NGC 2207/IC 2163: A Grazing Encounter with Large-Scale Shocks (original) (raw)

2012, The Astronomical Journal

Radio continuum, Spitzer infrared, optical, and XMM-Newton X-ray and ultraviolet observations (U V W 1 and U V M 2) 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. This large-scale shock lies behind NGC 2207 and is not prominent in X-rays. In the outer part of the companion side of NGC 2207, a large-scale front attributed to disk or 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 is simply due to compression of the magnetic field and may be mainly in the halo on the back side of NGC 2207, between the two galaxies. Values of the ratio of 8 µm to λ6 cm radio continuum flux density of prominent, kpc-sized, 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. On the other hand, 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. Highly absorbed, it is not detected in X-rays and is fainter in the ultraviolet than the clump containing SN 1999ec, 8 ′′ SSE of the core of feature i. 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 ULXs. One of these corresponds with the Type Ib SN 1999ec, and another may be a radio supernova or a background quasar. The X-ray luminosity of the NGC 2207 nucleus is log L 0.3−10.0keV = 40.6 [ergs s −1 ], which, together with its X-ray spectrum, suggests that this is a highly absorbed, low-luminosity, Active Galactic Nucleus.