arXiv subjects

Raghunathan Srianand

Publications and source records attributed to Raghunathan Srianand.

At least 37 records · Page 2Linked to original sources

arXiv2022

Host Galaxies of Ultra Strong Mg II absorbers at z $\sim$ 0.5

From a sample of 109 candidate Ultra-Strong Mg II (USMgII; having rest equivalent width of Mg II absorption, $W_{2796}>3.0$ Angstrom) systems at z=0.4-0.6, we confirm 27 and identify host galaxies of 20 systems based on associated nebular line emission from our SALT observations or from SDSS fiber spectra. The measured impact parameter, [O II] luminosity, star formation rate, B-band luminosity and stellar mass are in the ranges $7.3\le D[kpc]\le79$, $0.2\le L_{[O II]}[ 10^{41}~erg s^{-1}]$ $\le 4.5$, $2.59\le SFR[M_\odot yr^{-1} ]\le 33.51$, $0.15L_B^*\le L_B\le1.63L_B^*$ and $10.21\le log[M_*/M_\odot]\le11.62$ respectively. The impact parameters found are larger than that predicted by the $W_{2796}$ vs D relationship of the general population of Mg II absorbers. At a given D, USMgII host galaxies are more luminous and massive compared to typical Mg II absorbers. However, the measured SFRs are slightly lower than that of main-sequence galaxies with the same M$_\star$ at $z\sim0.5$. We report a correlation between $L_{[O II]}$ and W$_{2796}$ for the full population of Mg II absorbers, driven mainly by the host galaxies of weak Mg II absorbers that tend to have low $L_{[O II]}$ and large impact parameters. We find at least $\sim$33% of the USMgII host galaxies (with a limiting magnitude of $m_r<23.6$) are isolated and the large $W_{2796}$ in these cases may originate from gas flows (infall/outflow) in single halos of massive but not starburst galaxies. We also find galaxy interactions could be responsible for large velocity widths in at least $\sim$17% cases.

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arXiv2022

LADUMA: Discovery of a luminous OH megamaser at $z > 0.5$

In the local Universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (HI), radio surveys to probe the cosmic evolution of HI in galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep HI survey, we report the first untargeted detection of an OHM at $z > 0.5$, LADUMA J033046.20$-$275518.1 (nicknamed "Nkalakatha"). The host system, WISEA J033046.26$-$275518.3, is an infrared-luminous radio galaxy whose optical redshift $z \approx 0.52$ confirms the MeerKAT emission line detection as OH at a redshift $z_{\rm OH} = 0.5225 \pm 0.0001$ rather than HI at lower redshift. The detected spectral line has 18.4$σ$ peak significance, a width of $459 \pm 59\,{\rm km\,s^{-1}}$, and an integrated luminosity of $(6.31 \pm 0.18\,{\rm [statistical]}\,\pm 0.31\,{\rm [systematic]}) \times 10^3\,L_\odot$, placing it among the most luminous OHMs known. The galaxy's far-infrared luminosity $L_{\rm FIR} = (1.576 \pm 0.013) \times 10^{12}\,L_\odot$ marks it as an ultra-luminous infrared galaxy; its ratio of OH and infrared luminosities is similar to those for lower-redshift OHMs. A comparison between optical and OH redshifts offers a slight indication of an OH outflow. This detection represents the first step towards a systematic exploitation of OHMs as a tracer of galaxy growth at high redshifts.

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arXiv2022

Discovery of a damped Ly$α$ absorber originating in a spectacular interacting dwarf galaxy pair at $z = 0.026$

