Wheels of Fire. I. Massive Star Formation in the Cartwheel Ring Galaxy

Hα CCD imaging is used to map the distribution and intensity of massive star formation (MSF) in the Cartwheel ring galaxy. Line emission is restricted to the outer ring, with upper limits comparable to the disks of early-type galaxies found in the spokes, disk, inner ring, and nucleus. Systematic variations in Hα surface brightness are detected around the ring, with 80% of the total line luminosity (L_Hα = 7.4 x 10⁴² ergs s^-1, corrected for A_ν = 2.1) originating in one quadrant. The 29 identified H II region complexes of the ring are characterized by very large Hα luminosities and equivalent widths. In contrast to Hα, the outer ring is faint in red continuum and shows only small azimuthal variations in surface brightness. The systematic variation in line emission around the ring, high star formation rates, and low H II region abundances are evidence that the ring acts to both organize the interstellar medium and directly trigger MSF. Faint H II region complexes are found in advance of the outer ring and appear to be spawned by shocks and supernovae from nearby giant H II regions.

The overall distribution of MSF points to a small impact parameter collision with one of the three companion galaxies. The lack of Hα emission from the Cartwheel's inner ring and spokes suggests that they are gas poor. The observed azimuthal variations in L_Hα and EW_Hα in the outer ring are most simply explained by systematic changes in age or initial mass function.