Probing the inner kpc of massive galaxies with strong gravitational
  lensing
  We examine the prospects of detecting demagnified images of gravitational
lenses in observations of strongly lensed mm-wave molecular emission lines with
ALMA. We model the lensing galaxies as a superposition of a dark matter
component, a stellar component, and a central supermassive black hole and
assess the detectability of the central images for a range of relevant
parameters (e.g., stellar core, black hole mass, and source size). We find that
over a large range of plausible parameters, future deep observations of lensed
molecular lines with ALMA should enable detection of the central images at
$\gtrsim 3\sigma$ significance. We use a Fisher analysis to examine the
constraints that could be placed on these parameters in various scenarios and
find that for large stellar cores, both the core size and the mass of the
central SMBHs can be accurately measured. We also study the prospects for
detecting binary SMBHs with such observations and find that only under rare
conditions and with very long integrations ($\sim$40-hr) the masses of both
SMBHs may be measured using the distortions of central images.
