Radial B − V / V − K color gradients , extinction - free Q BVK combined color indices , and the history of star formation of the Cartwheel ring galaxy

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 QBVK 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 µ 0 V = 21.0 mag arcsec −2 and R0 = 2 kpc respectively. The pre-collision gaseous disk has a metallicity gradient ranging from z = z⊙/5 at the outer regions to z = z⊙ 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 16 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 AV = 1 m .3, might be responsible for the peculiar colors of the Cartwheel's nucleus.