Using Spitzer Colors as Diagnostics of Star Formation Regions: The Interacting Galaxy Arp 107

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.