The Cartwheel is the prototype of a class of galaxies known as ring galaxies. These galaxies are formed as a result of nearly a head-on collision of two galaxies. In the process, the bigger gas-rich galaxy transforms itself into a ring galaxy. This head-on collision model is able to explain the observed appearance of the ring of the Cartwheel.
Using the Very Large Array (VLA) in New Mexico, we have detected radial filaments of 20 cm radio continuum emission pointing towards the geometrical center of the ring or spokes of the wheel. These spokes have distinct origin as compared to the optical spokes, and there is no apparent positional coincidence between these two. Radio continuum spokes represent the trajectory of past star formation in the Cartwheel. The intensity along the spokes suggests that the star formation rate in the past was lower than the presently observed values in the Cartwheel.
|
|
|
| The Hubbble Space Telescope (HST) image of the Cartwheel. The two galaxies at the top of the image are companion galaxies, of which the bluer one along with another galaxy further outside the displayed image, are believed to be responsible for creating the observed ring. | VLA 20 cm images of the Cartwheel, over-plotted on a gray-scale Hα image. Radial filaments pointing towards the geometrical center of the ring are the newly detected spokes. The spokes are the regions of past star formation, whereas the bright knots in the ring are regions of current star formation. | 20 cm radio continuum contours showing spokes are over-plotted on the B-band HST image. The radio continuum spokes and optical spokes occur at different positions. |