Outflow from Outer-arm Starburst in a Grazing Collision between Galaxies

Gemini NIFS K-band spectra and Atacama Large Millimeter/submillimeter Array 12CO , HCO+, and 100 GHz continuum observations are used to study a bright starburst clump on an outer arm of the interacting galaxy NGC 2207. This clump emits 23% of the total 24 m flux of the galaxy pair and has an optically opaque dust cone extending out of its 170 pc core. The measured CO accounts for the dark cone extinction if almost all the gas and dust there are in front of the star clusters. An associated approaching CO outflow has vz ∼ 16 km s−1, an estimated molecular mass 8 × 106 M , and rises to heights ∼0.9 kpc. A receding CO outflow on the far side with vz ∼ 28 km s−1 is less extensive. The observed star formation in the core over 10 Myr can supply the dark cone kinetic energy of roughly 2 × 1052 erg via supernovae and stellar winds. Other signs of intense activity are a variable radio continuum, suggesting an embedded supernova or other outburst; X-ray emission possibly from an X-ray binary or intermediate-mass black hole, depending on the extinction; and Brγ and He i lines with 82 km s−1 line widths and fluxes consistent with excitation by embedded O-type stars. According to previous models, the retrograde encounter suffered by NGC 2207 caused the loss of angular momentum. This compressed its outer disk. We suggest that the resulting inward crashing stream hit a massive H i clump on the preexisting spiral arm and triggered the observed starburst.