arXiv subjects

Nissim Kanekar

Publications and source records attributed to Nissim Kanekar.

At least 55 records · Page 3Linked to original sources

arXiv2018

Statistical properties of Faraday rotation measure in external galaxies -- I: intervening disc galaxies

Deriving the Faraday rotation measure (RM) of quasar absorption line systems, which are tracers of high-redshift galaxies intervening background quasars, is a powerful tool for probing magnetic fields in distant galaxies. Statistically comparing the RM distributions of two quasar samples, with and without absorption line systems, allows one to infer magnetic field properties of the intervening galaxy population. Here, we have derived the analytical form of the probability distribution function (PDF) of RM produced by a single galaxy with an axisymmetric large-scale magnetic field. We then further determine the PDF of RM for one random sight line traversing each galaxy in a population with a large-scale magnetic field prescription. We find that the resulting PDF of RM is dominated by a Lorentzian with a width that is directly related to the mean axisymmetric large-scale field strength $\langle B_0 \rangle$ of the galaxy population if the dispersion of $B_0$ within the population is smaller than $\langle B_0 \rangle$. Provided that RMs produced by the intervening galaxies have been successfully isolated from other RM contributions along the line of sight, our simple model suggests that $\langle B_0 \rangle$ in galaxies probed by quasar absorption line systems can be measured within $\approx50$ per cent accuracy without additional constraints on the magneto-ionic medium properties of the galaxies. Finally, we discuss quasar sample selection criteria that are crucial to reliably interpret observations, and argue that within the limitations of the current database of absorption line systems, high-metallicity damped Lyman-$α$ absorbers are best suited to study galactic dynamo action in distant disc galaxies.

astro-ph.GA↗

arXiv2018

Molecular Emission from a Galaxy Associated with a z~2.2 Damped Lyman-alpha Absorber

Using the Atacama Large Millimeter/sub-millimeter Array, we have detected CO(3-2) line and far-infrared continuum emission from a galaxy associated with a high-metallicity ([M/H] = -0.27) damped Ly-alpha absorber (DLA) at z =2.19289. The galaxy is located 3.5" away from the quasar sightline, corresponding to a large impact parameter of 30 kpc at the DLA redshift. We use archival Very Large Telescope-SINFONI data to detect Halpha emission from the associated galaxy, and find that the object is dusty, with a dust-corrected star formation rate of 110 +60 -30 Msun/yr. The galaxy's molecular mass is large, Mmol = (1.4 +- 0.2) x 10^11 x (α_CO/4.3) x (0.57/r_31) Msun, supporting the hypothesis that high-metallicity DLAs arise predominantly near massive galaxies. The excellent agreement in redshift between the CO(3-2) line emission and low-ion metal absorption (~40 km/s) disfavors scenarios whereby the gas probed by the DLA shows bulk motion around the galaxy. We use Giant Metrewave Radio Telescope HI 21cm absorption spectroscopy to find that the HI along the DLA sightline must be warm, with a stringent lower limit on the spin temperature of T_s > 1895 x (f/0.93) K. The detection of CI absorption in the DLA, however, also indicates the presence of cold neutral gas. To reconcile these results requires that the cold components in the DLA contribute little to the HI column density, yet contain roughly 50% of the metals of the absorber, underlining the complex multi-phase nature of the gas surrounding high-z galaxies.

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arXiv2018

Stringent constraints on fundamental constant evolution using conjugate 18 cm satellite OH lines

We have used the Arecibo Telescope to carry out one of the deepest-ever integrations in radio astronomy, targetting the redshifted conjugate satellite OH 18 cm lines at $z \approx 0.247$ towards PKS1413+135. The satellite OH 1720 and 1612 MHz lines are respectively in emission and absorption, with exactly the same line shapes due to population inversion in the OH ground state levels. Since the 1720 and 1612 MHz line rest frequencies have different dependences on the fine structure constant $α$ and the proton-electron mass ratio $μ$, a comparison between their measured redshifts allows one to probe changes in $α$ and $μ$ with cosmological time. In the case of conjugate satellite OH 18 cm lines, the predicted perfect cancellation of the sum of the line optical depths provides a strong test for the presence of systematic effects that might limit their use in probing fundamental constant evolution. A non-parametric analysis of our new Arecibo data yields $\left[ΔX/X \right] = (+0.97 \pm 1.52) \times 10^{-6}$, where $X \equiv μα^2$. Combining this with our earlier results from the Arecibo Telescope and the Westerbork Synthesis Radio Telescope, we obtain $\left[ΔX/X \right] = (-1.0 \pm 1.3) \times 10^{-6}$, consistent with no changes in the quantity $μα^2$ over the last 2.9~Gyr. This is the most stringent present constraint on fractional changes in $μα^2$ from astronomical spectroscopy, and with no evidence for systematic effects.

