Detection of the 2175\AA~ extinction feature and 21-cm absorption in two MgII systems at z~1.3

TL;DR

This study reports the discovery of two MgII absorption systems at z~1.3 exhibiting the 2175 Å extinction feature and 21-cm absorption, indicating high column densities, near-solar metallicities, and cold neutral gas conditions.

Contribution

First detection of the 2175 Å extinction bump and 21-cm absorption in MgII systems at z~1.3, linking dust extinction features with neutral hydrogen properties at high redshift.

Findings

Detection of the 2175 Å UV extinction bump in two MgII systems.

21-cm absorption observed indicating cold neutral gas with T_s ≤ 500K.

High column densities and near-solar metallicities inferred from absorption lines.

Abstract

We have discovered two dusty intervening MgII 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 MgII systems. In particular, we detect clearly the presence of the UV extinction bump at $\lambda_{\rm rest}\sim 2175$~\AA. Absorption lines of weak transitions like SiII$\lambda$1808, CrII$\lambda$2056, CrII+ZnII$\lambda$2062, MnII$\lambda$2594, CaII$\lambda$3934 and TiII$\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 vs. N(HI) relationship we derive N(HI)~$6\times 10^{21}$ cm$^{-2}$ in both systems. Metallicities are close to solar. Giant Metrewave Radio Telescope (GMRT) observations of these two relatively weak radio loud QSOs (f$_\nu$~50mJy) 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 500K. These systems provide a unique opportunity to search for molecules and diffuse interstellar bands at z>1.