High Redshift Galaxies

(Jayaram N. Chengalur, Nissim Kanekar, Aditya Chowdhury, Yogesh Wadadekar, Omkar S. Bait, Preetish K. Mishra, Apurba Bera, Former members: D. J. Saikia, J. N. H. S. Aditya)

Understanding the nature of high-redshift galaxies, as well as galaxy evolution, is an important research area at NCRA-TIFR. Astronomers here use diverse methods, in both emission and absorption, and over a wide range of observing frequencies, to probe physical conditions in high-redshift galaxies. The techniques used include neutral hydrogen (HI) 21cm absorption studies, HI 21cm emission studies of individual galaxies, "stacking" of HI 21cm emission, millimetre-wave carbon monoxide (CO) emission and absorption studies, ionized carbon (C+) emission studies, hydroxyl (OH) 18cm absorption studies, radio continuum studies, optical and ultraviolet imaging and spectroscopy, etc.. Some of the questions being addressed by NCRA-TIFR astronomers are discussed in more detail below.


Damped Lyman-alpha Absorbers (DLAs)
Damped Lyman-alpha absorbers (DLAs) are absorption-selected galaxies, identified by the presence of strong Lyman-alpha absorption in quasar spectra. Since these galaxies are detected by their absorption signatures, DLA samples contain no bias towards brighter galaxies (unlike emission-selected galaxy samples); understanding the nature of DLAs has hence long been considered an important issue in galaxy evolution. 

For DLAs towards radio-loud quasars, HI 21cm absorption studies allow one to measure the spin temperature of the absorbing gas and the cold gas content of the absorber. NCRA-TIFR astronomers have long used low-frequency radio telescopes like the Giant Metrewave Radio Telescope (GMRT) and the Green Bank Telescope (GBT) to carry out deep searches for HI 21cm absorption in DLAs, obtaining the majority of the detections of HI 21cm absorption in high-z DLAs as well as lower limits to the spin temperature in a large number of these galaxies. They have used optical spectroscopy to measure the metallicity of the absorbers, and have connected the high spin temperatures in DLAs to the paucity of metals in these galaxies. They have also extended HI 21cm absorption studies to lower redshifts, targetting strong MgII absorption systems, allowing the identification of DLAs via their HI 21cm absorption and providing the first understanding of the redshift evolution of the cold gas fraction in normal galaxies.

HI 21cm emission studies of galaxies allow one to directly measure their atomic gas mass, a critical input to describing galaxies and understanding their evolution. Follow-up interferometric mapping studies allow one to measure the velocity field of the galaxies, and infer their dynamical mass. Unfortunately, the weakness of the HI 21cm line has meant that such studies have only been possible at very low redshifts. NCRA-TIFR astronomers have led searches for HI 21cm emission in low-redshift DLAs, finding HI 21cm emission in a number of DLAs at z<0.1. They are now using the GMRT to map the HI 21cm emission in some of these galaxies.

CO emission studies of galaxies allow one to estimate their molecular gas mass, and, with follow-up mapping studies, the velocity field. NCRA-TIFR astronomers have been using such CO emission studies of high-z DLAs with the Atacama Large Millimeter/submillimeter Array (ALMA), the Very Large Array (VLA) and the NOrth European Millimetre Array (NOEMA) to measure the molecular gas masses of the DLA host galaxies. Remarkably, the ALMA and VLA studies have yielded extremely high molecular gas masses for a number of high-metallicity DLAs at both intermediate and high redshifts, raising questions on our understanding of the absorber host galaxies. 

Identifying the host galaxies of high-redshift DLAs has been an important open problem in galaxy evolution for thirty years. The difficulty lies in the fact that one has to detect a faint foreground object (the absorber host) in the presence of a far brighter object (the background quasar). NCRA-TIFR astronomers have used ALMA studies in the redshifted ionized carbon (C+) 158-micron line and the dust continuum as a new tool to identify and study the absorber host galaxies of DLAs at z~4. Initial ALMA studies have shown that the DLA hosts have relatively high star formation rates and C+ line luminosities, and large sizes.


