2009ApJ...691.1921M

We combine Spitzer Infrared Array Camera mid-infrared and Chandra X-ray observations of the dominant galaxies NGC 6872 and NGC 6876 in the Pavo group with archival optical and H I data to study interaction-induced star formation. In the spiral galaxy NGC 6872, 8.0 µm and 5.8 µm nonstellar emission having colors consistent with polycyclic aromatic hydrocarbons (PAHs) is concentrated primarily 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 NGC 6872's northern spiral arm to IC 4970, and along the full extent of both NGC 6872's tidal arms. PAH emission is correlated with young star clusters and dense H I regions. We find no strong differences in the mid-infrared colors of the star-forming regions in the spiral galaxy NGC 6872 as a function of position relative to the tidally interacting companion galaxy IC 4970. We find 11 very luminous X-ray sources (≳ (0.5-5) x10³⁹ erg/s) clustered to the southwest in NGC 6872, near bright star-forming regions. In NGC 6872'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 NGC 6872 is a weak X-ray point source with 0.5-8 keV luminosity of 8.5x10³⁹ erg/s, but there is little evidence in the inner 1 kpc of NGC 6872 for PAH emission from recent star formation or nuclear activity. However, a 4 kpc "stream," leading from the outer ring of NGC 6872 to the nucleus, may signal the transport of interstellar matter into NGC 6872's nuclear region. Nonstellar emission, consistent with PAH emission, is also found in the central region of elliptical galaxy NGC 6877, companion to the dominant Pavo group elliptical galaxy NGC 6876. However, in the central region of NGC 6876, the dust emission is more likely due to silicate emission from old asymptotic giant branch stars.