Search for cold gas in strong MgII absorbers at 0.5<z<1.5: nature and
  evolution of 21-cm absorbers
  We report 4 new detections of 21-cm absorption from a systematic search of
21-cm absorption in a sample of 17 strong (Wr(MgII 2796)>1A) intervening MgII
absorbers at 0.5<z<1.5. We also present 20-cm milliarcsecond scale maps of 40
quasars having 42 intervening strong MgII absorbers for which we have searched
for 21-cm absorption. Combining 21-cm absorption measurements for 50 strong
MgII systems from our surveys with the measurements from literature, we obtain
a sample of 85 strong MgII absorbers at 0.5<z<1 and 1.1<z<1.5. We present
detailed analysis of this 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, DeltaV>100 km/s 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 or DeltaV 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 show that within the measurement
uncertainty, the 21-cm detection rate in strong MgII systems is constant over
0.5<z<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 MgII absorbers is intriguing.
[abridged]