Stacking HI 21cm emission from high-redshift galaxies
Measuring the atomic gas mass of high-z galaxies requires the detection of their HI 21cm line emission. Unfortunately, the weakness of the HI 21cm line has meant that HI 21cm emission has so far only been detected out to relatively low redshifts, z~0.38. NCRA-TIFR astronomers use "stacking" of HI 21cm emission from galaxies with known optical redshifts to extend such HI 21cm emission searches to much higher redshifts, to measure the average gas mass of galaxy samples, as well as the cosmological gas mass density in such galaxies. This is an exciting research area today at NCRA-TIFR, due to the new wideband receivers, new correlator and high sensitivity of the upgraded GMRT


Gas in AGN environments
HI 21cm absorption studies of radio-loud active galactic nuclei (AGNs) allow one to probe the presence of, and physical conditions in, neutral hydrogen in AGN environments. The detection of HI 21cm absorption from such "associated" gas allows one to measure the velocity of the gas relative to the AGN. Such studies thus also provide information on whether the gas is predominantly falling into (and thus, fuelling) the AGN, or is being driven out into the intergalactic medium by the AGN. The wide-band receivers of the upgraded GMRT will allow such associated HI 21cm absorption studies to track the redshift evolution of gas conditions in the AGN environments over a wide variety of AGN types.


Molecules in high-redshift galaxies
NCRA-TIFR astronomers use both molecular absorption and emission studies of high-redshift galaxies to obtain information on physical conditions in these objects. Such studies include searches for OH 18cm megamasers in ultra-luminous infra-red galaxies, blind surveys for CO and HCO+ absorption at high redshifts, searches for OH absorption in red quasars, searches for CO emission in Lyman-alpha emitters and Lyman-break galaxies, searches for molecular oxygen absorption in high-z galaxies, etc.

