Methanol absorption in PKS B1830-211 at milliarcsecond scales
  Observations of the frequencies of different rotational transitions of the
methanol molecule have provided the most sensitive probe to date for changes in
the proton-to-electron mass ratio, over space and time. Using methanol
absorption detected in the gravitational lens system PKS B1830-211, changes in
the proton-to-electron ratio over the last 7.5 billion years have been
constrained to a fractional change less than 1.1e-07. Molecular absorption
systems at cosmological distances present the best opportunity for constraining
or measuring changes in the fundamental constants of physics over time,
however, we are now at the stage where potential differences in the morphology
of the absorbing systems and the background source, combined with their
temporal evolution, provide the major source of uncertainty in some systems.
Here we present the first milliarcsecond resolution observations of the
molecular absorption system towards PKS B1830-211. We have imaged the
absorption from the 12.2-GHz transition of methanol (which is redshifted to
6.45 GHz) toward the southwestern component and show that it is possibly offset
from the peak of the continuum emission and partially resolved on
milliarcsecond scales. Future observations of other methanol transitions with
similar angular resolution offer the best prospects for reducing systematic
errors in investigations of possible changes in the proton-to-electron mass
ratio on cosmological scales.
