Browsing by Author "Gupta, N."

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21-cm absorbers at intermediate redshifts
(2009-02-01) Gupta, N.; Srianand, R.; Petitjean, Patrick; et al.
Damped Lyman-α systems (DLAs) seen in the spectra of high-z QSOs allow us to probe the physical conditions in protogalaxies. Our understanding of physical conditions in DLAs at high-z is primarily based on the absorption lines of H2 molecules and ﬁne-structure transitions. Another important way of probing the thermal state of interstellar medium in these systems is by studying the 21-cm absorption in the spectra of background quasars. Here we report the main results of our GMRT survey to search for 21-cm absorption in a representative and unbiased sample of 35 DLA candidates at 1.10≤z≤1.45. Our sample of DLA candidates is drawn from the strong Mg ii systems in SDSS DR5 and has resulted in discovery of 9 new 21-cm absorbers. Prior to our survey only one 21-cm absorber was known in the redshift range: 0.7≤z≤2. This survey has allowed us to investigate the dependence of detectability of 21-cm absorption on the properties of UV absorption lines detected in SDSS spectra and estimate the number per unit redshift of 21-cm absorbers. Our GMRT survey provides a representative sample of systems that can be used in combination with various follow-up observations: (1) for investigating the physical conditions in the absorbing gas using spin temperature (TS) measurements, (2) for investigating the eﬀect of metallicity and dust content on the detectability of 21-cm absorption, (3) for studying the morphology of the absorbing gas and (4) for probing the time evolution of various fundamental constants. Results from the ﬁrst phase of our survey are presented in Gupta et al. (2007). Detailed description of the entire sample and results from the survey are presented in Gupta et al. (2009).
Complete sample of 21-cm absorbers at z ~ 1.3: Giant Metrewave Radio Telescope Survey Using Mg II Systems
(2009-04-01) Gupta, N.; Srianand, R.; Petitjean, Patrick; et al.
We present the results of a systematic Giant Metrewave Radio Telescope (GMRT) survey of 21-cm absorption in a representative and unbiased sample of 35 strong Mg ii systems in the redshift range: zabs∼1.10−1.45, 33 of which have Wr ≥1˚ A. The survey using ∼400 hrs of telescope time has resulted in 9 new 21-cm detections and stringent 21-cm optical depth upper limits (median 3σ optical depth per 10 kms−1 of 0.017) for the remaining 26 systems. This is by far the largest number of 21-cm detections from any single survey of intervening absorbers. Prior to our survey no intervening 21-cm system was known in the above redshift range and only one system was known in the redshift range 0.7 ≤ z ≤ 1.5. We discuss the relation between the detectability of 21-cm absorption and various properties of UV absorption lines. We show that if Mg ii systems are selected with the following criteria, Mg ii doublet ratio ≤1.3 and Wr(Mg i)/Wr(Mg ii) ≥ 0.3, then a detection rate of 21-cm absorption up to 90% can be achieved. We estimate n21, the number per unit redshift of 21-cm absorbers with Wr(Mg ii) > Wo and integrated optical depth T21 > To and show that n21 decreases with increasing redshift. In particular, for Wo = 1.0 ˚ A and To > 0.3 km s−1, n21 falls by a factor 4 from < z > = 0.5 to < z > = 1.3. The evolution seems to be stronger for stronger Mg ii systems. Using a subsample of systems for which high frequency VLBA images are available, we show that the eﬀect is not related to the structure of the background radio sources and is most probably due to the evolution of the cold neutral medium ﬁlling factor in Mg ii systems. We ﬁnd no correlation between the velocity spread of the 21-cm absorption feature and Wr(Mg ii) at z ∼ 1.3
GMRT mini-survey to search for 21-cm absorption in quasar-galaxy pairs at z~0.1
(2010-07-05) Gupta, N.; et al.; Srianand, R.
We present the results from our 21-cm absorption survey of a sample of 5 quasar-galaxy pairs (QGPs), with the redshift of the galaxies in the range 0.03≤ zg ≤ 0.18, selected from the Sloan Digital Sky Survey (SDSS). The H i 21-cm absorption was searched towards the 9 sight lines with impact parameters ranging from ∼10 to ∼55 kpc using Giant Metrewave Radio Telescope (GMRT). 21-cm absorption was detected only in one case i.e. towards the Quasar (zq = 2.625SDSS J124157.54+633241.6) − galaxy (zg = 0.143 SDSS J124157.26+633237.6) pair with the impact parameter ∼11 kpc. The quasar sight line in this case pierces through the stellar disk of a galaxy having near solar metallicity (i.e (O/H)+12 = 8.7) and star formation rate uncorrected for dust attenuation of 0.1M⊙ yr−1. The quasar spectrum reddened by the foreground galaxy is well ﬁtted with the Milky Way extinction curve (with an AV of 0.44) and the estimated H i column density is similar to the value obtained from 21-cm absorption assuming spin temperature (TS) of 100K. In the remaining cases, our GMRT spectra provide upper limit on N(H i) in the range, (1017 −1018)×TS cm−2. Combining our sample with the z ≤ 0.1 data available in the literature, we ﬁnd the detectability of 21-cm absorption with integrated optical depth greater than 0.1 kms−1 to be 50% for the impact parameter less than 20 kpc. Using the surface brightness proﬁles and well established relationship between the optical size and extent of the H i disk known for nearby galaxies, we conclude that in most of the cases of 21-cm absorption non-detection, the sight lines may not be passing through the H i gas (1σ column density of few times 1019 cm−2). We also ﬁnd that in comparison to the absorption systems associated with these QGPs, z < 1 DLAs with 21-cm absorption detections have lower Ca ii equivalent widths despite having higher 21-cm optical depths and smaller impact parameters. This suggests that the current sample of DLAs may be a biased population that avoids sight lines through dusty star-forming galaxies. A systematic survey of QGPs over a wider redshift range using a large sample is needed to conﬁrm these ﬁndings and understand the nature of 21-cm absorbers.