Search for cold gas in strong Mg II absorbers at 0.5  <  z  <  1.5: nature and evolution of 21-cm absorbers^⋆

¹ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: gupta@astron.nl
² IUCAA, Post Bag 4, Ganeshkhind, Pune 411007, India
³ UPMC-CNRS, UMR 7095, Institut d’Astrophysique de Paris, 75014 Paris, France

Received: 2 March 2012
Accepted: 17 May 2012

Abstract

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, W_r(Mg iiλ2796)  ≥  1 Å) intervening Mg ii absorbers at 0.5  <  z_abs  <  1.5. We also present 20-cm milliarcsecond scale maps of 40 quasars having 42 intervening strong Mg ii absorbers for which we have searched for 21-cm absorption. These maps are used to understand the dependence of 21-cm detection rate on the radio morphology of the background quasar and address the issues related to the covering factor of absorbing gas. Combining 21-cm absorption measurements for 50 strong Mg ii systems from our surveys with the measurements from literature, we obtain a sample of 85 strong Mg ii absorbers at 0.5  <  z_abs  <  1 and 1.1  <  z_abs  <  1.5. We present detailed analysis of this 21-cm absorption sample, taking into account the effect of the varying 21-cm optical depth sensitivity and covering factor associated with the different quasar sight lines. We find that the 21-cm detection rate is higher towards the quasars with flat or inverted spectral index at cm wavelengths. About 70% of 21-cm detections are towards the quasars with linear size, LS  <  100 pc. The 21-cm absorption lines having velocity widths, ΔV  >  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  < 1.1, W(Mg ii)/W(Fe ii)  <  1.47 and W(Mg i)/W(Mg ii)  >  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  <  z_abs  <  1.5, i.e., over  ~30% of the total age of universe. We show that the detection rate can be underestimated by up to a factor 2 if 21-cm optical depths are not corrected for the partial coverage estimated using milliarcsecond scale maps. Since stellar feedback processes are expected to diminish the filling factor of cold neutral medium over 0.5  <  z  <  1, this lack of evolution in the 21-cm detection rate in strong Mg ii absorbers is intriguing. Large blind surveys of 21-cm absorption lines with the upcoming Square Kilometre Array pathfinders will provide a complete view of the evolution of cold gas in galaxies and shed light on the nature ofMg ii systems and DLAs, and their relationship with stellar feedback processes.