Recent Results
HI 21-centimetre emission from an ensemble of galaxies at an average redshift of one
Baryonic processes in galaxy evolution include the infall of gas onto galaxies to form neutral atomic hydrogen (HI), which is then converted to the molecular state (H2), and, finally, the conversion of H2 to stars. Understanding galaxy evolution thus requires an understanding of the evolution of stars and of neutral atomic and molecular hydrogen. For the stars, the cosmic star-formation rate density is known to peak at redshifts between 1 and 3, and to decline by an order of magnitude over the subsequent 8 billion years; the causes of this decline are not known. For the gas, the weakness of the hyperfine transition of HI at 21 cm wavelength — the main tracer of the HI content of galaxies—means that it has not hitherto been possible to measure the atomic gas mass of galaxies at redshifts higher than about 0.4; this is a critical gap in our understanding of galaxy evolution. Chowdhury et. al. report a measurement of the average HI mass of star-forming galaxies at a redshift of about one, obtained by stacking the HI 21 cm emission signals from 7,653 galaxies over a 1.2 square degree region of the sky. The figure shows [A] the stacked HI 21 cm emission map and [B] the stacked HI 21 cm emission spectrum; the detection of the average 21cm emission signal can be clearly seen in both panels. The measured average HI mass of the sample of galaxies at z~1 is similar to the average stellar mass of the sample but the HI mass can fuel the observed star-formation rates for only 1 to 2 billion years in the absence of fresh gas infall. This suggests that gas accretion onto galaxies at redshifts of less than one may have been insufficient to sustain high star-formation rates in star-forming galaxies. This is likely to be the cause of the decline in the cosmic star-formation rate density at redshifts below one.
High Molecular Gas Masses in Absorption-selected Galaxies at z ~ 2
Kanekar et al. used the Atacama Large Millimeter/submillimeter Array (ALMA) to carry out a search for CO (3-2) or (4-3) emission from the fields of 12 high-metallicity damped Lyman-alpha absorbers (DLAs) at z~1.7-2.6. They detected CO emission from galaxies in the fields of five DLAs, obtaining high molecular gas masses, in the range (13 - 210) billion solar masses. The impact parameters of the CO emitters to the QSO sightline lie in the range 5.6-100 kpc, with the three new CO detections having impact parameters <~ 15 kpc. The highest CO line luminosities and inferred molecular gas masses are associated with the highest-metallicity DLAs, with metallicities within a factor of 2 of the solar metallicity. The high inferred molecular gas masses may be explained by a combination of a stellar mass-metallicity relation and a high molecular gas-to-stars mass ratio in high-redshift galaxies; the DLA galaxies identified by the authors' CO searches have properties consistent with those of emission-selected samples. None of the DLA galaxies detected in CO emission were identified in earlier optical or near-IR searches and vice-versa; DLA galaxies earlier identified in optical/near-IR searches were not detected in CO emission. The high ALMA CO and [CII] 158-micron line detection rate in high-redshift, high-metallicity DLA galaxies has revolutionized the field, allowing the identification of dusty, massive galaxies associated with high-redshift DLAs. The HI-absorption criterion identifying DLAs selects the entire high-redshift galaxy population, including dusty and UV-bright galaxies, in a wide range of environments. The left panel of the figure shows the CO line luminosity (in logarithmic units) plotted against the absorber metallicity; the higher CO line luminosity at [M/H]>= -0.3 dex is clear. The right panel plots metallicity against stellar mass (assumed to be equal to the molecular gas mass), with CO detections shown as filled blue circles and CO non-detections as open blue circles. The filled black squares show the (binned) emission metallicity plotted against the (binned) stellar mass for the UV-selected galaxies of Erb et al. (2006), while the dashed red curve shows the mass-metallicity relation of these galaxies. Three DLA galaxies identified via optical spectroscopy are shown as red stars, with stellar mass estimates from the optical/near-IR photometry.
High-opacity associated HI 21cm absorbers at z~1.2
HI 21cm absorption arising from neutral hydrogen in the environments of Active Galactic Nuclei (AGNs) can be used to probe physical conditions in the AGN’s vicinity and how these conditions change over time. However, despite a large number of searches spanning many decades, only 7 such ``associated'' HI 21cm absorbers have been detected at redshifts greater than one. Chowdhury et al. used the new wide-band GMRT Band-4 receivers to discover two remarkable HI 21cm absorbers at a high redshift, z~1.2, against faint AGNs. The two absorbers were found in an unbiased search for HI 21cm absorption against all radio-continuum sources over a 1.2 square degree region of the sky, the first time that such a search has been carried out with a high sensitivity at high redshifts. The velocity-integrated HI 21cm optical depths of the two systems, shown in the figure, are greater than those of any known associated HI 21cm absorbers, and the two AGNs are very faint in both the radio and the ultraviolet wavebands. The discovery of these two systems is consistent with an earlier hypothesis that the dearth of associated HI 21cm absorbers at high redshifts, z>1, may be due to an observational bias wherein high-redshift AGNs targeted in surveys for associated HI 21cm absorbers are bright in the ultraviolet and radio wavelengths. The high AGN radio and/or ultraviolet luminosity may ionize or excite the HI in its vicinity, reducing the strength of the HI 21cm absorption. The two new HI 21cm absorbers emphasize the need to carry out unbiased HI 21cm absorption surveys and to extend future searches to low luminosity AGNs.
