The Detection of Nonthermal Radio Continuum Spokes and the Study of Star Formation in the Cartwheel

Abstract

New sensitive Very Large Array 20 cm continuum observations of the Cartwheel, the prototypical collisional ring galaxy, were carried out with the principal aim of tracing supernova remnants that are expected to lie in the wake of the expanding ring and in the ring itself. We detect predominantly nonthermal radio continuum emission from regions associated with 13 ring H II complexes. The emission interior to the ring is confined to structures that resemble spokes of the wheel. The spokes start near bright H II complexes and extend to around 6'' (4 kpc) inward in the direction of the geometrical center of the ring. There is no apparent positional coincidence between the radio continuum and optical spokes. Radial distribution of intensity along the spokes suggests that the past star formation rate (SFR) in the Cartwheel was much lower than the current SFR. New Hα observations were used to revise the current SFR in the Cartwheel. The revised value is 18 M☉ yr-1 , which is a factor of 4 lower than the value reported previously but is in good agreement with the SFR estimated from far-infrared luminosity. About 30% of the observed 20 cm continuum nonthermal emission seems to originate in processes that are not related to star formation. Revised SFR in the Cartwheel is comparable to that in the rest of the ring galaxies.