Unlocking the secrets of stellar haloes using combined star counts and
  surface photometry
  The stellar haloes of galaxies can currently be studied either through
observations of resolved halo stars or through surface photometry. Curiously,
the two methods appear to give conflicting results, as a number of surface
photometry measurements have revealed integrated colours that are too red to be
reconciled with the halo properties inferred from the study of resolved stars.
Several explanations for this anomaly have been proposed - including dust
photoluminescence, extinction of extragalactic background light and a
bottom-heavy stellar initial mass function. A decisive test is, however, still
lacking. Here, we explain how observations of the halo of a nearby galaxy,
involving a combination of both surface photometry and bright star counts, can
be used to distinguish between the proposed explanations. We derive the
observational requirements for this endeavour and find that star counts in
filters VI and surface photometry in filters VIJ appears to be the optimal
strategy. Since the required halo star counts are already available for many
nearby galaxies, the most challenging part of this test is likely to be the
optical surface photometry, which requires several nights of exposure time on a
4-8 m telescope, and the near-IR surface photometry, which is most readily
carried out using the upcoming James Webb Space Telescope.
