T.H. Jarrett 1, G. Helou 1, D. Van Buren 1, E. Valjavec 1, & J. Condon 2
1 IPAC/Caltech
2 NRAO
We are conducting a multi-wavelength study of the nearby (cz = 4600 km/s) interacting spiral galaxy system, UGC 12914/12915 (also known as VV 254), using ground-based near infrared and radio observations, and mid-infrared imaging and spectral observations using ISOCAM and Phot-S on the Infrared Space Observatory (ISO). This rarely occurring interacting system consists of two counter-rotating spirals having suffered a nearly face-on collision, and exhibits a prominent 1.4 GHz radio ``bridge'' connecting the two galaxies. The collision has stripped H I gas from the respective disks leaving a gas material trail in the post-collision trajectory (time since collision estimated at 10**7 years), while the magnetic fields associated with disk giant molecular clouds appear to have been entangled and stretched out bye the interaction - thus creating the bridge or ``taffy''-like appearance as seen in the radio. Optical and near-infrared imaging data do not trace the gas bridge (a giant H II region is seen in H-alpha just south of UGC 12915); moreover, the galactic nuclei are heavily obscured by dust for wavelengths less than 3 microns and so are of limited value. In the far-infrared, IRAS does not have the spatial resolution to separate the two galactic nuclei or clearly resolve the bridge. Warm/hot dust in the nuclei and bridge is, however, resolved and detected with our ISOCAM data using the LW1, LW2, LW8 and LW3 filters, covering 4 to 17 microns in wavelength. Very small dust grains are detected via strong emission features at 7.7 and 11.3 microns (aromatic hydrocarbons) seen in both the mid-IR imaging and Phot-S spectro-photometry centered on the nuclei and their connecting bridge. The mid-infrared morphology of the nuclei and bridge closely resembles that of the radio continuum and the hydrogen gas column density map, particularly for the intermediary bridge. Since massive stars (or any stars for that matter) do not appear to be located within the bridge and little if any H-alpha ionization is observed, the heating mechanism for the warm dust must be either non-thermal processes (electron cosmic-ray generated synchrotron & inverse-Compton) or gas- particle collisions and shocks, or a combination of the two. We will compare the mid-IR and radio energetics. Finally, our mid- IR data supports the hypothesis that the restricted form of galaxy-galaxy interaction - counter-rotating direct head-on collision between comparably massed spirals - has produced a large-scale dynamically expanding ``ring'' of recent star-formation (also hinted at in the optical/near-IR) and gas ``bar'' structures near the respective nuclei. For the interacting system and local environment, we will present sectro/imaging data spanning optical, near-infrared, mid-infrared, and the radio spectrum.