In this paper we model and analyse the B - V / V - K radial color gradients observed in the Cartwheel ring galaxy .
Along with the color-color diagrams , we use the Q _ { BVK } combined color indices , which minimise the uncertainties in the observed B - V and V - K colors introduced by dust extinction .
To model the optical and near-infrared color properties of the Cartwheel galaxy , we assume that an intruder galaxy generates an expanding ring density wave in the Cartwheel ’ s disk , which in its turn triggers massive star formation along the wave ’ s perimeter according to the Schmidt law .
We use the population synthesis to calculate the color properties of stellar populations formed in the expanding density wave .
The results of color modelling suggest that the pre-collision Cartwheel was a late-type spiral , embedded in an extensive gaseous disk of sub-critical surface density .
The properties of the old stellar disk are typical for the late-type Freeman disks , with the central surface brightness in V -band and the scale length being \mu _ { \mathrm { V } } ^ { 0 } = 21.0 mag arcsec ^ { -2 } and R _ { 0 } = 2 Â kpc respectively .
The pre-collision gaseous disk has a metallicity gradient ranging from z = z _ { \odot } / 5 at the outer regions to z = z _ { \odot } in the central regions .
At present , the wave of star formation has passed the initial extent of the pre-collision , old stellar disk and is currently moving in the predominantly gaseous , low-metallicity disk at the radius of 16Â kpc .
Neither young stellar populations formed in an expanding density wave , nor their mixture with the old , pre-collision stellar populations can reproduce the B - V and V - K colors of the Cartwheel ’ s nucleus+inner ring .
We find that an additional 10-Myr-old burst of star formation in the nuclear regions , along with the visual extinction of A _ { \mathrm { V } } = 1 ^ { \mathrm { m } } .3 , might be responsible for the peculiar colors of the Cartwheel ’ s nucleus .