Detection of the 2175 angstrom extinction feature and 21-cm absorption in two Mg II systems at z similar to 1.3

作者：Srianand R^*; Gupta N; Petitjean P; Noterdaeme P; Saikia D J

来源：Monthly Notices of the Royal Astronomical Society, 2008, 391(1): L69-L73.

DOI：10.1111/j.1745-3933.2008.00558.x

• quasars: absorption lines
• quasar: individual: J085042.21+515911.7
• quasar: individual: J085244.74+343540.46

We have discovered two dusty intervening MgII absorption systems at z similar to 1.3 in the Sloan Digital Sky Survey (SDSS) data base. The overall spectra of both quasi-stellar objects (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 MgII systems. In particular, we detect clearly the presence of the ultraviolet (UV) extinction bump at lambda(rest) similar to 2175 angstrom. Absorption lines of weak transitions like Si II lambda 1808, Cr II lambda 2056, Cr II + Zn II lambda 2062, Mn II lambda 2594, Ca II lambda 3934 and Ti II lambda 1910 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 A V versus N(H I) relationship, we derive N(H I) similar to 6 x 10(21) cm(-2) in both systems. Metallicities are close to solar. Giant Metrewave Radio Telescope (GMRT) observations of these two relatively weak radio QSOs (f(nu) similar to 50 mJy) resulted in the detection of 21-cm absorption in both the 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.

• 出版日期2008-11-21
• 单位CSIRO