arXiv subjects

R. Srianand

Publications and source records attributed to R. Srianand.

At least 37 records · Page 2Linked to original sources

arXiv2021

Estimation of the CMB temperature from atomic C\,{\sc i} and molecular CO lines in the interstellar medium of early galaxies

The linear increase of the cosmic microwave background (CMB) temperature with cosmological redshift, $T_{\rm CMB} = T_0(1 + z)$, is a prediction of the standard cosmological $Λ$CDM model. There are currently two methods to measure this dependence at redshift $z>0$, and that is equally important to estimate the CMB temperature $T_0$ at the present epoch $z=0$. The first method is based on the Sunyaev-Zeldovich (SZ) effect for a galaxy cluster. aThe second method is based on the analysis of the populations of atomic and molecular energy levels observed in the absorption spectra of quasars. This method allows $T_{\rm CMB}(z)$ to be measured directly. We present new estimates of $T_{\rm CMB}(z_i)$ in the redshift range $1.7\le z_i \le3.3$ based on the analysis of excitation of the CO rotational levels and C\,{\sc i} fine-structure levels in 15 absorption systems. We take into account collisional excitation of CO and C\,{\sc i} with hydrogen atoms and H$_2$ and radiative pumping of C\,{\sc i} by the interstellar ultraviolet radiation. Applying this corrections leads to a systematic decrease in the previously obtained estimates of $T_{\rm CMB}(z_i)$ (for some systems the magnitude of the effect is $\sim$10\%). Combining our measurements with the measurements of $T_{\rm CMB}(z)$ in galaxy clusters we have obtained a constraint on the parameter $β=+0.010\pm0.013$, which characterizes the deviation of the CMB temperature from the standard relation, $T_{\rm CMB} = T_0(1 + z)^{1-β}$, and an independent estimate of the CMB temperature at the present epoch, $T_0 = 2.719\pm0.009$\,K, which agrees well with the estimate from orbital measurements, $T_0 = 2.7255\pm0.0006$\,K. This independent estimate is very important because it was obtained using cosmological data, in contrast to satellite measurements, which are obtained "here" and "now".

astro-ph.CO↗

arXiv2021

Evolution of cold gas at 2<z<5: a blind search for HI and OH absorption lines towards mid-infrared color selected radio-loud AGNs

We present results from a spectroscopically blind search for associated and intervening HI 21-cm and OH 18-cm absorption lines towards 88 AGNs at $2\le z\le5$ using the upgraded Giant Metrewave Radio Telescope (uGMRT). The sample of AGNs with 1.4 GHz spectral luminosity in the range, $10^{27 - 29.3}$ W/Hz, is selected using mid-infrared colors and closely resembles the distribution of the underlying quasar population. The search for associated or proximate absorption, defined to be within 3000 km/s of the AGN redshift, led to one HI 21-cm absorption detection (M1540-1453; $z_{abs}$= 2.1139). This is only the fourth known absorption at $z>2$. The detection rate ($1.6^{+3.8}_{-1.4}$%) suggests low covering factor of cold neutral medium (CNM; T$\sim$100 K) associated with these powerful AGNs. The intervening absorption line search, with a sensitivity to detect CNM in damped Ly$α$ systems (DLAs), has comoving absorption path lengths of $Δ$X = 130.1 and 167.7 for HI and OH, respectively. The corresponding number of absorber per unit comoving path lengths are $\le$0.014 and $\le$0.011, respectively. The former is at least 4.5 times lower than that of DLAs and consistent with the CNM cross-section estimated using H$_2$ and CI absorbers at $z>2$. Our AGN sample is optically fainter compared to the quasars used to search for DLAs in the past. In our optical spectra obtained using the Southern African Large Telescope (SALT) and the Nordic Optical Telescope (NOT), we detect 5 intervening (redshift path$\sim9.3$) and 2 proximate DLAs. This is slightly excessive compared to the statistics based on optically selected quasars. The non-detection of HI 21-cm absorption from these DLAs suggests small CNM covering fraction around galaxies at $z>2$.