astro-ph.CO↗

arXiv2017

A Giant Metrewave Radio Telescope search for associated H{\sc i} 21\,cm absorption in GHz-peaked-spectrum sources

We report the first detections of associated H{\sc i} 21\,cm absorption in Gigahertz-peaked-spectrum (GPS) sources at high redshifts, $z > 1$, using the Giant Metrewave Radio Telescope (GMRT). Our GMRT search for associated H{\sc i} 21\,cm absorption in a sample of 12 GPS sources yielded two new detections of absorption, towards TXS~1200+045 at $z = 1.226$ and TXS~1245$-$197 at $z = 1.275$, and five non-detections. These are only the sixth and seventh detections of associated H{\sc i} 21\,cm absorption in active galactic nuclei (AGNs) at $z > 1$. Both H{\sc i} 21\,cm absorption profiles are wide, with velocity spans between nulls of $\approx 600$~km~s$^{-1}$ (TXS~1200+045) and $\approx 1100$~km~s$^{-1}$ (TXS~1245$-$197). In both absorbers, the large velocity spread of the absorption and its blueshift from the AGN, suggests that it arises in outflowing neutral gas, perhaps driven by the radio jets to high velocities. We derive mass outflow rates of ${\dot M} \approx 32 \; {\rm M}_\odot$~yr$^{-1}$ (TXS~1200+045) and ${\dot M} \approx 18 \; {\rm M}_\odot$~yr$^{-1}$ (TXS~1245$-$197), comparable to the mass outflow rates seen earlier in low-redshift active galactic nuclei.

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arXiv2017

The gas and stellar mass of low-redshift damped Lyman-$α$ absorbers

We report Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet and Arecibo Telescope H{\sc i} 21cm spectroscopic studies of six damped and sub-damped Lyman-$α$ absorbers (DLAs and sub-DLAs, respectively) at $z \lesssim 0.1$, that have yielded estimates of their H{\sc i} column density, metallicity and atomic gas mass. This significantly increases the number of DLAs with gas mass estimates, allowing the first comparison between the gas masses of DLAs and local galaxies. Including three absorbers from the literature, we obtain H{\sc i} masses $\approx (0.24 - 5.2) \times 10^9 \: {\rm M}_\odot$, lower than the knee of the local H{\sc i} mass function. This implies that massive galaxies do not dominate the absorption cross-section for low-$z$ DLAs. We use Sloan Digital Sky Survey photometry and spectroscopy to identify the likely hosts of four absorbers, obtaining low stellar masses, $\approx 10^7-10^{8.7} M_\odot$, in all cases, consistent with the hosts being dwarf galaxies. We obtain high H{\sc i} 21\,cm or CO emission line widths, $ΔV_{20} \approx 100-290$~km~s$^{-1}$, and high gas fractions, $f_{\rm HI} \approx 5-100$, suggesting that the absorber hosts are gas-rich galaxies with low star formation efficiencies. However, the H{\sc i} 21\,cm velocity spreads ($\gtrsim 100$~km~s$^{-1}$) appear systematically larger than the velocity spreads in typical dwarf galaxies.

astro-ph.GA↗

arXiv2017

Giant Metrewave Radio Telescope monitoring of the black hole X-ray binary, V404 Cygni, during its June 2015 outburst

We report results from a Giant Metrewave Radio Telescope (GMRT) monitoring campaign on the black hole X-ray binary V404 Cygni during its 2015 June outburst. The GMRT observations were carried out at observing frequencies of 1280, 610, 325 and 235 MHz, and extended from June 26.89 UT (a day after the strongest radio/X-ray outburst) to July 12.93 UT. We find the low-frequency radio emission of V404 Cygni to be extremely bright and fast-decaying in the outburst phase, with an inverted spectrum below 1.5 GHz and an intermediate X-ray state. The radio emission settles to a weak, quiescent state $\approx 11$ days after the outburst, with a flat radio spectrum and a soft X-ray state. Combining the GMRT measurements with flux density estimates from the literature, we identify a spectral turnover in the radio spectrum at $\approx 1.5$ GHz on $\approx$ June 26.9 UT, indicating the presence of a synchrotron self-absorbed emitting region. We use the measured flux density at the turnover frequency with the assumption of equipartition of energy between the particles and the magnetic field to infer the jet radius ($\approx 4.0 \times 10^{13}$ cm), magnetic field ($\approx 0.5$ G), minimum total energy ($\approx 7 \times 10^{39}$ ergs) and transient jet power ($\approx 8 \times 10^{34}$ erg s$^{-1}$). The relatively low value of the jet power, despite V404 Cygni's high black hole spin parameter, suggests that the radio jet power does not correlate with the spin parameter.

