A near- and mid-infrared study of the interacting galaxy pair UGC 12914/12915: "Taffy"

We report on an infrared 1 to 17 mu m study of the nearby (cz = 4600 km s(- 1)) interacting spiral galaxy system UGC 12914/12915, using the ground-base d Palomar 200 inch (5 m) telescope and PFIRCAM near-infrared detector and s pace-based mid-infrared imaging and spectral observations using ISOCAM and PHT-S on the Infrared Space Observatory. The system consists of two counter rotating spirals having suffered a nearly face-on collision only similar to 2 x 10(7) yr ago. In conjunction with radio observations we explore the co mplex gas/dust morphology of the postcollision disks, ring structures, curr ent epoch of star formation, and the remnant connecting bridge: It is the u nusual radio synchrotron bridge that this study was largely aimed at unders tanding. Strong line emission from aromatic band features at 6.2, 7.7, and 11.3 mu m are seen in both the mid-IR imaging and PHT-S spectrophotometry c entered on the nuclei. The average mid-IR (5-17 mu m) flux density is simil ar to 0.42 Jy, or about 7% of the 60 mu m IRAS flux density. Our near-/mid- IR data support the hypothesis that the restricted form of galaxy-galaxy in teraction-counterrotating direct head-on collision among comparably massed spirals-has produced a large-scale dynamically expanding "ring" of recent s tar formation and gas "bar" structure within the disk of UGC 12914. The nuc leus and northwestern disk of UGC 12915 are undergoing vigorous star format ion probably triggered by the interaction. UGC 12914 appears to be more qui escent in comparison, although there are signatures of massive star formati on as revealed in direct comparison between the radio, mid-IR, and H alpha imaging. Within the connecting bridge region, the mid-IR imaging reveals du st grains intermixed with the atomic hydrogen gas. The heating mechanism fo r the hot dust is likely to be UV photons diffusing out from the galactic d isks and the H II, complex located along the extreme northeastern portion o f the bridge: The dust emission, or mid-IR intensity per atomic hydrogen co lumn density ratio, is consistent with heating from the local (bridge) inte rstellar radiation field.