astro-ph.GA↗

arXiv2021

The distribution of HI velocity profiles in a $Λ$CDM universe

We model the distribution of the observed profiles of 21 cm line emission from neutral hydrogen (HI) in central galaxies selected from a statistically representative mock catalog of the local Universe in the Lambda-cold dark matter framework. The distribution of these HI velocity profiles (specifically, their widths $W_{50}$) has been observationally constrained, but has not been systematically studied theoretically. Our model profiles derive from rotation curves of realistically baryonified haloes in an N-body simulation, including the quasi-adiabatic relaxation of the dark matter profile of each halo in response to its baryons. We study the predicted $W_{50}$ distribution using a realistic pipeline applied to noisy profiles extracted from our luminosity-complete mock catalog with an ALFALFA-like survey geometry and redshift selection. Our default mock is in good agreement with observed ALFALFA results for $W_{50}\gtrsim700$ km/s, being incomplete at lower widths due to the intrinsic threshold of $M_r\leq-19$. Variations around the default model show that the velocity width function at $W_{50}\gtrsim300$ km/s is most sensitive to a possible correlation between galaxy inclination and host concentration, followed by the physics of quasi-adiabatic relaxation. We also study the excess kurtosis of noiseless velocity profiles, obtaining a distribution which tightly correlates with $W_{50}$, with a shape and scatter that depend on the properties of the turbulent HI disk. Our results open the door towards using the shapes of HI velocity profiles as a novel statistical probe of the baryon-dark matter connection.

astro-ph.GA↗

arXiv2021

Remarkably high mass and high velocity dispersion of molecular gas associated with a regular, absorption-selected type-I quasar

We present 3-mm observations of the quasar J0015+1842 at z=2.63 with the NOrthern Extended Millimeter Array (NOEMA). Our data reveals molecular gas, traced via a Gaussian CO(3-2) line, with a remarkably large velocity dispersion (FWHM=1010+/-120 km/s) and corresponding to a total molecular mass MH2~(3.4-17)x10^10 Msun, depending on the adopted CO-to-H2 conversion factor alphaCO=(0.8-4.0) Msun (km/s pc^2)^-1. Assuming the 3-mm continuum emission is thermal, we derive a dust mass of the order of Mdust ~5x10^8 Msun. J0015+1842 is located in the molecular gas-rich region in the IR vs CO line luminosity diagram, in-between the main locus of main-sequence and sub-millimetre galaxies and that of most other AGNs targeted so far for CO measurements. While the large velocity dispersion of the CO line suggests a merging system, J0015+1842 is observed to be a regular, only very moderately dust-reddened (Av~0.3-0.4) type-I quasar from its UV-optical spectrum, from which we infer a mass of the super-massive black hole be around MBH~6x10^8 Msun. We suggest that J0015+1842 is observed at a galaxy evolutionary stage where a massive merger has brought significant amounts of gas towards an actively accreting super-massive black hole (quasar). While the host still contains a large amount of dust and molecular gas with high velocity dispersion, the quasar has already cleared the way towards the observer, likely through powerful outflows as recently revealed by optical observations of the same object. High angular resolution observations of this and similar systems} should help determining better the respective importance of evolution and orientation in the appearance of quasars and their host galaxies and have the potential to investigate early feedback and star-formation processes in galaxies in their quasar phases.

astro-ph.GA↗

arXiv2021

Correlated time variability of multi-component high velocity outflows in J162122.54+075808.4

We present a detailed analysis of time variability of two distinct C IV broad absorption line (BAL) components seen in the spectrum of J162122.54+075808.4 ($z_{em}$ = 2.1394) using observations from SDSS, NTT and SALT taken at seven different epochs spanning about 15 years. The blue-BAL component (with an ejection velocity, $v_{\rm e}\sim37,500$ kms$^{-1}$) is an emerging absorption that shows equivalent width variations and kinematic shifts consistent with acceleration. The red-BAL component ($v_{\rm e} \sim 15,400$ kms$^{-1}$) is a three component absorption. One of the components is emerging and subsequently disappearing. The two other components show kinematic shifts consistent with acceleration coupled with equivalent width variability. Interestingly, we find the kinematic shifts and equivalent width variability of the blue- and red-BAL components to be correlated. While the C IV emission line flux varies by more than 17% during our monitoring period, the available light-curves (covering rest frame 1300-2300 angstrom do not show more than a 0.1 mag variability in the continuum. This suggests that the variations in the ionizing flux are larger than that of the near-UV flux. However, the correlated variability seen between different BAL components cannot be explained solely by photoionization models without structural changes. In the framework of disk wind models, any changes in the radial profiles of density and/or velocity triggered either by disk instabilities or by changes in the ionizing radiation can explain our observations. High resolution spectroscopic monitoring of J1621+0758 is important to understand the physical conditions of the absorbing gas and thereby to constrain the parameters of disk-wind models.

