which [O/C] ∼ 0.18 ± 0.18 and [O/Si] ∼ 0. This shows that even in the very early stages of chemical evolution, the carbon or silicon to oxygen ratios can be close to solar. Using Voigt proﬁle ﬁtting we derive log(N(D i)/N(H i)) = −(4.93 ± 0.15) in this system. This is a factor of two smaller than the value expected from the best ﬁtted value of Ωb from theWilkinson Microwave Anisotropy Probe (WMAP) 5 year data. This conﬁrms the presence of astration of deuterium even at very low metallicity.
Detection of molecular hydrogen in a near solar-metallicity damped Lyman - alpha system at Zabc ~ 2 toward Q 0551 - 366 Authors: Ledoux, C.
(2002-05-01) Ledoux, C.; Srianand, R.; Petitjean, Patrick
We report the detection of H2, C I, C 1*, C I ** and Cl I lines in a near Solar-metallicity ([Zn/H] = -0.13) damped Lyman-a (DLA) system at Zabs = 1.962 observed on the line of sight to the quasar Q 0551-366. 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 Hs-bearing clouds, spanning 55 km S-l in velocity space, we derive a total molecular hydrogen column density N(H2) = 2.6 X 1017 cm- 2 and a mean molecular fraction f = 2N(H2)/(2N(H2) + 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 H2. The gas clouds in which H2 is detected always have large densities, nH > 30 cm- 3 , and low temperatures, T01 ~ 100 K. This shows that presence of dust, high particle density and/or low temperature are required for molecules to be present. The photo-dissociation rate derived in the components where H2 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 H2-bearing clouds.
Detection of the 2175\AA~ extinction feature and 21-cm absorption in two MgII systems at z~1.3
(2008-09) Srianand, R.
We have discovered two dusty intervening Mg ii absorption systems at z ∼ 1.3 in the Sloan Digital Sky Survey (SDSS) database. The overall spectra of both QSOs are red (u-K>4.5 mag) and are well modelled by the composite QSO spectrum reddened by the extinction curve from the Large Magellanic Cloud(LMC2) Supershell redshifted to the rest-frame of the Mg ii systems. In particular, we detect clearly the presence of the UV extinction bump at λrest ∼ 2175 ˚ A. Absorption lines of weak transitions like Si iiλ1808, Cr iiλ2056, Cr ii+Zn iiλ2062, Mn iiλ2594, Ca iiλ3934 and Ti iiλ1910 from these systems are detected even in the low signal-to-noise ratio and low resolution SDSS spectra, suggesting high column densities of these species. The depletion pattern inferred from these absorption lines is consistent with that seen in the cold neutral medium of the LMC. Using the LMC AV vs. N(H i) relationship we derive N(H i)∼ 6×1021 cm−2 in both systems. Metallicities are close to solar. Giant Metrewave Radio Telescope (GMRT) observations of these two relatively weak radio loud QSOs (fν ∼ 50 mJy) resulted in the detection of 21-cm absorption in both cases.We show that the spin temperature of the gas is of the order of or smaller than 500 K. These systems provide a unique opportunity to search for molecules and diﬀuse interstellar bands at z > 1.
Diffuse molecular gas at high redshift: Detection of CO molecules and the 2175 A* dust feature at z=1.64
(2009-06-01) Noterdaeme, P.; Ledoux, C.; Srianand, R.
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. CO absorption is detected in at least two components in the ﬁrst six A-X bands and one d-X(5-0) inter-band system. This is the second detection of this kind along a quasar line of sight. The CO absorption proﬁles are well modelled assuming a 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 × 1014 cm−2 . The measured column densities of S i, Mg i, Zn ii, Fe ii and Si ii 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 presents a pronounced 2175 Å dust absorption feature at the redshift of the CO absorber. Using a control sample of ∼500 quasars we ﬁnd the chance probability for this feature to be spurious is ∼0.3%. We show that the spectral energy distribution (SED) of the quasar is well ﬁtted with 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 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 physical conditions and chemistry of diﬀuse molecular gas in high redshift galaxies.
Diffuse molecular gas at high redshift: Detection of CO molecules and the 2175 A* dust feature at z=1.64
(2009-06-01) Noterdaeme, P.; Ledoux, C.; Srianand, R.; et al.
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. CO absorption is detected in at least two components in the ﬁrst six A-X bands and one d-X(5-0) inter-band system. This is the second detection of this kind along a quasar line of sight. The CO absorption proﬁles are well modelled assuming a 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 × 1014 cm−2 . The measured column densities of S i, Mg i, Zn ii, Fe ii and Si ii 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 presents a pronounced 2175 Å dust absorption feature at the redshift of the CO absorber. Using a control sample of ∼500 quasars we ﬁnd the chance probability for this feature to be spurious is ∼0.3%. We show that the spectral energy distribution (SED) of the quasar is well ﬁtted with 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 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 physical co
Do the central engines of quasars evolve by accretion
(2015-03-11) Srianand, R.; Krishna-Gopal
Dust and molecules at high-z
(2011-11-22) Srianand, R.
Dynamical history of Ly α clouds
(2015-02-07) Srianand, R.
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.
First detection of CO in a high-redshift damped Lyman - Alpha system
(2008-04) Srianand, R.
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, H2, and 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, is 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.