astro-ph.HE↗

arXiv2017

[CII] 158 micron Emission from the Host Galaxies of Damped Lyman Alpha Systems

Gas surrounding high redshift galaxies has been studied through observations of absorption line systems toward background quasars for decades. However, it has proven difficult to identify and characterize the galaxies associated with these absorbers due to the intrinsic faintness of the galaxies compared to the quasars at optical wavelengths. Utilizing the Atacama Large Millimeter/Submillimeter Array, we report on detections of [CII] 158 micron line and dust continuum emission from two galaxies associated with two such absorbers at a redshift of z~4. Our results indicate that the hosts of these high-metallicity absorbers have physical properties similar to massive star-forming galaxies and are embedded in enriched neutral hydrogen gas reservoirs that extend well beyond the star-forming interstellar medium of these galaxies.

astro-ph.GA↗

arXiv2016

Giant Metrewave Radio Telescope detection of associated HI 21cm absorption at $z = 1.2230$ towards TXS1954+513

We have used the 610 MHz receivers of the Giant Metrewave Radio Telescope (GMRT) to detect associated HI 21cm absorption from the $z = 1.2230$ blazar TXS1954+513. The GMRT HI 21cm absorption is likely to arise against either the milli-arcsecond-scale core or the one-sided milli-arcsecond-scale radio jet, and is blueshifted by $\approx 328$ km s$^{-1}$ from the blazar redshift. This is consistent with a scenario in which the HI cloud giving rise to the absorption is being driven outward by the radio jet. The integrated HI 21cm optical depth is $(0.716 \pm 0.037)$ km s$^{-1}$, implying a high HI column density, $N_{\rm HI} = (1.305 \pm 0.067) \times ({\rm T_s/100\: K}) \times 10^{20}$ cm$^{-2}$, for an assumed HI spin temperature of 100 K. We use Nickel Telescope photometry of TXS1954+513 to infer a high rest-frame 1216 Å luminosity of $(4.1 \pm 1.2) \times 10^{23}$ W Hz$^{-1}$. The $z = 1.2230$ absorber towards TXS1954+513 is only the fifth case of a detection of associated HI 21cm absorption at $z > 1$, and is also the first case of such a detection towards an active galactic nucleus (AGN) with a rest-frame ultraviolet luminosity $\gg 10^{23}$ W Hz$^{-1}$, demonstrating that neutral hydrogen can survive in AGN environments in the presence of high ultraviolet luminosities.

astro-ph.GA↗

arXiv2016

The HI Content of the Universe over the Past 10 Gyrs

We use the Hubble Space Telescope (HST) archive of ultraviolet (UV) quasar spectroscopy to conduct the first blind survey for damped Ly-alpha absorbers (DLAs) at low redshift (z < 1.6). Our statistical sample includes 463 quasars with spectral coverage spanning a total redshift path, dz = 123.3 or an absorption path, dX = 229.7. Within this survey path, we identify 4 DLAs, defined as absorbers with HI column density N(HI) >= 10^20.3cm-2, which implies an incidence per absorption length, l(X)= 0.017(+0.014-0.008) at a median survey path redshift of z=0.623. While our estimate of l(X) is lower than earlier estimates at z ~ 0 from HI 21cm emission studies, the results are consistent within the measurement uncertainties. Our dataset is too small to properly sample the N(HI) frequency distribution function f(N(HI),X), but the observed distribution agrees with previous estimates at z > 2. Adopting the z > 2 shape of f(N(HI),X), we infer an HI mass density at z ~ 0.6 of rho_HI = 0.25(+0.20-0.12) x 10^8 Msol Mpc-3. This is significantly lower than previous estimates from targeted DLA surveys with the HST, but consistent with results from low-z HI 21cm observations, and suggests that the neutral gas density of the universe has been decreasing over the past 10 Gyrs.