astro-ph.GA↗

arXiv2021

HD molecules at high redshift: cosmic-ray ionization rate in the diffuse interstellar medium

We present a systematic study of deuterated molecular hydrogen (HD) at high redshift, detected in absorption in the spectra of quasars. We present four new identifications of HD lines associated with known $\rm H_2$-bearing Damped Lyman-$α$ systems. In addition, we measure upper limits on the $\rm HD$ column density in twelve recently identified $\rm H_2$-bearing DLAs. We find that the new $\rm HD$ detections have similar $N({\rm HD})/N(\rm H_2)$ ratios as previously found, further strengthening a marked difference with measurements through the Galaxy. This is likely due to differences in physical conditions and metallicity between the local and the high-redshift interstellar media. Using the measured $N({\rm HD})/N({\rm H_2})$ ratios together with priors on the UV flux ($χ$) and number densities ($n$), obtained from analysis of $\rm H_2$ and associated CI lines, we are able to constrain the cosmic-ray ionization rate (CRIR, $ζ$) for the new $\rm HD$ detections and for eight known HD-bearing systems where priors on $n$ and $χ$ are available. We find significant dispersion in $ζ$, from a few $\times 10^{-18}$ s$^{-1}$ to a few $\times 10^{-15}$ s$^{-1}$. We also find that $ζ$ strongly correlates with $χ$ -- showing almost quadratic dependence, slightly correlates with $Z$, and does not correlate with $n$, which probably reflects a physical connection between cosmic rays and star-forming regions.

astro-ph.GA↗

arXiv2021

PKS1830-211: OH and HI at z=0.89 and the first MeerKAT UHF spectrum

The Large Survey Project (LSP) "MeerKAT Absorption Line Survey" (MALS) is a blind HI 21-cm and OH 18-cm absorption line survey in the L- and UHF-bands, with the primary goal to better determine the occurrence of atomic and molecular gas in the circum-galactic and inter-galactic medium, and its redshift evolution. Here we present the first results using the UHF-band, obtained towards the strongly lensed radio source PKS1830, detecting absorption in the lens galaxy. With merely 90min of data acquired on-source for science verification and processed using the Automated Radio Telescope Imaging Pipeline (ARTIP), we detect in absorption the known HI 21-cm and OH 18-cm main lines at z=0.89 at an unprecedented signal-to-noise ratio (4000 in the continuum, with 6km/s channels). For the first time we report the detection at z=0.89 of OH satellite lines, so far not detected at z $>$ 0.25. We decompose the OH lines into a thermal and a stimulated contribution, where the 1612 and 1720MHz lines are conjugate. The total OH 1720MHz emission line luminosity is 6100Lsun. This is the most luminous known 1720MHz maser line. The absorption components of the different images of the background source sample different light paths in the lensing galaxy, and their weights in the total absorption spectrum are expected to vary in time, on daily and monthly time scales. We compare our normalized spectra with those obtained more than 20 yrs ago, and find no variation. We interpret the absorption spectra with the help of a lens galaxy model, derived from an N-body hydro-dynamical simulation, with a morphology similar to its optical HST image. It is possible to reproduce the observations without invoking any central gas outflows. There are, however, distinct and faint high-velocity features, most likely high-velocity clouds. These clouds may contribute to broaden the HI and OH spectra.

astro-ph.GA↗

arXiv2021

Down-the-barrel observations of a multiphase quasar outflow at high redshift: VLT/X-shooter spectroscopy of the proximate molecular absorber at z=2.631 towards SDSS J001514+184212