ALMA [CII] 158um imaging of an HI-Selected Major Merger at z~4
Prochaska et al. used the Atacama Large Millimeter/submillimeter Array (ALMA) to obtain high spatial resolution (~ 2 kpc) observations of [CII] 158-micron and dust-continuum emission from a galaxy at z=3.7978 selected by its strong Lyman-alpha absorption (a damped Ly-alpha absorber, DLA) against a background QSO. Their ALMA images reveal a pair of star-forming galaxies separated by approximately 6 kpc (projected) undergoing a major merger. Between these galaxies is a third emission component with highly elevated [CII] 158-micron A emission relative to the dust continuum (by a factor of ~2), which is likely to arise from stripped gas associated with the merger. This merger of two otherwise-normal galaxies is not accompanied by enhanced star-formation, contrary to mergers detected in most luminosity-selected samples. The DLA associated with the merger exhibits extreme kinematics, with a velocity width for the low-ionization metal lines of roughly 470 km/s, that spans the velocity spread revealed in the [CII] 158-micron emission. The authors propose that DLAs with high low-ionization metal line widths are a signpost of major mergers in normal galaxies at high redshifts, and use the distribution of the velocity widths of metal lines in high-z DLAs to provide a rough estimate of the fraction of z>3 galaxies that are undergoing a major merger. Figure: (Left) The zeroth-moment image showing the integrated [CII] 158-micron flux density of the galaxies A and B associated with the absorption-selected galaxy. The axes are labeled in physical units (kpc) at z=3.7978. The lowest contour corresponds to 3-sigma significance, with the contours increasing by a factor of sqrt(2). (right) The first-moment image showing the flux density-weighted velocity field, restricted to regions where the integrated flux density exceeds 2.5-sigma significance. There are two kinematically distinct components which we associate with a pair of merging galaxies, labeled A and B. There is additional emission between these components, referred to as component C, which appears to be gas stripped during the merger.
A cold, massive, rotating disk galaxy 1.5 billion years after the Big Bang
Massive disk galaxies like the Milky Way are expected to form at late times in traditional models of galaxy formation, but recent numerical simulations suggest that such galaxies could form as early as a billion years after the Big Bang through the accretion of cold material and mergers. Observationally, it has been difficult to identify disk galaxies in emission at high redshift in order to discern between competing models of galaxy formation. Neeleman et al. report Atacama Large Millimeter/submillimeter Array (ALMA) imaging, with a resolution of about 1.3 kiloparsecs, of the 158-micrometre emission line from singly ionized carbon ([CII]), the far-infrared dust continuum and the near-ultraviolet continuum emission from a galaxy at a redshift of 4.2603, identified by detecting its absorption of quasar light. The ALMA observations show that the emission arises from gas inside a cold, dusty, rotating disk with a rotational velocity of about 272 kilometres per second. The detection of emission from carbon monoxide in the galaxy yields a molecular mass that is consistent with the estimate from the ionized carbon emission of about 72 billion solar masses. The existence of such a massive, rotationally supported, cold disk galaxy when the Universe was only 1.5 billion years old favours formation through either cold-mode accretion or mergers, although its large rotational velocity and large content of cold gas remain challenging to reproduce with most numerical simulations. Figure: The top row shows the velocity-integrated [CII] flux density for the data (left panel), the constant rotational velocity model (middle panel) and the residual after subtracting the model from the data (right panel). The outer contour is at 3 sigma, where sigma is the standard deviation of the noise in the observations, with contours increasing in powers of sqrt(2). No negative contours at the same levels are observed in the image. The synthesized beam of the observations is shown in the bottom left corner of the leftmost plot. The bottom row shows the mean velocity of the [CII] emission, for the data (left panel), the model (middle) and the residuals (right). Velocities are relative to the systemic velocity of the [CII] emission, corresponding to z = 4.2603.
Atomic hydrogen in star-forming galaxies at intermediate redshifts