We present the discovery of neutral gas detected in both damped Ly$α$ absorption (DLA) and HI 21-cm emission outside of the stellar body of a galaxy, the first such detection in the literature. A joint analysis between the Cosmic Ultraviolet Baryon Survey and the MeerKAT Absorption Line Survey reveals an HI bridge connecting two interacting dwarf galaxies (log$(M_{\text{star}}/\text{M}_{\odot}) = 8.5 \pm 0.2$) that host a $z = 0.026$ DLA with log[$N$(HI)/cm$^{-2}$]$ = 20.60 \pm 0.05$ toward the QSO J2339-5523 ($z_{\text{QSO}} = 1.35$). At impact parameters of $d = 6$ and $33$ kpc, the dwarf galaxies have no companions more luminous than $\approx 0.05L_{*}$ within at least $Δv = \pm 300$ km s$^{-1}$ and $d \approx 350$ kpc. HI 21-cm emission is spatially coincident with the DLA at the 2-3$σ$ level per spectral channel over several adjacent beams. However, HI 21-cm absorption is not detected against the radio-bright QSO; if the background UV and radio sources are spatially aligned, the gas is either warm or clumpy (with spin temperature to covering factor ratio $T_{s}/f_{c} > 1880$ K). VLT-MUSE observations demonstrate that the $α$-element abundance of the ionized ISM is consistent with the DLA ($\approx 10$% solar), suggesting that the neutral gas envelope is perturbed ISM gas. This study showcases the impact of dwarf-dwarf interactions on the physical and chemical state of neutral gas outside of star-forming regions. In the SKA era, joint UV and HI 21-cm analyses will be critical for connecting the cosmic neutral gas content to galaxy environments.

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arXiv2022

The Hydrogen Intensity and Real-time Analysis eXperiment: 256-Element Array Status and Overview

The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a radio interferometer array currently in development, with an initial 256-element array to be deployed at the South African Radio Astronomy Observatory (SARAO) Square Kilometer Array (SKA) site in South Africa. Each of the 6m, $f/0.23$ dishes will be instrumented with dual-polarisation feeds operating over a frequency range of 400-800 MHz. Through intensity mapping of the 21 cm emission line of neutral hydrogen, HIRAX will provide a cosmological survey of the distribution of large-scale structure over the redshift range of $0.775 < z < 2.55$ over $\sim$15,000 square degrees of the southern sky. The statistical power of such a survey is sufficient to produce $\sim$7 percent constraints on the dark energy equation of state parameter when combined with measurements from the Planck satellite. Additionally, HIRAX will provide a highly competitive platform for radio transient and HI absorber science while enabling a multitude of cross-correlation studies. In this paper, we describe the science goals of the experiment, overview of the design and status of the sub-components of the telescope system, and describe the expected performance of the initial 256-element array as well as the planned future expansion to the final, 1024-element array.

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arXiv2021

Damped Ly-alpha Absorbers in Star-forming Galaxies at z < 0.15 Detected with the Hubble Space Telescope and Implications for Galaxy Evolution

We report {\it HST} COS spectroscopy of 10 quasars with foreground star-forming galaxies at 0.02$<$$z$$<$ 0.14 within impact parameters of $\sim$1-7 kpc. We detect damped/sub-damped Ly$α$ absorption in 100$\%$ of cases where no higher-redshift Lyman-limit systems extinguish the flux at the expected wavelength of Ly$α$ absorption, obtaining the largest targeted sample of DLA/sub-DLAs in low-redshift galaxies. We present absorption measurements of neutral hydrogen and metals. Additionally, we present GBT 21-cm emission measurements for 5 of the galaxies (including 2 detections). Combining our sample with the literature, we construct a sample of 115 galaxies associated with DLA/sub-DLAs spanning 0$<$$z$$<$4.4, and examine trends between gas and stellar properties, and with redshift. The H~I column density is anti-correlated with impact parameter and stellar mass. More massive galaxies appear to have gas-rich regions out to larger distances. The specific SFR (sSFR) of absorbing galaxies increases with redshift and decreases with $M^{\ast}$, consistent with evolution of the star-formation main sequence (SFMS). However, $\sim$20$\%$ of absorbing galaxies lie below the SFMS, indicating that some DLA/sub-DLAs trace galaxies with longer-than-typical gas-depletion time-scales. Most DLA/sub-DLA galaxies with 21-cm emission have higher H I masses than typical galaxies with comparable $M^{\ast}$. High $M_{\rm H I}/M^{\ast}$ ratios and high sSFRs in DLA/sub-DLA galaxies with $M^{\ast}$$<$$10^{9}$$M_{\odot}$ suggest these galaxies may be gas-rich because of recent gas accretion rather than inefficient star formation. Our study demonstrates the power of absorption and emission studies of DLA/sub-DLA galaxies for extending galaxy-evolution studies to previously under-explored regimes of low $M^{\ast}$ and low SFR.