We present UV to NIR spectroscopic observations of the quasar J0015+1842 and its proximate molecular absorber at z=2.631. The [OIII] emission line of the quasar is composed of a broad (FWHM~1600 km/s), spatially-unresolved component, blueshifted by ~600 km/s from a narrow, spatially-resolved component (FWHM~650 km/s). The wide, blueshifted, unresolved component is consistent with the presence of outflowing gas in the nuclear region. The narrow component can be further decomposed into a blue and a red blob with velocity width of several hundred km/s each, seen ~5 pkpc on opposite spatial locations from the nuclear emission, indicating outflows over galactic scales. The presence of ionised gas over kpc-scales is also seen from a weak CIV emission component, detected in the trough of a saturated CIV absorption that removes the strong nuclear emission from the quasar. Towards the nuclear emission, we observe absorption lines from atomic species in various ionisation and excitation stages and confirm the presence of strong H2 lines. The overall absorption profile is very wide, spread over ~600 km/s, roughly matching in velocities the blue narrow [OIII] blob. From detailed investigation of the chemical and physical conditions in the absorbing gas, we infer densities of about nH ~ 10^4-10^5 cm^-3 in the cold (T~100 K) H2-bearing gas, which we find to be located at ~10 kpc distances from the central UV source. We conjecture that we are witnessing different manifestations of a same AGN-driven multi-phase outflow, where approaching gas is intercepted by the line of sight to the nucleus. We corroborate this picture by modelling the scattering of Ly-a photons from the central source through the outflowing gas, reproducing the peculiar Ly-a absorption-emission profile, with a damped Ly-a absorption in which red-peaked, spatially offset and extended Ly-a emission is seen. [abridged]

astro-ph.GA↗

arXiv2020

Physical conditions and redshift evolution of optically thin C III absorbers: Low-z sample

We present a detailed analysis of 99 optically thin C III absorption systems at redshift, $0.2 \le z \le 0.9$ associated with neutral hydrogen column densities in the range, $15 \le {\rm log}$ $N_{\rm H\,I}$ ($cm^{-2}$) $\le 16.2$. Using photoionization models, we infer the number density ($n_{\rm H}$), C-abundance ($[C/H]$) and line-of-sight thickness ($L$) of these systems in the ranges, $-3.4 \le$ log $n_{\rm H}$ (in $cm^{-3}$) $\le -1.6$, $-1.6 \le [C/H] \le 0.4$, and 1.3 pc $\le L \le$ 10 kpc, respectively with most of the systems having sub-kpc scale thickness. We combine the low$-z$ and previously reported high$-z$ ($2.1\le z\le 3.3$) optically thin C III systems to study the redshift evolution and various correlation between the derived physical parameters. We see a significant redshift evolution in $n_{\rm H}$, $[C/H]$ and $L$. We compare the redshift evolution of metallicity in C III systems with those of various types of absorption systems. We find that the slope of $[C/H]$ vs. $z$ for C III absorbers is stepper compared to the redshift evolution of cosmic metallicity of the damped \lya\ sample (DLAs) but consistent with that of sub$-$DLAs. We find the existence of strong anti-correlation between $L$ vs. $[C/H]$ for the combined sample with a significance level of 8.39$σ$. We see evidence of two distinct $[C/H]$ branch C III populations (low$-[C/H]$ branch, $[C/H]$ $\le -1.2$ and high$-[C/H]$ branch, $[C/H]$ $> -1.2$) in the combined C III sample when divided appropriately in the $L$ vs. $N_{\rm C\,III}$ plane. Further studies of C III absorbers in the redshift range, $1.0 \le z \le 2.0$ is important to map the redshift evolution of these absorbers and gain insights into the time evolution physical conditions of the circumgalactic medium.

astro-ph.GA↗

arXiv2020

Blind HI and OH absorption line search: first results with MALS and uGMRT processed using ARTIP

We present details of the Automated Radio Telescope Imaging Pipeline (ARTIP) and results of a sensitive blind search for HI and OH absorbers at $z<0.4$ and $z<0.7$, respectively. ARTIP is written in Python 3.6, extensively uses the Common Astronomy Software Application (CASA) tools and tasks, and is designed to enable the geographically-distributed MeerKAT Absorption Line Survey (MALS) team to collaboratively process large volumes of radio interferometric data. We apply it to the first MALS dataset obtained using the 64-dish MeerKAT radio telescope and 32K channel mode of the correlator. With merely 40 minutes on target, we present the most sensitive spectrum of PKS1830-211 ever obtained and characterize the known HI ($z=0.19$) and OH ($z=0.89$) absorbers. We further demonstrate ARTIP's capabilities to handle realistic observing scenarios by applying it to a sample of 72 bright radio sources observed with the upgraded Giant Metrewave Radio Telescope (uGMRT) to blindly search for HI and OH absorbers. We estimate the numbers of HI and OH absorbers per unit redshift to be $n_{21}(z\sim0.18)<$0.14 and $n_{\rm OH}(z\sim0.40)<$0.12, respectively, and constrain the cold gas covering factor of galaxies at large impact parameters (50 kpc $<ρ<$ 150 kpc) to be less than 0.022. Due to the small redshift path, $Δz\sim$13 for HI with column density$>5.4\times10^{19}$ cm$^{-2}$, the survey has probed only the outskirts of star-forming galaxies at $ρ>30$ kpc. MALS with the expected $Δz\sim10^{3-4}$ will overcome this limitation and provide stringent constraints on the cold gas fraction of galaxies in diverse environments over $0<z<1.5$.

