ALMA CO Observations of Shocks and Star Formation in the Interacting Galaxies IC 2163 and NGC 2207
Abstract
The spiral galaxies IC 2163 and NGC 2207 are a well-studied pair undergoing a grazing collision. ALMA CO observations of masses, column densities, and velocities are combined with HI, Hα, optical, and 24 micron data to study the star formation rates and efficiencies. The close encounter of the galaxies produced in-plane tidal forces in IC 2163, resulting in a large shock with high molecular velocity gradients and both radial and azimuthal streaming (100 km/s) that formed a pile-up of molecular gas in the resulting cuspy-oval or ``eyelid'' structure at mid-radius. The encounter also produced forces nearly orthogonal to the plane of NGC 2207, resulting in a warp. By comparing with the Kennicutt-Schmidt relation for star formation, we find that some regions of NGC 2207 with unusually high turbulent speeds (40-50 km/s) and high star formation rates (>0.01 Mo/pc2/Myr) have gas that is predominantly atomic with high density cores. Half of the CO mass is in 300 clouds each more massive than 4.0x105 Mo. The mass distribution functions for the CO clouds and star complexes in the eyelid in IC 2163 both have a slope similar to what is observed in Milky Way clouds; the CO slope is steeper in NGC 2207. The CO distribution in NGC 2207 also includes a nuclear ring, a mini-bar, and a mini-starburst region that dominates the 24 micron, radio, and Hα emission in both galaxies. Dust extinction, molecular column densities, and slightly negative molecular velocities indicate the mini-starburst region has ejected a jet of molecular gas nearly perpendicular to the plane of NGC 2207 on the near side with a kinetic energy of 1052 ergs. The large scale star formation efficiency, measured as the ratio of the summed masses of the star complexes near molecular clouds to the combined star complex and cloud masses, is 7% overall; it is 23% in the mini-starburst. The maximum age of star complexes in the galactic-scale shock front at the eyelid is about the same as the time since closest approach of the galaxies, suggesting a triggering process related to tidal compression.
- Publication:
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American Astronomical Society Meeting Abstracts #229
- Pub Date:
- January 2017
- Bibcode:
- 2017AAS...22914421E