  We present Spitzer infrared imaging of the peculiar galaxy pair Arp 107, and
compare with an optical H-alpha map and a numerical model of the interaction.
The [3.6 micron] - [4.5 micron] colors of clumps in the galaxy do not vary
around the ring-like primary spiral arm and are consistent with those of stars,
thus these bands are dominated by starlight. In contrast, the [5.8 micron] -
[8.0 micron] colors are consistent with those of interstellar dust, and vary by
about 0.2 magnitudes around the ring/spiral, with redder colors associated with
regions with stronger star formation as indicated by H-alpha and mid-infrared
luminosity. The [4.5 micron] - [5.8 micron] colors for clumps in this arm are
bluer than dust and redder than stars, and vary by 1.3 magnitudes around the
arm. This color is therefore a measure of the relative number of young stars to
old stars, with a redder color indicating a higher proportion of young stars.
There is a clear azimuthal sequence in the [4.5] - [5.8] color around the arm,
indicating a sequence in average stellar age. The L(H-alpha)/L(8.0 micron)
ratio varies around the arm by a factor of ~7; this variation may be due to
extinction or to PAH excitation by non-ionizing photons. Our model of Arp 107
accounts for the general morphology of the system, and explains the age
variation along the arm as the result of differences in the time of maximum
compression in the arm. Using Spitzer colors, we are able to distinguish
background quasars and foreground stars from star forming regions associated
with Arp 107.