astro-ph.GA↗

arXiv2020

Nature of the DLA towards Q0528-250: High pressure and strong UV field revealed by excitation of CI, H2 and SiII

We present the detection of excited fine-structure energy levels of singly-ionized silicon and neutral carbon associated with the proximate damped Lyman-$α$ system at $z_{\rm abs}=2.811$ towards \qso. This absorber has an apparent relative velocity that is inconsistent with the Hubble flow indicating motion along the line-of-sight towards the quasar, i.e., $z_{\rm abs}>z_{\rm em}$. We measure the metallicity of the system to be ${\rm [Zn/H]}=-0.68\pm 0.02$. Using the relative populations of the fine-structure levels of SiII and CI, as well as the populations of H$_2$ rotational levels, we constrain the physical conditions of the gas. We derive hydrogen number densities of $n_{\rm H}=190^{+70}_{-50}$ cm$^{-3}$ and $260^{+30}_{-20}$ cm$^{-3}$ in two velocity components where both CI and H$_2$ are detected. Taking into account the kinetic temperature in each component, $\sim 150$K, we infer high values of thermal pressure in the cold neutral medium probed by the observations. The strengths of the UV field in Draine's unit are $I_{\rm UV} = 10^{+5}_{-3}$ and $14^{+3}_{-3}$ in each of these two components, respectively. Such enhanced UV fluxes and thermal pressure compared to intervening DLAs are likely due to the proximity of the quasar. The typical size of the absorber is $\sim 10^4$ a.u. Assuming the UV flux is dominated by the quasar, we constrain the distance between the quasar and the absorber to be $\sim 150-200$ kpc. This favours a scenario where the absorption occurs in a companion galaxy located in the group where the quasar-host galaxy resides. This is in line with studies in emission that revealed the presence of several galaxies around the quasar.

astro-ph.GA↗

arXiv2019

Chemical enrichment and host galaxies of extremely-strong intervening DLAs towards quasars

We present the results from VLT/X-shooter spectroscopic observations of 11 extremely strong intervening damped Lyman-alpha absorbers (ESDLAs) initially selected as high N(Hi) (i.e.>=5x10^21 cm-2) candidates from the Sloan Digital Sky Survey (SDSS). We confirm the high Hi column densities which we measure to be in the range log N(Hi) = 21.6-22.4. Molecular hydrogen is detected with high column densities (N(H_2)>=10^18 cm-2) in five out of eleven systems, three of which are reported here for the first time. We compare the chemical properties of this sample of ESDLAs, supplemented with literature measurements, to that of DLAs located at the redshift of long-duration gamma-ray bursts (GRB-DLAs). We confirm that the two populations are almost indistinguishable in terms of chemical enrichment, H_2 column density and gas kinematics. All this suggests that ESDLAs and GRB-DLAs probe similar galactic environments. We search for the galaxy counterparts of ESDLAs and find associated emission lines in three out of eleven systems, two of which are reported here for the first time (towards the quasars SDSS J002503.03+114547.80 and SDSS J114347.21+142021.60, respectively). The measured separations between the quasar sightlines and the emission associated with the ESDLA galaxy are all very small (rho < 3 kpc). While the small impact parameters are similar to what is observed for GRB-DLAs, the associated star-formation rates are on average lower than seen for GRB host galaxies. This is explained by long-duration GRBs being associated with the death of massive stars, hence pinpointing regions of active star formation in the GRB host galaxies. Our observations support the suggestion from the literature that ESDLAs could act as blind analogues of GRB-DLAs, probing high column density neutral gas in the heart of high-redshift galaxies, without any prior on the instantaneous star-formation rate.

