We combine Spitzer IRAC mid-infrared and Chandra X-ray observations of the dominant galaxies NGC 6872 and NGC 6876 in the Pavo group with archival optical and HI data to study interaction-induced star formation .
In the spiral galaxy NGC 6872 , 8.0 \micron and 5.8 \micron 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 HI 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 eleven very luminous X-ray sources ( \gtrsim ( 0.5 - 5 ) \times 10 ^ { 39 } erg s ^ { -1 } ) 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 8.5 \times 10 ^ { 39 } erg s ^ { -1 } , 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 AGB stars .