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arXiv2021

Measurement of redshift-space two- and three-point correlation of Ly$α$ absorbers at $1.7< z <3.5$: Implications on evolution of the physical properties of IGM

We present redshift space two-point ($ξ$), three-point ($ζ$) and reduced three-point (Q) correlations of Ly$α$ absorbers (Voigt profile components having HI column density, $N_{\rm HI}>10^{13.5}$cm$^{-2}$) over three redshift bins spanning $1.7< z<3.5$ using high-resolution spectra of 292 quasars. We detect positive $ξ$ up to 8 $h^{-1}$ cMpc in all three redshift bins. The strongest detection of $ζ=1.81\pm 0.59$ (with Q$=0.68\pm 0.23$), is in $z=1.7-2.3$ bin at $1-2h^{-1}$ cMpc. The measured $ξ$ and $ζ$ values show an increasing trend with $N_{\rm HI}$, while Q remains relatively independent of $N_{\rm HI}$. We find $ξ$ and $ζ$ to evolve strongly with redshift. Using simulations, we find that $ξ$ and $ζ$ seen in real space may be strongly amplified by peculiar velocities in redshift space. Simulations suggest that while feedback, thermal and pressure smoothing effects influence the clustering of Ly$α$ absorbers at small scales, i.e $<0.5h^{-1}$ cMpc, the HI photo-ionization rate ($Γ_{\rm HI}$) has a strong influence at all scales. The strong redshift evolution of $ξ$ and $ζ$ (for a fixed $N_{\rm HI}$-cutoff) is driven by the redshift evolution of the relationship between $N_{\rm HI}$ and baryon overdensity. Our simulation using best-fitted $Γ_{\rm HI}(z)$ measurements produces consistent clustering signals with observations at $z\sim 2$ but under-predicts the clustering at higher redshifts. One possible remedy is to have higher values of $Γ_{\rm HI}$ at higher redshifts. Alternatively the discrepancy could be related to non-equilibrium and inhomogeneous conditions prevailing during HeII reionization not captured by our simulations.

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arXiv2021

Spatially resolved Lyman-$α$ emission around radio bright quasars

We use Southern African Large Telescope (SALT) to perform long-slit spectroscopic observations of 23 newly discovered radio-loud quasars (RLQs) at $2.7 $1000 km/s in all, except one). Using the samples of high-$z$ radio-loud quasars and galaxies from literature, we confirm the correlation between the Ly$α$ halo luminosity and its size with radio power ($L_{\rm 420MHz}$). The same quantities are found to be correlating weakly with the projected linear size of the radio emission. Our sample is the second largest sample of RLQs being studied for the presence of diffuse Ly$α$ emission and fills in a redshift gap between previous such studies. Integral Field Spectroscopy is required to fully understand the relationship between the large scale radio emission and the overall distribution, kinematics and over density of Ly$α$ emission in the field of these RLQs.

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arXiv2021

A consistent and robust measurement of the thermal state of the IGM at $2 \leq z \leq 4$ from a large sample of Ly$α$ forest spectra: Evidence for late and rapid HeII reionization

We characterise the thermal state of the intergalactic medium (IGM) in ten redshift bins in the redshift range $2 \leq z \leq 4$ with a sample of 103 high resolution, high S/N Ly$α$ forest spectra using four different flux distribution statistics. Our measurements are calibrated with mock spectra from a large suite of hydrodynamical simulations post-processed with our thermal IGM evolution code CITE, finely sampling amplitude and slope of the expected temperature-density relation. The thermal parameters inferred from our measurements of the flux power spectrum, Doppler parameter distribution, as well as wavelet and curvature statistics agree well within their respective errors and all clearly show the peak in temperature and minimum in slope of the temperature density relation expected from HeII reionization. Combining our measurements from the different flux statistics gives $T_0=(14750 \pm 1322)$K for the peak temperature at mean density and a corresponding minimum slope $γ= 1.225 \pm 0.120$. The peak in the temperature evolution occurs at $z \approx 3$, in agreement with previous measurements that had suggested the presence of such a peak, albeit with a large scatter. Using CITE, we also calculate the thermal state of the IGM predicted by five widely used (spatially homogeneous) UV-background models. The rather rapid thermal evolution inferred by our measurements is well reproduced by two of the models, if we assume (physically well motivated) non-equilibrium evolution with photo-heating rates that are reduced by a moderate factor of $\sim 0.7-0.8$. The other three models predict HeII reionization to be more extended with a somewhat earlier as well as higher temperature peak than our measurements suggest.