astro-ph.GA↗

arXiv2019

X-shooter observations of strong H$_2$-bearing DLAs at high redshift

We present results from spectroscopic observations with X-shooter at the Very Large Telescope of seven H2-bearing DLAs at high redshifts (z$_{\rm abs}\sim 2.5-3$). These DLAs were originally selected from the presence of strong H$_2$ lines directly seen at the DLA redshift in low-resolution, low S/N SDSS spectra. We confirm the detection of molecular hydrogen in all of them. We measure the column densities of HI, H$_2$ in various rotational levels, and metal species, and associated dust extinction. The metallicities, obtained from undepleted species, are in the range logZ=-0.8 to -0.2. We discuss the chemical enrichment in these clouds and compare their properties with that of other molecular-rich systems selected by other means. In particular, we show that three different methods of pre-selection of H$_2$-bearing DLAs in the SDSS have their own biases but complement each other mostly in terms of chemical enrichment. We use the rotational excitation of H$_2$ molecules together with the fine-structure energy levels of neutral carbon to constrain the physical conditions in the gas with the help of numerical modeling as well as analytical expressions for the surface density at which atomic to molecular conversion happens. We find that the H$_2$-bearing medium revealed by the studied DLAs has typical values for the kinetic temperature, hydrogen density, and UV radiation field of, respectively, T$\sim$100K, $n_{\rm H}\sim 100$ cm$^{-3}$, and I$_{\rm UV}$ about twice the intensity of the Draine field. Detailed studies combining different selections should, therefore, bring important clues to understand the HI-H$_2$ transition at high redshift.

astro-ph.GA↗

arXiv2019

Proximate Molecular Quasar Absorbers: Excess of damped H2 systems at zabs~zQSO in SDSS DR14

We present results from a search for strong H2 absorption systems proximate to quasars (zabs~zem) in the Sloan Digital Sky Survey (SDSS) Data Release 14. The search is based on the Lyman-Werner band signature of damped H2 absorption lines without any prior on the associated metal or neutral hydrogen content. This has resulted in the detection of 81 systems with log N(H2)~19-20 located within a few thousand km/s from the quasar. Compared to a control sample of intervening systems, this implies an excess of proximate H2 systems by about a factor of 4 to 5. The incidence of H2 systems increases steeply with decreasing relative velocity, reaching an order of magnitude higher than expected from intervening statistics at Delta_v<1000 km/s. The most striking feature of the proximate systems compared to the intervening ones is the presence of Ly-alpha emission in the core of the associated damped HI absorption line in about half of the sample. This puts constraints on the relative projected sizes of the absorbing clouds to those of the quasar line emitting regions. Using the SDSS spectra, we estimate the HI, metal and dust content of the systems, which are found to have typical metallicities of one tenth Solar, albeit with a large spread among individual systems. We observe trends between the fraction of leaking Ly-alpha emission and the relative absorber-quasar velocity as well as with the excitation of several metal species, similar to what has been seen in metal-selected proximate DLAs. With the help of theoretical HI-H2 transition relations, we show that the presence of H2 helps to break the degeneracy between density and strength of the UV field as main sources of excitation and hence provides unique constraints on the possible origin and location of the absorbing clouds. We suggest that most of these systems originate from galaxies in the quasar group. [truncated]

astro-ph.GA↗

arXiv2019

Physical conditions in high$-$z optically thin C III absorbers: Origin of cloud sizes and associated correlations

We present detailed photoionization models of well aligned optically thin C III absorption components at $2.1 < z < 3.4$. Using our models we estimate density ($n_{\rm \tiny H}$), metallicity ($[C/H]$), total hydrogen column density and line-of-sight thickness ($L$) in each C III components. We estimate the systematic errors in these quantities contributed by the allowed range of the quasar spectral index used in the ultraviolet background radiation calculations. Our inferred $n_{\rm \tiny H}$ and overdensity ($Δ$) are much higher than the measurements available in the literature and favor the absorption originating from gas associated with circumgalactic medium and probably not in hydrostatic equilibrium. We also notice $n_{\rm \tiny H}$, $L$ and $[C/H]$ associated with C III components show statistically significant redshift evolution. To some extent, these redshift evolutions are driven by the appearance of compact, high $n_{\rm \tiny H}$ and high $[C/H]$ components only in the low$-z$ end. We find more than 5$σ$ level correlation between $[C/H]$ and $L$, $L$ and neutral hydrogen column density (N (HI)), N (HI) and $[C/H]$. We show $L$ versus $[C/H]$ correlation can be well reproduced if $L$ is governed by the product of gas cooling time and sound speed as expected in the case of cloud fragmentation under thermal instabilities. This allows us to explain other observed correlations by simple photoionization considerations. Studying the optically thin C III absorbers over a large $z$ range and probably correlating their $z$ evolution with global star formation rate density evolution can shed light into the physics of cold clump formation and their evolution in the circumgalactic medium.

