Search for cold gas in strong Mg?II absorbers at 0.5 ?

摘要

We report four new detections of 21-cm absorption from a systematic search of 21-cm absorption in a sample of 17?strong (rest equivalent width, Wr(Mg?iiλ2796)??≥??1??) intervening Mg?ii?absorbers at 0.5????100?km?s-1 are mainly seen towards the quasars with extended radio morphology at arcsecond scales. However, we do not find any correlation between the integrated 21-cm optical depth,?∫τdv, or?the width of 21-cm absorption line, ΔV, with the LS?measured from the milliarcsecond scale images. All?this can be understood if the absorbing gas is patchy with a typical correlation length of??~30?100?pc. We confirm our previous finding that the 21-cm detection rate for a given optical depth threshold can be increased by up to a factor?2 by imposing the following additional constraints:?Mg?ii?doublet ratio????0.27. This suggests that the probability of detecting 21-cm absorption is higher in the systems with high?N(H?i). We show that within the measurement uncertainty, the 21-cm detection rate in strong Mg?ii?systems is constant over 0.5??