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arXiv2021

Redshift space three-point correlation function of IGM at $z<0.48$

The Ly$α$ forest decomposed into Voigt profile components allow us to study clustering properties of the intergalactic medium and its dependence on various physical quantities. Here, we report the first detections of probability excess of low-z (i.e z<0.48) Ly$α$ absorber triplets over a scale of $r_\parallel\leq8$pMpc with a maximum amplitude of $8.76^{+1.96}_{-1.65}$ at a longitudinal separation of 1-2pMpc. We measure non-zero three-point correlation ($ζ=4.76^{+1.98}_{-1.67}$) only at this scale with reduced three-point correlation value of Q=$0.95^{+0.39}_{-0.38}$. The measured $ζ$ shows an increasing trend with increasing HI column density ($N_{HI}$) while Q does not show any $N_{HI}$ dependence. About 88% of the triplets contributing to $ζ$ (at z$\le0.2$) have nearby galaxies (whose distribution is known to be complete for 0.1L$_*$ at z<0.1 and for L$_*$ at z~0.25 and within 20' to the quasar sightlines) within a velocity separation of 500$kms^{-1}$ and a median impact parameter of 405pkpc. The measured impact parameters are consistent with appreciable number of triplets at z$\le0.2$ not originating from individual galaxies but tracing the underlying galaxy distribution. Frequency of occurrence of high-b absorbers in triplets (~85%) is a factor~3 higher than that found among the full sample (~32%). Using four different cosmological simulations, we quantify the effect of peculiar velocities, feedback effects and show that most of the observed trends are broadly reproduced. However, $ζ$ at small scales ($r_\parallel<1$pMpc) and b-dependence of $ζ$ in simulations are found inconsistent with the observations. This could either be related to the fact that none of these simulations reproduce the observed b-distribution and $N_{HI}$ distribution for $N_{HI}>10^{14}$cm$^{-2}$ self-consistently or to the widespread of signal-to-noise ratio in the observed data.

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arXiv2021

The COS-legacy survey of C IV absorbers: properties and origins of the intervening systems

We present here results from a survey of intervening C IV absorbers at $z < 0.16$ conducted using 223 sightlines from the Hubble Spectroscopic Legacy Archive. Most systems (83%) out of the total sample of 69 have simple kinematics with 1 or 2 C IV components. In the 22 C IV systems with well constrained H I column densities, the temperatures from the $b$-values imply predominantly photoionized plasma ($T\leq 10^5$ K) and non-thermal dynamics. These systems also have solar or higher metallicities. We obtain a C IV line density of $d\mathcal{N}/dX = 5.1\pm 1.0$ for $\log [N(C~IV)~(cm^{-2})]\geq12.9$, and $Ω_{C~IV}=(8.01\pm 1.62) \times 10^{-8}$ for $12.9 \leq \log [N(C~IV)~(cm^{-2})] \leq 15.0$. The C IV bearing diffuse gas in the $z < 0.16$ Universe has a metallicity of $(2.07~{\pm}~0.43)~\times~10^{-3}$ Z$_{\odot}$, an order of magnitude more than the metal abundances in the IGM at high redshifts ($z \gtrsim 5$), and consistent with the slow build-up of metals in the diffuse circum/intergalactic space with cosmic time. For $z<0.015$ (complete above $L>0.01L^\star$), the Sloan Digital Sky Survey provides a tentative evidence of declining covering fraction for strong C IV ($N>10^{13.5}~cm^{-2}$) with $ρ$ (impact parameter) and $ρ/R_\mathrm{vir}$. However, the increase at high separations suggests that strong systems are not necessarily coincident with such galaxies. We also find that strong C IV absorption at $z<0.051$ is not coincident with galaxy over-dense regions complete for $L>0.13L^\star$

