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Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society. DOI: 10.1086/176374 [Full text of this article is not available in the UHRA]High-resolution H I observations taken at the VLA of the interacting pair of galaxies NGC 2207 and IC 2163 are presented, compared with optical and radio continuum images, and analyzed with detailed computer simulations (in Paper II) in order to determine the orbits and study the effects of in-plane and out-of-plane tidal forcing during a recent, close, nonmerging encounter between galaxies of comparable mass. IC 2163 has an ocular shape (an eye-shaped central oval with a sharp apex at each end) and a double- parallel arm structure on the side opposite NGC 2207. Our observations of IC 2163 find (1) an intrinsically oval shape to the disk, (2) large streaming motions along the oval, (3) H I tidal arms located symmetrically on opposite sides of the nucleus (even though one side is obscured by NGC 2207 at optical wavelengths), and (4) a line-of-sight velocity difference of 70-100 km s-1 between the two components of the double-parallel arm. Optical surface photometry of IC 2163 indicates that most of the stars in the interarm region have been cleared away and put into the central oval and tidal arms. The kinematic and structural anomalies of IC 2163 are consistent with the predictions of N-body galaxy encounter simulations if NGC 2207 moved approximately in the plane of IC 2163 in a prograde sense. The companion, NGC 2207, shows different types of disturbances. The main body of H I gas in NGC 2207 forms a broad, clumpy ring that contains relatively thin stellar arms and corresponds to a plateau in the radial distribution of optical light. The most massive H I clouds in the ring do not always coincide with the stellar arms. The ring is broken in the south, and a filamentary pool of H I extends 40 kpc farther south. The velocity field in the main disk of NGC 2207 is highly distorted with isovelocity contours that are shaped like an open trailing spiral, and the H I line profiles in this region are very broad with Gaussian dispersions of 40-50 km s-1. The kinematic disturbances in NGC 2207 suggest that the main tidal force on NGC 2207 was perpendicular to its disk. The companion side of the H I ring is unusually bright in ??20 cm radio continuum emission, perhaps indicating a shock front. There is no significant excess of star formation in these galaxies, but there are several 108 Msun gas clouds in each disk, some of which may eventually become detached dwarf galaxies

IC 2163 and NGC 2207 are interacting galaxies that have been well studied at optical and radio wavelengths and simulated in numerical models to reproduce the observed kinematics and morphological features. Spitzer IRAC and MIPS observations reported here show over 200 bright clumps from young star complexes. The brightest IR clump is a morphologically peculiar region of star formation in the western arm of NGC 2207. This clump, which dominates the H and radio continuum emission from both galaxies, accounts for $12% of the total 24 m flux. Nearly half of the clumps are regularly spaced along some filamentary structure, whether in the starburst oval of IC 2163 or in the thin spiral arms of NGC 2207. This regularity appears to influence the clump luminosity function, making it peaked at a value nearly a factor of 10 above the completeness limit, particularly in the starburst oval. This is unlike the optical clusters inside the clumps, which have a luminosity function consistent with the usual power-law form. The giant IR clumps presumably formed by gravitational instabilities in the compressed gas of the oval and the spiral arms, whereas the individual clusters formed by more chaotic processes, such as turbulence compression, inside these larger scale structures.

Observations with the Hubble Space Telescope reveal an irregular network of dust spiral arms in the nuclear region of the interacting disk galaxy NGC 2207. The spirals extend from ∼50 to ∼300 pc in galactocentric radius, with a projected width of ∼20 pc. Radiative transfer calculations determine the gas properties of the spirals and the inner disk and imply a factor of ∼4 local gas compression in the spirals. The gas is not strongly self-gravitating, nor is there a nuclear bar, so the spirals could not have formed by the usual mechanisms applied to main galaxy disks. Instead, they may result from acoustic instabilities that amplify at small galactic radii. Such instabilities may promote gas accretion into the nucleus.

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