2008 (IPP)

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    Detection of the 2175\AA~ extinction feature and 21-cm absorption in two MgII systems at z~1.3
    (2008-09) Srianand, R.
    We have discovered two dusty intervening Mg ii absorption systems at z ∼ 1.3 in the Sloan Digital Sky Survey (SDSS) database. The overall spectra of both QSOs are red (u-K>4.5 mag) and are well modelled by the composite QSO spectrum reddened by the extinction curve from the Large Magellanic Cloud(LMC2) Supershell redshifted to the rest-frame of the Mg ii systems. In particular, we detect clearly the presence of the UV extinction bump at λrest ∼ 2175 ˚ A. Absorption lines of weak transitions like Si iiλ1808, Cr iiλ2056, Cr ii+Zn iiλ2062, Mn iiλ2594, Ca iiλ3934 and Ti iiλ1910 from these systems are detected even in the low signal-to-noise ratio and low resolution SDSS spectra, suggesting high column densities of these species. The depletion pattern inferred from these absorption lines is consistent with that seen in the cold neutral medium of the LMC. Using the LMC AV vs. N(H i) relationship we derive N(H i)∼ 6×1021 cm−2 in both systems. Metallicities are close to solar. Giant Metrewave Radio Telescope (GMRT) observations of these two relatively weak radio loud QSOs (fν ∼ 50 mJy) resulted in the detection of 21-cm absorption in both cases.We show that the spin temperature of the gas is of the order of or smaller than 500 K. These systems provide a unique opportunity to search for molecules and diffuse interstellar bands at z > 1.
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    Dielectronic recombination and stability of warm gas in active galactic nuclei
    (2008-09) Chakravorty, Susmita; Kembhavi, A.K.
    Understanding the thermal equilibrium (stability) curve may offer insights into the nature of the warm absorbers often found in active galactic nuclei. Its shape is determined by factors such as the spectrum of the ionizing continuum and the chemical composition of the gas. We find that the stability curves obtained under the same set of the above-mentioned physical factors, but using recently derived dielectronic recombination rates, give significantly different results, especially in the regions corresponding towarmabsorbers, leading to different physical predictions. Using the current rates we find a larger probability of having a thermally stable warmabsorber at 105 Kthan previous predictions and also a greater possibility for itsmultiphase nature. The results obtained with the current dielectronic recombination rate coefficients are more reliable because the warm absorber models along the stability curve have computed coefficient values, whereas previous calculations relied on guessed averages for these because of a lack of available data.