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arXiv2021

Constraining the ratio of median pixel optical depth profile around z ~ 4 quasars using the longitudinal proximity effect

We present a detailed study of the longitudinal proximity effect using a sample of 85 quasars spanning an emission redshift range of $3.5 \leq z_{em} \leq 4.5$ and Lyman continuum luminosity ($L_{912}$) ranging from 1.06$\times 10^{31}$ to 2.24$\times 10^{32}$ erg s$^{-1}$ Hz$^{-1}$. We use the high-quality spectra of these quasars obtained at a spectral resolution of $R\sim$ 5100 and S/N $\sim$ 30 using X-SHOOTER spectrograph mounted on the Very Large Telescope (VLT). In our analysis, we compared the transmitted flux and pixel optical depth of the Ly$α$ absorption originating from the vicinity of quasars to those from the general intergalactic medium by using a redshift matched control sample. The longitudinal proximity effect is found up to $r \leq 12$ Mpc (proper) from quasars. By appropriately scaling up the pixel optical depth in the vicinity of quasars to account for the excess ionization by quasars, we constrain the ratio of median HI optical depth in the vicinity of the quasar to that of the IGM ($R_τ(r)$). The $R_τ(r)$ is found to be significantly higher than unity up to 6 Mpc from the quasar with a typical radial profile of the form $R_τ(r) = 1+A \times exp(-r/r_0)$ with $A=9.16\pm 0.68$ and $r_0= 1.27\pm 0.08$ Mpc. The integrated value of the scaled pixel optical depth over the radial bin of 0-6 Mpc is found to be higher by a factor of $2.55 \pm 0.17$ than the corresponding integrated value of the median pixel optical depth of the IGM. We also found $R_τ(r)$ to be luminosity dependent.

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arXiv2021

Lyman-$α$ emission from a WISE-selected optically faint powerful radio galaxy M151304.72-252439.7 at $z$ = 3.132

We report the detection of a large ($\sim90$ kpc) and luminous $\mathrm{Lyα}$ nebula [$L\mathrm{_{Lyα}}$ = $(6.80\pm0.08)\times 10^{44}$] $\rm{\,erg\,s^{-1}}$ around an optically faint (r$>23$ mag) radio galaxy M1513-2524 at $z\mathrm{_{em}}$=3.132. The double-lobed radio emission has an extent of 184 kpc, but the radio core, i.e., emission associated with the active galactic nucleus (AGN) itself, is barely detected. This object was found as part of our survey to identify high-$z$ quasars based on Wide-field Infrared Survey Explorer (WISE) colors. The optical spectrum has revealed $\mathrm{Lyα}$, NV, CIV and HeII emission lines with a very weak continuum. Based on long-slit spectroscopy and narrow band imaging centered on the $\mathrm{Lyα}$ emission, we identify two spatial components: a "compact component" with high velocity dispersion ($\sim 1500$$\rm{\,km\,s^{-1}}$) seen in all three lines, and an "extended component", having low velocity dispersion (i.e., 700-1000$\rm{\,km\,s^{-1}}$). The emission line ratios are consistent with the compact component being in photoionization equilibrium with an AGN. We also detect spatially extended associated $\mathrm{Lyα}$ absorption, which is blue-shifted within 250-400$\rm{\,km\,s^{-1}}$ of the $\mathrm{Lyα}$ peak. The probability of $\mathrm{Lyα}$ absorption detection in such large radio sources is found to be low ($\sim$10%) in the literature. M1513-2524 belongs to the top few percent of the population in terms of $\mathrm{Lyα}$ and radio luminosities. Deep integral field spectroscopy is essential for probing this interesting source and its surroundings in more detail.

