NGC 2207/IC 2163 : A Grazing Encounter with Large-scale Shocks

Abstract

Radio continuum, Spitzer infrared, optical, and XMM-Newton X-ray and ultraviolet observations (UVW1 and UVM2) are used to study large-scale shock fronts, young star complexes, and the galactic nuclei in the interacting galaxies NGC 2207/IC 2163. There are two types of large-scale shock fronts in this galaxy pair. The large-scale shock front along the rim of the ocular oval in IC 2163 has produced vigorous star formation in a dusty environment, bright in the Spitzer 8 μm and 24 μm images. In the outer part of the companion side of NGC 2207, a large-scale front attributed to halo scraping is particularly bright in the λ6 cm and λ20 cm radio continuum but not in any tracers of recent star formation (Hα, 8 μm, 24 μm, or ultraviolet emission) or in X-rays. This radio-continuum front may be from compression of the halo magnetic field on the back side of NGC 2207, between the two galaxies. The X-ray emission sets an upper limit to the gas density in the halo. Values of the flux density ratio S ν(8 μm)/S ν(6 cm) of prominent, kiloparsec-size, Spitzer/IRAC star-forming clumps in NGC 2207/IC 2163 are compared with those of giant radio H II regions in M81. For the bright clumps in NGC 2207, the mean value of this ratio is the same as for the M81 H II regions, whereas for the bright clumps on the rim of the IC 2163 ocular oval, the mean value is nearly a factor of two greater. Possible explanations for this are discussed. The galaxy pair has global values of the ratios of infrared-to-radio continuum flux density in the Spitzer 8 μm, 24 μm, and 70 μm bands, and the IRAS FIR significantly below the medians/means for large samples of galaxies. Feature i, a mini-starburst on an outer arm of NGC 2207 on its anti-companion side, is the most luminous 8 μm, 24 μm, 70 μm, radio continuum, and Hα source in the galaxy pair. We find evidence that a radio supernova was present in the core of feature i in 2001. X-ray emission is detected from the nucleus of NGC 2207 and from nine discrete sources whose X-ray luminosities make them possible candidates for Ultraluminous X-ray sources. One of these corresponds with the Type Ib SN 1999ec, which is also bright in the ultraviolet, and another may be a radio supernova or a background quasar. The X-ray luminosity of the NGC 2207 nucleus is log L0.3-10.0 keV = 40.6 erg s–1, which, together with its X-ray spectrum, suggests that this is a highly absorbed, low-luminosity, active galactic nucleus