We report 4 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 ii \lambda 2796 ) \geq 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 , \Delta V > 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 , \int \tau dv , or the width of 21-cm absorption line , \Delta V , 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 \sim 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 \sim 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 of Mg ii systems and DLAs , and their relationship with stellar feedback processes .