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arXiv2020

Discovery of a damped Ly$α$ galaxy at z $\sim$ 3 towards the quasar SDSS J011852+040644

We report the detection of the host galaxy of a damped Ly$α$ system (DLA) with log N(HI) $ [\rm cm^{-2}]$ = $21.0 \pm 0.10$ at $z \approx 3.0091$ towards the background quasar SDSS J011852+040644 using the Palomar Cosmic Web Imager (PCWI) at the Hale (P200) telescope. We detect Ly$α$ emission in the dark core of the DLA trough at a 3.3$σ$ confidence level, with Ly$α$ luminosity of $L_{\rm Lyα}$ $\rm = (3.8 \pm 0.8) \times 10^{42}\ erg\ s^{-1}$, corresponding to a star formation rate of $\gtrsim 2\ \rm M_{\odot}\ yr^{-1}$ (considering a lower limit on Ly$α$ escape fraction $f_{esc}^{Lyα} \sim 2\%$) as typical for Lyman break galaxies at these redshifts. The Ly$α$ emission is blueshifted with respect to the systemic redshift derived from metal absorption lines by $281 \pm 43$ km/s. The associated galaxy is at very small impact parameter of $\lesssim 12 \rm\ kpc$ from the background quasar, which is in line with the observed anticorrelation between column density and impact parameter in spectroscopic searches tracing the large-scale environments of DLA host galaxies.

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arXiv2020

A Carbon-enhanced Lyman Limit System: Signature of the First Generation of Stars?

We present the study of a Lyman limit system (LLS) at $z_{\rm abs}$ = 1.5441 towards quasar J134122.50+185213.9 observed with VLT X-shooter. This is a very peculiar system with strong C I absorption seen associated with a neutral hydrogen column density of log $N$(H I) (cm$^{-2}$) = 18.10, too small to shield the gas from any external UV flux. The low ionization absorption lines exhibit a simple kinematic structure consistent with a single component. Using CLOUDY models to correct for ionization, we find that the ionization parameter of the gas is in the range $-$ 4.5 $<$ log $U$ $<$ $-$4.2 and the gas density $-$1.5 $<$ log $n$(H) (cm$^{-3}$) $<$ $-$1.2. The models suggest that carbon is overabundant relative to iron, [C/Fe] $>$ +2.2 at [Fe/H] $\sim$ $-$1.6. Such a metal abundance pattern is reminiscent of carbon-enhanced metal-poor stars detected in the Galaxy halo. Metal enrichment by the first generation of supernovae provides a plausible explanation for the inferred abundance pattern in this system.

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arXiv2020

Three- and two-point spatial correlations of IGM at $z\sim 2$: Cloud based analysis using simulations

Ly$α$ forest absorption spectra decomposed into multiple Voigt profile components (clouds) allow us to study clustering of intergalactic medium (IGM) as a function of HI column density ($N_{\rm HI}$). Here, we explore the transverse three-point correlation ($ζ$) of these Ly$α$ clouds using mock triplet spectra obtained from hydrodynamical simulations at $z \sim 2$ on scales of 1-5 $h^{-1}$cMpc. We find $ζ$ to depend strongly on $N_{\rm HI}$ and scale and weakly on angle ($θ$) of the triplet configuration. We show that the "hierarchical ansatz" is applicable for scales $\ge~ 3h^{-1}$cMpc, and obtain a median reduced three-point correlation (Q) in the range 0.2-0.7. We show, $ζ$ is influenced strongly by the thermal and ionization state of the gas. As found in the case of galaxies, the influence of physical parameters on Q is weaker compared to that of $ζ$. We show difference in $ζ$ and Q between different simulations are minimized if we use appropriate $N_{\rm HI}$ cut-offs corresponding to a given baryon over-density ($Δ$) using the measured $N_{\rm HI}~vs~Δ$ relationship obtained from individual simulations. Additionally, we see the effect of pressure broadening on $ζ$ in a model with artificially boosted heating rates. However, for models with realistic thermal and ionization histories the effect of pressure broadening on $ζ$ is weak and sub-dominant compared to other local effects. We find strong redshift evolution shown by $ζ$, mainly originating from the redshift evolution of thermal and ionization state of the IGM. We discuss the observational requirements for the detection of three-point correlation, specifically, in small intervals of configuration parameters and redshift.

