Deconstructing a galaxy: colour distributions of point sources in
  Messier 83
  What do we see when we look at a nearby, well-resolved galaxy? Thousands of
individual sources are detected in multiband imaging observations of even a
fraction of a nearby galaxy, and characterizing those sources is a complex
process. This work analyses a ten-band photometric catalogue of nearly 70 000
point sources in a 7.3 square arcmin region of the nearby spiral galaxy Messier
83, made as part of the Early Release Science programme with the Hubble Space
Telescope's Wide Field Camera 3. Colour distributions were measured for both
broad-band and broad-and-narrow-band colours; colours made from broad-bands
with large wavelength differences generally had broader distributions although
B - V was an exception. Two- and three-dimensional colour spaces were generated
using various combinations of four bands and clustered with the K-Means and
Mean Shift algorithms. Neither algorithm was able to consistently segment the
colour distributions: while some distinct features in colour space were
apparent in visual examinations, these features were not compact or isolated
enough to be recognized as clusters in colour space. K-Means clustering of the
UBVI colour space was able to identify a group of objects more likely to be
star clusters. Mean Shift was successful in identifying outlying groups at the
edges of colour distributions. For identifying objects whose emission is
dominated by spectral lines, there was no clear benefit from combining
narrow-band photometry in multiple bands compared to a simple continuum
subtraction. The clustering analysis results are used to inform recommendations
for future surveys of nearby galaxies.
