A Multiwavelength View of Star Formation in Interacting Galaxies in the   Pavo Group
  We combine Spitzer IRAC mid-infrared (MIR) and Chandra X-ray observations of the dominant galaxies NGC6872 and NGC6876 in the Pavo group with archival optical and HI data to study interaction-induced star formation. In spiral galaxy NGC6872, 8.0 and 5.8 micron nonstellar emission having colors consistent with polycyclic aromatic hydrocarbons (PAHs) is concentrated in clumps in three regions: in a 5 kpc radius outer ring about the center of the spiral galaxy, in a bridge of emission connecting NGC6872's northern spiral arm to IC4970, and along the full extent of NGC6872's tidal arms. PAH emission is correlated with young star clusters and dense HI regions. We find no strong differences in the MIR colors of star-forming regions in the spiral galaxy NGC6872 as a function of position relative to the tidally interacting companion galaxy IC4970. We find 11 very luminous X-ray sources (>~ (0.5 - 5) x 10^{39} ergs/s) clustered to the southwest in NGC6872, near bright star-forming regions. In NGC6872's tidal features, young star clusters form at the boundaries of diffuse X-ray gas, suggesting that stars form as gas stripped by the interactions cools. The nucleus of NGC6872 is a weak X-ray point source (0.5-8 keV luminosity of 8.5 x 10^{39} ergs/s), but there is little evidence in the inner 1 kpc of NGC6872 for PAH emission from recent star formation or nuclear activity. However, a 4 kpc `stream', leading from the outer ring of NGC6872 to the nucleus, may signal transport of interstellar matter into NGC6872's nuclear region. Nonstellar emission, consistent with PAH emission, is also found in the central region of elliptical galaxy NGC6877, companion to dominant Pavo group elliptical NGC6876. However, in the central region of NGC6876, the dust emission is more likely due to silicate emission from old AGB stars. 
M. Machacek, M.L.N. Ashby, C. Jones, W. R. Forman (CfA) and N. Bastian   (IoA, Cambridge Univ.)