Resolved optical-infrared SEDs of galaxies: universal relations and
  their break-down on local scales
  A large body of evidence has demonstrated that the global rest-frame optical
and IR colours of galaxies correlate well with each other, as well as with
other galactic properties such as surface brightness and morphology. However
the processes that lead to the observed correlations are contrary; the stellar
light that contributes to the optical is readily absorbed by dust which emits
in the IR. Thus on small scales we expect these correlations to break down. We
examine seven nearby galaxies ranging from early- to late-types, on a
pixel-by-pixel basis and we demonstrate that there is disconnect between the
optical and IR when normalized to the near-IR (H-band). We can decompose this
disconnect into two distinct components through a Principal Component Analysis
of the H-band normalized SED of the pixels: one mainly correlated with
variations in the IR, the other correlated with variations in the optical. By
mapping these two components it is clear they arise from distinct spatial
regions. The IR dominated component is strongly associated with the specific
star-formation rate, while the optical-dominated component is broadly
associated with the stellar mass density. However, when the pixels of all
galaxies are compared, the well known optical-IR colour correlations return,
demonstrating that the variance observed within galaxies is around a mean which
follows the well-known trend. We also examine the extremely strong correlations
between the IRAC-NIR colours and demonstrate that they are tight enough to use
a single IRAC-NIR colour (i.e. 8mum-H) to determine the fluxes in the other
IRAC bands. These correlations arise from the differing contribution of stellar
light and dust to the IRAC bands, enabling us to determine pure stellar colours
for these bands, but still demonstrating the need for dust (or stellar)
corrections in these bands when being used as stellar (dust) tracers.
