Reconstructing the stellar mass distributions of galaxies using S4G IRAC
  3.6 and 4.5 micron images: I. Correcting for contamination by PAH, hot dust,
  and intermediate age stars
  With the aim of constructing accurate 2D maps of the stellar mass
distribution in nearby galaxies from S4G 3.6 and 4.5 micron images, we report
on the separation of the light from old stars from the emission contributed by
contaminants (e.g. hot dust and the 3.3 micron PAH feature). Results for a
small sample of six disk galaxies (NGC 1566, NGC 2976, NGC 3031, NGC 3184, NGC
4321, and NGC 5194) with a range of morphological properties, dust contents and
star formation histories are presented to demonstrate our approach. We use an
Independent Component Analysis (ICA) technique designed to separate
statistically independent source distributions, maximizing the distinction in
the [3.6]-[4.5] colors of the sources. The technique also removes emission from
intermediate-age evolved red objects with a low mass-to-light ratio, such as
asymptotic giant branch (AGB) and red supergiant (RSG) stars, revealing maps of
the underlying old distribution of light with [3.6]-[4.5] colors consistent
with the colors of K and M giants. Contaminants are identified via comparison
to the non-stellar emission imaged at 8 microns, which is dominated by the
broad PAH feature. Using the measured 3.6/8 micron ratio to select the
individual contaminants, we find that hot dust and PAH together contribute
between ~5-15% to the integrated light at 3.6 microns, while light from regions
dominated by intermediate-age stars accounts for only 1-5%. Locally, however,
the contribution from either contaminant can reach much higher levels; dust
contributes on average 22% to the emission in star-forming regions throughout
the sample, while intermediate age-stars contribute upwards of 50% in localized
knots. The removal of these contaminants with ICA leaves maps of the old
stellar disk that retain a high degree of structural information and are
ideally suited for tracing the stellar mass, as will be the focus in a
companion paper.
