Detection of the 2175 Å extinction feature and 21-cm absorption in two Mg II systems at z ∼ 1.3

Abstract

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 ultraviolet (UV) extinction bump at λ_rest∼ 2175Å. 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 versus 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 QSOs (f_υ∼ 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 Κ. These systems provide a unique opportunity to search for molecules and diffuse interstellar bands at z > 1.