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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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    Quasars probing intermediate redshift star-forming galaxies
    (2009-12-01) Noterdaeme, P.; Srianand, R.; Mohan, V.; et al.
    We present a sample of 46 [Oiii]-emitting galaxies at z < 0.8 detected in the fibre spectra of quasars from the Sloan Digital Sky Survey, Data Release 7 (SDSS-DR7) through an automatic search procedure. We also detect [Oii] and Hβ emission lines from most of these galaxies in the SDSS spectra. We study both the emission and absorption properties of a sub sample of 17 galaxies in the redshift range z = 0.4-0.7, where Mg ii lines are covered by the SDSS spectra. The measured lower-limits on the star-formation rates of these galaxies are in the range 0.2-20 M⊙ yr −1 . The emission line luminosities and (O/H) metallicities from R23 measured in this sample are similar to what is found in normal galaxies at these redshifts. Thus, this constitutes a unique sample of intermediate redshift star-forming galaxies where we can study the QSO absorber - galaxy connection. Strong Mg ii (Wλ2796> ∼ 1 Å) as well as Mg i absorption lines are detected in the QSO spectra at the redshift of most of these galaxies. Strong Fe ii (Wλ2600 > 1 Å) absorption lines are also generally detected whenever the appropriate wavelength ranges are covered. This suggests that most of these systems could be bona-fide Damped Lyman-α systems. We investigate various possible relations between the Mg ii rest equivalent widths and the emission line properties.We find a possible (2σ) correlation between the emission-line metallicity of the galaxies and the Mg ii rest equivalent width of the absorbers (log(O/H) + 12 = 0.1Wλ2796 + 8.27), which could be a consequence of an underlying mass-metallicity relation. However, [Oiii]-selected Mg ii systems represent only a minor fraction of the strongMg ii absorbers.We find this cannot be attributed to biases related either to the spectral signal-to-noise ratio or to the brightness of the QSOs. We measure the average observed fluxes (collected into the SDSS fibre) of the [Oii] and [Oiii] lines associated to Mg ii-selected systems through stacking technique.We find that the average lumiosities of emission lines are higher for systems with larger Wλ2796. The stacked luminosities are found to be below the typical detection limit in individual spectra, indicating that faint galaxies can contribute appreciably to the observed population of strong Mg ii absorbers at intermediate redshifts.We also present long-slit spectroscopic observations of SDSS J113108+202151, the most luminous line-emitting galaxy in our z ≥ 0.4 sample. Surprisingly, we find that the line-emitting region does not coincide with the nearby extended bright galaxy with consistent photometric redshift seen in the SDSS image.