astro-ph.GA↗

arXiv2016

The gas mass of star-forming galaxies at $z \approx 1.3$

We report a Giant Metrewave Radio Telescope (GMRT) search for HI 21cm emission from a large sample of star-forming galaxies at $z \approx 1.18 - 1.34$, lying in sub-fields of the DEEP2 Redshift Survey. The search was carried out by co-adding ("stacking") the HI 21cm emission spectra of 857 galaxies, after shifting each galaxy's HI 21cm spectrum to its rest frame. We obtain the $3σ$ upper limit S$_{\rm{HI}} < 2.5 μ$Jy on the average HI 21cm flux density of the 857 galaxies, at a velocity resolution of $\approx 315$ km s$^{-1}$. This yields the $3σ$ constraint M$_{\rm{HI}} < 2.1 \times 10^{10} \times \left[Δ{\rm V}/315 \rm{km/s} \right]^{1/2} \textrm{M}_\odot$ on the average HI mass of the 857 stacked galaxies, the first direct constraint on the atomic gas mass of galaxies at $z > 1$. The implied limit on the average atomic gas mass fraction (relative to stars) is ${\rm M}_{\rm GAS}/{\rm M}_* < 0.5$, comparable to the cold molecular gas mass fraction in similar star-forming galaxies at these redshifts. We find that the cosmological mass density of neutral atomic gas in massive star-forming galaxies at $z \approx 1.3$ is $Ω_{\rm GAS} < 3.7 \times 10^{-4}$, significantly lower than $Ω_{\rm GAS}$ estimates in both galaxies in the local Universe and damped Lyman-$α$ absorbers at $z \geq 2.2$. Massive blue star-forming galaxies thus do not appear to dominate the neutral atomic gas content of the Universe at $z \approx 1.3$.

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arXiv2015

Invisible Active Galactic Nuclei. II Radio Morphologies & Five New HI 21 cm Absorption Line Detections

We have selected a sample of 80 candidates for obscured radio-loud active galactic nuclei and presented their basic optical/near-infrared (NIR) properties in Paper 1. In this paper, we present both high-resolution radio continuum images for all of these sources and HI 21cm absorption spectroscopy for a few selected sources in this sample. A-configuration 4.9 and 8.5 GHz VLA continuum observations find that 52 sources are compact or have substantial compact components with size <0.5" and flux density >0.1 Jy at 4.9 GHz. The most compact 36 sources were then observed with the VLBA at 1.4 GHz. One definite and 10 candidate Compact Symmetric Objects (CSOs) are newly identified, a detection rate of CSOs ~3 times higher than the detection rate previously found in purely flux-limited samples. Based on possessing compact components with high flux densities, 60 of these sources are good candidates for absorption-line searches. Twenty seven sources were observed for HI 21cm absorption at their photometric or spectroscopic redshifts with only 6 detections made (one detection is tentative). However, five of these were from a small subset of six CSOs with pure galaxy optical/NIR spectra and for which accurate spectroscopic redshifts place the redshifted 21cm line in a RFI-free spectral window. It is likely that the presence of ubiquitous RFI and the absence of accurate spectroscopic redshifts preclude HI detections in similar sources (only one detection out of the remaining 22 sources observed, 14 of which have only photometric redshifts). Future searches for highly-redshifted HI and molecular absorption can easily find more distant CSOs among bright, blank field' radio sources but will be severely hampered by an inability to determine accurate spectroscopic redshifts for them due to their lack of rest-frame UV continuum.

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arXiv2015

A Giant Metrewave Radio Telescope search for associated HI 21cm absorption in high-redshift flat-spectrum sources

We report results from a Giant Metrewave Radio Telescope search for "associated" redshifted HI 21cm absorption from 24 active galactic nuclei (AGNs), at $1.1 < z < 3.6$, selected from the Caltech-Jodrell Bank Flat-spectrum (CJF) sample. 22 out of 23 sources with usable data showed no evidence of absorption, with typical $3σ$ optical depth detection limits of $\approx 0.01$ at a velocity resolution of $\approx 30$~km~s$^{-1}$. A single tentative absorption detection was obtained at $z \approx 3.530$ towards TXS0604+728. If confirmed, this would be the highest redshift at which HI 21cm absorption has ever been detected. Including 29 CJF sources with searches for redshifted HI 21cm absorption in the literature, mostly at $z < 1$, we construct a sample of 52 uniformly-selected flat-spectrum sources. A Peto-Prentice two-sample test for censored data finds (at $\approx 3σ$ significance) that the strength of HI 21cm absorption is weaker in the high-$z$ sample than in the low-$z$ sample, this is the first statistically significant evidence for redshift evolution in the strength of HI 21cm absorption in a uniformly selected AGN sample. However, the two-sample test also finds that the HI 21cm absorption strength is higher in AGNs with low ultraviolet or radio luminosities, at $\approx 3.4 σ$ significance. The fact that the higher-luminosity AGNs of the sample typically lie at high redshifts implies that it is currently not possible to break the degeneracy between AGN luminosity and redshift evolution as the primary cause of the low HI 21cm opacities in high-redshift, high-luminosity active galactic nuclei.

