1999AJ....118.2132J

We report on an infrared 1 to 17 µm study of the nearby (cz=4600 km.s^–1) interacting spiral galaxy system UGC 12914/12915, using the ground-based Palomar 200 inch (5 m) telescope and PFIRCAM near-infrared detector and space-based mid-infrared imaging and spectral observations using ISOCAM and PHT-S on the Infrared Space Observatory. The system consists of two counterrotating spirals having suffered a nearly face-on collision only ∼2x10⁷ yr ago. In conjunction with radio observations we explore the complex gas/dust morphology of the postcollision disks, ring structures, current epoch of star formation, and the remnant connecting bridge: It is the unusual radio synchrotron bridge that this study was largely aimed at understanding. Strong line emission from aromatic band features at 6.2, 7.7, and 11.3 µm are seen in both the mid-IR imaging and PHT-S spectrophotometry centered on the nuclei. Theaverage mid-IR (5-17 µm) flux density is ∼0.42 Jy, or about 7% of the 60 µm IRAS flux density. Ournear-/mid-IR data support the hypothesis that the restricted form of galaxy-galaxy interaction–counterrotating direct head-on collision among comparably massed spirals–has produced a large-scale dynamically expanding ``ring'' of recent star formation and gas ``bar'' structure within the disk of UGC 12914. The nucleus and northwestern disk of UGC 12915 are undergoing vigorous star formation probably triggered by the interaction. UGC 12914 appears to be more quiescent in comparison, although there are signatures of massive star formation as revealed in direct comparison between the radio, mid-IR, and Hα imaging. Within the connecting bridge region, the mid-IR imaging reveals dust grains intermixed with the atomic hydrogen gas. The heating mechanism for the hot dust is likely to be UV photons diffusing out from the galactic disks and the H II complex located along the extreme northeastern portion of the bridge: The dust emission, or mid-IR intensity per atomic hydrogen column density ratio, is consistent with heating from the local (bridge) interstellar radiation field.