Hubble Space Telescope images of the galaxies NGC 2207 and IC 2163 show star formation and dust structures in a system that has experienced a recent grazing encounter .
Tidal forces from NGC 2207 compressed and elongated the disk of IC 2163 , forming an oval ridge of star formation along a caustic where the perturbed gas rebounded after its inward excursion .
Gas flowing away from this ridge has a peculiar structure characterized by thin parallel dust filaments transverse to the direction of motion .
The filaments get thicker and longer as the gas approaches the tidal arm .
Star formation that occurs in the filaments consistently lags behind , as if the exponential disk pressure gradient pushes outward on the gas but not the young stars .
Numerical models suggest that the filaments come from flocculent spiral arms that were present before the interaction .
The arms stretch out into parallel filaments as the tidal tail forms .
A dust lane at the outer edge of the tidal tail is a shock front where the flow abruptly changes direction .
Gas at small to intermediate radii along this edge flows back toward the galaxy , while elsewhere in the tidal arm , the gas flows outward .

A spiral arm of NGC 2207 that is backlit by IC 2163 is seen with HST to contain several parallel , knotty filaments spanning the full width of the arm .
These filaments are probably shock fronts in a density wave .
The parallel structure suggests that the shocks occur in several places throughout the arm , or that the interarm gas is composed of spiral-like wisps that merge together in the arms .
Blue clusters of star formation inside the clumps of these dust lanes show density wave triggering in unprecedented detail .
The star-formation process seems to be one of local gravitational collapse , rather than cloud collisions .

Spiral arms inside the oval of IC 2163 have a familiar geometry reminiscent of a bar , although there is no obvious stellar bar .
The shape and orientation of these arms suggest they could be the result of ILR-related orbits in the \cos ( 2 \theta ) tidal potential that formed the oval .
Their presence suggests that tidal forces alone may initiate a temporary nuclear gas flow and eventual starburst without first forming a stellar bar .

Several emission structures resembling jets or conical flows that are 100 - 1000 pc long appear in these galaxies .
In the western arm of NGC 2207 there is a dense dark cloud with a conical shape 400 pc long and a bright compact cluster at the tip , and there is a conical emission nebula of the same length that points away from the cluster in the other direction .
This region also coincides with a non-thermal radio continuum source that is \sim 1000 times the luminosity of Cas A at \lambda = 20 cm .
Surrounding clusters in arc-like patterns may have been triggered by enormous explosions .