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arXiv2015

Next Generation Very Large Array Memo No. 9 Science Working Group 4: Time Domain, Fundamental Physics, and Cosmology

We report here on key science topics for the Next Generation Very Large Array in the areas of time domain, fundamental physics, and cosmology. Key science cases considered are pulsars in orbit around the Galactic Center massive black hole, Sagittarius A*, electromagnetic counterparts to gravitational waves, and astrometric cosmology. These areas all have the potential for ground-breaking and transformative discovery. Numerous other topics were discussed during the preparation of this report and some of those discussions are summarized here, as well. There is no doubt that further investigation of the science case will reveal rich and compelling opportunities.

astro-ph.HE↗

arXiv2015

A new constraint on the molecular oxygen abundance at $z \sim 0.886$

We report Karl G. Jansky Very Large Array (VLA) and Atacama Large Millimeter Array (ALMA) spectroscopy in the redshifted molecular oxygen (O$_2$) 56.265~GHz and 424.763~GHz transitions from the $z=0.88582$ gravitational lens towards PKS\,1830$-$21. The ALMA non-detection of O$_2$ 424.763~GHz absorption yields the $3σ$ upper limit $N({\rm O}_2) \leq 5.8 \times 10^{17}$~cm$^{-2}$ on the O$_2$ column density, assuming that the O$_2$ level populations are thermalized at the gas kinetic temperature of 80~K. The VLA spectrum shows absorption by the CH$_3$CHO 56.185~GHz and 56.265~GHz lines, with the latter strongly blended with the O$_2$ 56.265~GHz line. Since the two CH$_3$CHO lines have the same equilibrium strength, we used the known CH$_3$CHO 56.185~GHz line profile to subtract out the CH$_3$CHO 56.265~GHz feature from the VLA spectrum, and then carried out a search for O$_2$ 56.265~GHz absorption in the residual spectrum. The non-detection of redshifted O$_2$ 56.265~GHz absorption in the CH$_3$CHO-subtracted VLA spectrum yields $N({\rm O}_2) \leq 2.3 \times 10^{17}$~cm$^{-2}$. Our $3σ$ limits on the O$_2$ abundance relative to H$_2$ are then $X({\rm O}_2) \leq 9.1 \times 10^{-6}$ (VLA) and $X({\rm O}_2) \leq 2.3 \times 10^{-5}$ (ALMA). These are $5-15$ times lower than the best previous constraint on the O$_2$ abundance in an external galaxy. The low O$_2$ abundance in the $z= 0.88582$ absorber may arise due to its high neutral carbon abundance and the fact that its molecular clouds appear to be diffuse or translucent clouds with low number density and high kinetic temperature.

astro-ph.GA↗

arXiv2015

First measurement of HI 21cm emission from a GRB host galaxy indicates a post-merger system

We report the detection and mapping of atomic hydrogen in HI 21cm emission from ESO 184-G82, the host galaxy of the gamma ray burst 980425. This is the first instance where HI in emission has been detected from a galaxy hosting a gamma ray burst. ESO 184-G82 is an isolated galaxy and contains a Wolf-Rayet region close to the location of the gamma ray burst and the associated supernova, SN 1998bw. This is one of the most luminous HII regions identified in the local Universe, with a very high inferred density of star formation. The HI 21cm observations reveal a high HI mass for the galaxy, twice as large as the stellar mass. The spatial and velocity distribution of the HI 21cm emission reveals a disturbed rotating gas disk, which suggests that the galaxy has undergone a recent minor merger that disrupted its rotation. We find that the Wolf-Rayet region and the gamma ray burst are both located in the highest HI column density region of the galaxy. We speculate that the merger event has resulted in shock compression of the gas, triggering extreme star formation activity, and resulting in the formation of both the Wolf-Rayet region and the gamma ray burst. The high HI column density environment of the GRB is consistent with the high HI column densities seen in absorption in the host galaxies of high redshift gamma ray bursts.