astro-ph.CO↗

arXiv2019

uGMRT search for cold gas at z~1-1.4 towards red quasars

We present results from our search for HI 21-cm and OH 18-cm absorption at z~1-1.4 towards red quasars showing strong MgII absorption using uGMRT. The quasars J1501+1822 and J1521+5508 show multiple strong associated MgII absorption at z~1.1 and signature of reddening in their optical spectra. We report the detection of HI 21-cm absorption towards J1521+5508 at the systemic redshift of the quasar, with N(HI) =(1.2 +/- 0.2) x 10^{20} cm^{-2} for spin temperature of 100 K and unit covering factor. The HI 21-cm absorption is offset from the blueshifted strong MgII absorbers by >~1500 km/s. We do not detect HI 21-cm absorption at the redshift of the associated MgII absorption and at the systemic redshift towards J1501+1822. We argue that lack of one-to-one correspondence between MgII and HI 21-cm absorption could be related with clumpiness of the neutral gas and the radio and optical sightlines probing different volume of the gas. We find that the presence of strong associated MgII absorption and reddening in the optical spectra of the quasars lead to an increased detection rate of associated HI 21-cm absorption at z>=1. We also report non-detection of intervening OH absorption ([OH]/[HI] <= (1-4) x 10^{-8}) at z = 1.3 towards two red quasars, J0850+5159 and J0852+3435, which show strong metal and HI 21-cm absorption and the 2175 A dust extinction bump.

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arXiv2019

Effect of non-equilibrium ionization on derived physical conditions of the high-$z$ intergalactic medium

Non-equilibrium ionization effects are important in cosmological hydrodynamical simulations but are computationally expensive. We study the effect of non-equilibrium ionization evolution and UV ionizing background (UVB) generated with different quasar spectral energy distribution (SED) on the derived physical conditions of the intergalactic medium (IGM) at $2\leq z \leq 6$ using our post-processing tool 'Code for Ionization and Temperature Evolution' (CITE). CITE produces results matching well with self-consistent simulations more efficiently. The HeII reionization progresses more rapidly in non-equilibrium model as compared to equilibrium models. The redshift of HeII reionization strongly depends on the quasar SED and occurs earlier for UVB models with flatter quasar SEDs. During this epoch the normalization of temperature-density relation, $T_0(z)$, has a maximum while the slope, $γ(z)$, has a minimum, but occurring at different redshifts. The $T_0$ is higher in non-equilibrium models using UVB obtained with flatter quasar SEDs. While our models produce the observed median HeII effective optical depth evolution and its scatter for equilibrium and non-equilibrium considerations, to explain the observed cumulative distributions we may need to consider fluctuating UVB. For a given UVB model, the redshift dependence of the HI photo-ionization rate derived from the observed HI effective optical depth ($τ_{\rm eff,HI}$) for the equilibrium model is different from that for the non-equilibrium model. This may lead to different requirements on the evolution of ionizing emissivities of sources. We show that, in the absence of strong differential pressure smoothing effects, it is possible to recover the $T_0$ and $γ$ realised in non-equilibrium model from the equilibrium models generated by rescaling photo-heating rates while producing the same $τ_{\rm eff,HI}$.

astro-ph.CO↗

arXiv2019

Probing the environment of high-z quasars using the proximity effect in projected quasar pairs

We have used spectra of 181 projected quasar pairs at separations $\le 1.5$ arcmins from the Sloan Digital Sky-Survey Data Release 12 in the redshift range of 2.5 to 3.5 to probe the proximity regions of the foreground quasars. We study the proximity effect both in the longitudinal as well as in the transverse directions, by carrying out a comparison of the \lya absorption lines originating from the vicinity of quasars to those originating from the general inter-galactic medium at the same redshift. We found an enhancement in the transmitted flux within 4 Mpc to the quasar in the longitudinal direction. However, the trend is found to be reversed in the transverse direction. In the longitudinal direction, we derived an excess overdensity profile showing an excess up to $r\leq 5$ Mpc after correcting for the quasar's ionization, taking into account the effect of low spectral resolution. This excess overdensity profile matches with the average overdensity profile in the transverse direction without applying any correction for the effect of the quasar's ionization. Among various possible interpretations, we found that the anisotropic obscuration of the quasar's ionization seems to be the most probable explanation. This is also supported by the fact that all of our foreground quasars happen to be Type-I AGNs. Finally, we constrain the average quasar's illumination along the transverse direction as compared to that along the longitudinal direction to be $\leq$27\% (3$σ$ confidence level).

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