Illuminating the past 8 billion years of cold gas towards two
  gravitationally lensed quasars
  Using the Boolardy Engineering Test Array of the Australian Square Kilometre
Array Pathfinder (ASKAP BETA), we have carried out the first $z = 0 - 1$ survey
for HI and OH absorption towards the gravitationally lensed quasars
PKSB1830$-$211 and MGJ0414$+$0534. Although we detected all previously reported
intervening systems towards PKSB1830$-$211, in the case of MGJ0414+0534 three
systems were not found, indicating that the original identifications may have
been confused with radio frequency interference. Given the sensitivity of our
data, we find that our detection yield is consistent with the expected
frequency of intervening HI systems estimated from previous surveys for 21-cm
emission in nearby galaxies and $z \sim 3$ damped Lyman $\alpha$ absorbers. We
find spectral variability in the $z = 0.886$ face-on spiral galaxy towards
PKSB1830$-$211, from observations undertaken with the Westerbork Synthesis
Radio Telescope in 1997/1998 and ASKAP BETA in 2014/2015. The HI equivalent
width varies by a few per cent over approximately yearly time-scales. This
long-term spectral variability is correlated between the north-east and
south-west images of the core, and with the total flux density of the source,
implying that it is observationally coupled to intrinsic changes in the quasar.
The absence of any detectable variability in the ratio of HI associated with
the two core images is in stark contrast to the behaviour previously seen in
the molecular lines. We therefore infer that coherent opaque HI structures in
this galaxy are larger than the parsec-scale molecular clouds found at
mm-wavelengths.
