2009 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/332

Browse

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    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).
  • No Thumbnail Available
    Item
    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