2013A&A...558A..84G

We report the detection of 21-cm absorption from foreground galaxies towards quasars, specifically z_gal=0.3120 towards SDSS J084957.97+510829.0 (z_qso=0.584; Pair-I) and z_gal=0.3714 towards SDSS J144304.53+021419.3 (z_qso=1.82; Pair-II). In both the cases, the integrated 21-cm optical depth is consistent with the absorbing gas being a damped Lyman-α (DLA) system. In the case of Pair-I, strong NaI and CaII absorption lines are also detected at z_gal in the QSO spectrum. We identify an early-type galaxy at an impact parameter of b∼14kpc whose photometric redshift is consistent with that of the detected metal and 21-cm absorption lines. This would be the first example of an early-type galaxy associated with an intervening 21-cm absorber. The gas detected in 21-cm and metal absorption lines on the outskirts of this luminous red galaxy could be associated with the reservoir of cold HI gas with a low level of star formation activity in the outer regions of the galaxy as reported in the literature for z∼0.1 early-type galaxies. In the case of Pair-II, the absorption is associated with a low surface brightness galaxy that, unlike most other known quasar-galaxy pairs (QGPs), i.e. QSO sight lines passing through disks or halos of foreground galaxies, is identified only via narrow optical emission lines detected on top of the QSO spectra. Using SDSS spectra we infer that the emission lines originate within ∼5kpc of the QSO sight line, and the gas has metallicity [12+O/H]∼8.4 and star formation rate ∼0.7-0.8M_☉/yr. The measured 21-cm optical depth can be reconciled with the N(HI) we derive from the measured extinction (A_V=0.6) if either the HI gas is warm or the extinction per hydrogen atom in this galaxy is much higher than the mean value of the Small Magellanic Cloud. Finally, using a sample of 9 QGPs with 21-cm absorption detection from our observations and literature, we report a weak anti-correlation (Spearman rank, r_s=-0.3) between the 21-cm optical depth and galaxy impact parameter. Milliarcsecond scale images and spectra are required to understand the implications of this.