Outflow from Outer-arm Starburst in a Grazing Collision between Galaxies
  New ALMA CO, HCO+, and 100 GHz continuum observations and Gemini NIFS K-band
spectra are combined with previous radio to X-ray data to study Feature i, a
highly luminous starburst clump on an outer arm of the interacting galaxy NGC
2207. This clump 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 is in front of the star clusters. An associated approaching
CO outflow has v_z ~ 16 km/s, an estimated molecular mass 8 x 10^6 Msun, and
rises to heights ~ 0.9 kpc. A receding CO outflow on the far side with v_z ~ 28
km/s is less extensive. The observed star formation in the core over 10 Myr can
supply the dark cone kinetic energy via supernovae and stellar winds. Other
signs of intense activity are variable radio continuum suggesting an embedded
supernova or other outburst, X-ray emission, and Br-gamma and He I lines with
82 km/s line widths. According to previous models, the retrograde encounter
suffered by NGC 2207 caused loss of angular momentum. This compressed its outer
disk. We suggest that the resulting inward crashing gas stream hit a massive HI
cloud as it was being compressed by a spiral density wave and triggered the
observed starburst.