astro-ph.GA↗

arXiv2015

On Detecting Millisecond Pulsars at the Galactic Center

The lack of detected pulsars at the Galactic Center (GC) region is a long-standing mystery. We argue that the high stellar density in the central parsec around the GC is likely to result in a pulsar population dominated by millisecond pulsars (MSPs), similar to the situation in globular cluster environments. Earlier GC pulsar searches have been largely insensitive to such an MSP population, accounting for the lack of pulsar detections. We estimate the best search frequency for such an MSP population with present and upcoming broad-band radio telescopes for two possible scattering scenarios, the "weak-scattering" case suggested by the recent detection of a magnetar close to the GC, and the "strong-scattering" case, with the scattering screen located close to the GC. The optimal search frequencies are $\approx 8$ GHz (weak-scattering) and $\approx 25$ GHz (strong-scattering), for pulsars with periods 1-20 ms, assuming that GC pulsars have a luminosity distribution similar to that those in the rest of the Milky Way. We find that 10-30 hour integrations with the Very Large Array and the Green Bank Telescope would be sufficient to detect MSPs at the GC distance in the weak-scattering case. However, if the strong-scattering case is indeed applicable to the GC, observations with the full Square Kilometre Array would be needed to detect the putative MSP population.

astro-ph.HE↗

arXiv2015

A search for H$α$ emission in high-metallicity damped Lyman-$α$ systems at $z \sim 2.4$

We report on a sensitive search for redshifted H$α$ line-emission from three high-metallicity damped Ly$α$ absorbers (DLAs) at $z \approx 2.4$ with the Near-infrared Integral Field Spectrometer (NIFS) on the Gemini-North telescope, assisted by the ALTtitude conjugate Adaptive optics for the InfraRed (ALTAIR) system with a laser guide star. Within the NIFS field-of-view, $\approx 3.22" \times 2.92"$ corresponding to $\approx 25$ kpc $ \times 23$ kpc at $z=2.4$, we detect no statistically significant line-emission at the expected redshifted H$α$ wavelengths. The measured root-mean-square noise fluctuations in $0.4"$ apertures are $1-3\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$. Our analysis of simulated, compact, line-emitting sources yields stringent limits on the star-formation rates (SFRs) of the three DLAs, $< 2.2$~M$_{\odot}$ yr$^{-1}$ ($3σ$) for two absorbers, and $< 11$~M$_{\odot}$ yr$^{-1}$ ($3σ$) for the third, at all impact parameters within $\approx 12.5$~kpc to the quasar sightline at the DLA redshift. For the third absorber, the SFR limit is $< 4.4$~M$_\odot$ yr$^{-1}$ for locations away from the quasar sightline. These results demonstrate the potential of adaptive optics-assisted, integral field unit searches for galaxies associated with high-$z$ DLAs.

astro-ph.GA↗

arXiv2014

Giant Metrewave Radio Telescope detection of two new HI 21cm absorbers at $z \approx 2$

I report the detection of HI 21cm absorption in two high column density damped Lyman-$α$ absorbers (DLAs) at $z \approx 2$ using new wide-band $250-500$ MHz receivers onboard the Giant Metrewave Radio Telescope. The integrated HI 21cm optical depths are $0.85 \pm 0.16$ km/s (TXS1755+578) and $2.95 \pm 0.15$ km/s (TXS1850+402). For the $z=1.9698$ DLA towards TXS1755+578, the difference in HI 21cm and CI profiles and the weakness of the radio core suggest that the HI 21cm absorption arises towards radio components in the jet, and that the optical and radio sightlines are not the same. This precludes an estimate of the DLA spin temperature. For the $z = 1.9888$ DLA towards TXS1850+402, the absorber covering factor is likely to be close to unity, as the background source is extremely compact, with all the 5 GHz emission arising from a region of size $\leq 1.4$ mas. This yields a DLA spin temperature of ${\rm T_s} = (372 \pm 18) \times (f/1.0)$ K, lower than typical ${\rm T_s}$ values in high-$z$ DLAs. This low spin temperature and the relatively high metallicity of the $z = 1.9888$ DLA ([Zn/H] $= (-0.68 \pm 0.04)$) are consistent with the anti-correlation between metallicity and spin temperature that has been earlier found in damped Lyman-$α$ systems.

astro-ph.GA↗