Bera et al. used the upgraded Giant Metrewave Radio Telescope to carry out a deep observation of one of the well-known optical deep fields, the Extended Groth Strip, (EGS) covering the frequency range 1000-1370 MHz. This enabled a sensitive search for the hyperfine HI 21cm line from neutral atomic hydrogen (HI) in galaxies in the EGS, in the redshift range z~0.05-0.4. Bera et al. stacked (i.e. averaged) the HI 21cm emission signals from 445 blue star-forming galaxies in the EGS at 0.2<z<0.4 to infer their average HI gas mass, obtaining an average HI mass of (4.93 +/- 0.70) × 10^9 solar masses at a mean redshift of <z>=0.34. This implies a ratio of average gas mass to average stellar mass of ~1.2 for star-forming galaxies at these redshifts, higher than the corresponding value in the local Universe. The author also stacked the rest-frame 1.4 GHz radio continuum emission of the same galaxies, and then used a relation between the 1.4 GHz radio luminosity and the star formation rate (SFR) to obtain a median SFR of (0.54 +/- 0.06) solar masses per year for the galaxies of the sample. If the galaxies continue to form stars at the same rate, their average HI content would be exhausted on a timescale of ~9 Gyr, consistent with values in star-forming galaxies in the local Universe. This suggests that the star-formation efficiency in blue star-forming galaxies has not changed significantly over the last ~4 Gyr. Finally, Bera et al. used the stacked HI 21 cm emission signal to infer the cosmic HI mass density in star-forming galaxies at z=0.2-0.4, obtaining a normalized cosmic HI density of (4.81 +/- 0.75) x 10^−4 at <z>=0.34. This is the first accurate measurement of the cosmic HI density at intermediate redshifts z~0.2-1.8, and indicates no significant evolution in the cosmic HI density from z~0.4 to the present epoch. The top panel of the figure shows the average HI 21cm emission profile of the 445 blue star-forming galaxies whose spectra were stacked together. The bottom panel shows the evolution of cosmic HI density from z~5 to today, with the blue star showing the measurement from the present study.
Ionized carbon 158 micron Emission from z~4 HI Absorption-selected Galaxies
Neeleman, Kanekar et al. have used the Atacama Large Millimeter/sub-millimeter Array (ALMA) to carry out a search for the ionized carbon ([CII]) 158 micron emission line from galaxies associated with four high-metallicity damped Ly-alpha absorbers (DLAs) at z~4. They detected [CII] 158 micron emission from galaxies at the DLA redshift in three fields, with one field showing two [CII] emitters. Combined with previous results, Neeleman et al. have now detected [CII] 158 micron emission from five of six galaxies associated with targeted high-metallicity DLAs at z~4. The galaxies have relatively large impact parameters, ~ 16-45 kpc, [CII] 158 micron line luminosities of 0.04-3 billion solar luminosities, and rest-frame far-infrared properties similar to those of luminous Lyman-break galaxies, with star formation rates of ~7-110 solar masses per year. Comparing the absorption and emission line profiles yields a remarkable agreement between the line centroids, indicating that the DLA traces gas at velocities similar to that of the [CII] 158 micron emission. This disfavours a scenario where the DLA arises from gas in a companion galaxy. These observations highlight ALMA’s unique ability to uncover a high-redshift galaxy population that has largely eluded detection for decades. The figure shows (top panels) the integrated [CII] 158 micron flux density maps over channels containing line emission and (bottom panels) 350 GHz continuum images of the four quasar fields (with the plus sign indicating the quasar position). For the sole [CII] 158 micron non-detection, the [CII] line flux density is integrated over the central 100 km/s around the DLA redshift.
Massive Absorption-selected Galaxies at Intermediate Redshifts
The nature of absorption-selected galaxies and their connection to the general galaxy population have been open issues for more than three decades, with little information available on their gas properties. Kanekar et al. used detections of carbon monoxide (CO) emission with the Atacama Large Millimeter/submillimeter Array to show that five of seven high-metallicity, absorption-selected galaxies at intermediate redshifts, z ~ 0.5–0.8, have extremely large molecular gas masses and high molecular gas fractions relative to stars. Their modest star formation rates then imply long gas depletion timescales, an order of magnitude larger than typical in star-forming galaxies. High-metallicity absorption-selected galaxies at z ~ 0.5–0.8 thus appear distinct from populations of star-forming galaxies at both z ~ 1.3–2.5, during the peak of star formation activity in the Universe, and low redshifts, z < 0.05. Their relatively low SFRs, despite the large molecular gas reservoirs, may indicate a transition in the nature of star formation at intermediate redshifts, z ~ 0.7. The figure shows the five CO detections (in contours), with the position of the background quasar indicated by a star in each panel.
The gas and stellar mass of low-redshift damped Lyman-alpha absorbers
Kanekar et al. report Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet and Arecibo Telescope HI 21cm spectroscopy of six damped and sub-damped Lyman-alpha absorbers (DLAs and sub-DLAs, respectively) at z<~0.1, which have yielded estimates of their HI 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, they obtain HI masses ~(0.24-5.2) billion solar masses, lower than the knee of the local HI mass function. This implies that massive galaxies do not dominate the absorption cross-section for low-z DLAs. Kanekar et al. use Sloan Digital Sky Survey photometry and spectroscopy to identify the likely hosts of four absorbers, obtaining low stellar masses, ~(0.01-0.3) billion solar masses in all cases, consistent with the hosts being dwarf galaxies. They obtain high HI 21cm or CO emission line widths, ~ 100-290 km/s, and high gas fractions, ~5-100, suggesting that the absorber hosts are gas-rich galaxies with low star formation efficiencies. However, the HI 21cm velocity spreads (>~ 100 km/s) appear systematically larger than the velocity spreads in typical dwarf galaxies. The figure shows the Arecibo HI 21cm spectra for the six galaxies of the paper.