astro-ph.GA↗

arXiv2018

Rapidly varying Mg II broad absorption line in SDSS J133356.02+001229.1

We report the discovery of rapid variations of a high-velocity Mg ii broad absorption line (BAL) trough in the quasar SDSS J133356.02+001229.1 (zem ~ 0.9197). Vivek et al. (2012a) revealed the emergence and subsequent near disappearance of a BAL component in this source having an ejection velocity of ~ 28000 km s-1. Our further follow up studies with South African Large Telescope (SALT) reveal the dramatic nature of the absorption line variability in this source. The absorption line emerged again at the same velocity and nearly disappeared within the SALT observations. Our observations allow us to probe variability over timescales of the order of few days to 4.2 years in the QSO rest-frame. The observed velocity stability of BAL absorption does not point to any line of sight acceleration/deceleration of BAL clouds. The ionization parameter of the absorbing cloud is constrained from the column density ratio of Mg II to Fe II ground state absorption. In the absence of strong optical continuum variability, we suggest that photoionization driven BAL variability due to changes in the shielding, multiple streaming clouds across our line of sight in a co-rotating wind or a combination of both as possible explanations for the observed strong equivalent width variations.

astro-ph.GA↗

arXiv2018

High resolution spectroscopy of the relatively hydrogen-poor metal rich giants in the globular cluster Omega Centauri

High-resolution optical spectra are analyzed for two of the four metal rich mildly hydrogen-poor or helium-enhanced giants discovered by Hema and Pandey (2014) along with their comparison normal (hydrogen-rich) giants of Omega Cen. The strengths of the MgH bands in the spectra of the program stars are analyzed for their derived stellar parameters. The observed spectra of the sample (hydrogen-poor) stars (LEID 39048 and LEID 34225) show weaker MgH bands unlike in the spectra of the normal comparison giants (LEID 61067 and LEID 32169). The magnesium abundance derived from MgH bands is less by 0.3 dex or more for LEID 39048 and LEID 34225, than that derived from Mg I lines. This difference, cannot be reconciled by making the changes to the stellar parameters within the uncertainties. This difference in the magnesium abundances derived from Mg I lines and from the MgH band is unacceptable. This difference is attributed to the hydrogen-deficiency or helium-enhancement in their atmospheres. These metal rich hydrogen-poor or helium-rich giants provide an important link to the evolution of the metal-rich sub population of Omega Cen. These stars provide the first direct spectroscopic evidence for the presence of the He-enhancement in the metal rich giants of Omega Cen.

astro-ph.SR↗

arXiv2018

Polarimetric and spectroscopic study of radio-quiet weak emission line quasars

A small subset of optically selected radio-quiet quasars showing weak or no emission lines may turn out to be the elusive radio-quiet BL Lac objects, or simply be radio-quiet QSOs with a still-forming/shielded broad line region (BLR). High polarisation ($p$ $>$ 3$-$4$\%$), a hallmark of BL Lacs, can be used to test whether some optically selected `radio-quiet weak emission line quasars' (RQWLQs) show a fractional polarisation high enough to qualify as radio-quiet analogs of BL Lac objects. Out of the observed six RQWLQs candidates showing an insignificant proper motion, only two are found to have $p$ $>$ 1$\%$. For these two RQWLQs, namely J142505.59$+$035336.2, J154515.77+003235.2, we found polarisation of 1.03$\pm$0.36$\%$, 1.59$\pm$0.53$\%$ respectively, which again is too modest to justify a (radio-quiet) BL Lac classification. We also present here a statistical comparison of the optical spectral index, for a set of 40 RQWLQs with redshift-luminosity matched control sample of 800 QSOs and an equivalent sample of 120 blazars. The spectral index distribution of RQWLQs is found to differ, at a high significance level, from that of blazars and is consistent with that of the ordinary QSOs. Likewise, a structure-function analysis of photometric light curves presented here suggests that the mechanism driving optical variability in RQWLQs is similar to that operating in QSOs and different from that of blazars. These findings are consistent with the common view that the central engine in RQWLQs, as a population, is akin to that operating in normal QSOs and the primary differences between them might be related to differences in the BLR.

astro-ph.HE↗