The Gas Mass of Star-forming Galaxies at z~1.3
Kanekar et al. used the GMRT 610 MHz receivers to carry out a search for HI 21cm emission from a large sample of massive star-forming galaxies at z~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. The non-detection of a signal in the stacked HI 21cm spectrum yielded a stringent upper limit of 2.5 microJy on the average HI 21cm flux density of the 857 galaxies, at a velocity resolution of 315 km/s. This implies an upper limit of 20 billion solar masses on the average HI mass of the 857 galaxies, the first direct constraint on the atomic gas mass of galaxies at z>0.5. The upper limit to the ratio of the atomic gas mass to the stellar mass, i.e. the gas fraction, is 0.5, comparable to the cold molecular gas fraction in similar galaxies at these redshifts. Kanekar et al. find that the cosmological mass density of neutral atomic gas in massive star-forming galaxies at z~1.3 is significantly lower than the mass density estimates in both galaxies in the local Universe and damped Lyman-alpha absorbers at z>2. This implies that massive blue star-forming galaxies do not dominate the neutral atomic gas content of the Universe at z~1.3. The figure shows the cosmological mass density in neutral gas plotted as a function of redshift. The open star shows the new GMRT result, for blue star-forming galaxies at z~1.3. See the paper for more details.
Ionized carbon and dust emission from high-redshift galaxies
Gas surrounding high-redshift galaxies has been studied through observations of damped Lyman-alpha absorbers 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 (at optical wavelengths) of the foreground galaxies compared with the background quasars. Neeleman et al. used the Atacama Large Millimeter/Submillimeter Array to obtain the first detections of ionized carbon ([CII]) 158-micron line and dust-continuum emission from two galaxies associated with damped Lyman-alpha absorbers at very high redshifts, z~4. The two upper panels of the figure show the dust continuum emission from the galaxies, while the lower panels show the [CII] 158-micron line emission. The results indicate that the host galaxies of the two absorbers are massive, dusty and rapidly star-forming systems. The hosts appear to be embedded in enriched neutral hydrogen gas reservoirs that extend well beyond the star-forming interstellar medium of the galaxies. The figure shows the two detections of ionized carbon (bottom panels) and dust continuum emission (top panels) from the two DLAs at z~4.
The spin temperature of high-redshift damped Lyman-alpha systems
Kanekar et al. report results from a large programme aimed at investigating the temperature of neutral gas in high-redshift damped Lyman-alpha absorbers (DLAs). This involved (1) HI 21cm absorption studies of a large sample of DLAs towards radio-loud quasars, to measure the spin temperature (2) very long baseline interferometric studies to measure the low-frequency quasar core fractions, and (3) optical/ultraviolet spectroscopy to determine DLA metallicities and the velocity widths of low-ionization metal lines. Kanekar et al. found a statistically significant difference between the spin temperature distributions in the high-redshift (z > 2.4) and low-redshift (z < 2.4) DLA samples: the high-z sample contains more DLAs with high spin temperature, >~ 1000 K. The high DLA spin temperatures arise due to low fractions of the cold neutral medium (CNM): only two of 23 DLAs at z > 1.7 have CNM fractions > 20%, comparable to the median value (~ 27%) in the Milky Way. Kanekar et al. robustly confirmed the presence of an anti-correlation between spin temperature and metallicity [Z/H], via a non-parametric Kendall-tau test. The data thus appear to indicate that high-redshift DLAs have significantly larger fractions of the warm phase of neutral hydrogen than is present in the Milky Way and local spiral galaxies, probably because the paucity of metals in the absorbers implies a lack of cooling routes in the absorber host galaxies. The figure shows the spin temperature of the DLAs of the sample plotted versus redshift (left panel) and metallicity (right panel). The left panel shows that there is a higher fraction of DLAs with high spin temperatures at high redshifts, z>1.7. The anti-correlation between spin temperature and metallicity is clearly visible in the right panel.
GMRT detections of HI 21cm absorption in two damped Lyman-alpha absorbers at z~2
Kanekar used the new 250-500 MHz receivers of the upgraded GMRT to detect redshifted HI 21cm absorption in two high column density damped Lyα absorbers (DLAs) at z ~ 2. Both absorbers have high inferred integrated HI 21cm optical depths, and hence low spin temperatures. However, for the z=1.9698 DLA toward TXS1755+578, the difference in HI 21cm and C I profiles and the weakness of the radio core suggest that the HI 21cm absorption arises toward 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 toward TXS1850+402, the HI 21cm absorption yields a DLA spin temperature ~ 372 K, lower than typical spin temperature values in high-z DLAs. This low spin temperature and the relatively high metallicity of the z=1.9888 DLA are consistent with the anti-correlation between metallicity and spin temperature that has been found earlier in damped Lyα systems. The figure shows the two new GMRT HI 21cm absorption detections.

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