Abstract:
We present the results obtained by analysing the new AstroSat Ultra-Violet Imaging Telescope (UVIT) far-ultraviolet (FUV) image of the collisional-ring galaxy Cartwheel. The FUV emission is principally associated with the star-forming outer ring, with no ultraviolet detection from the nucleus and inner ring. A few sources are detected in the region between the inner and the outer rings, all of which lie along the spokes. The FUV fluxes from the detected sources are combined with aperture-matched multiband photometric data from archival images to explore the post-collision star formation history of the Cartwheel. The data were corrected for extinction using AV derived from the Balmer decrement ratios and commonly used extinction curves. We find that the ring regions contain stellar populations of a wide range of ages, with the bulk of the FUV emission coming from non-ionizing stars, formed over the last 20–150 Myr, that are ∼25 times more massive than the ionizing populations. On the other hand, regions belonging to the spokes have negligible current star formation, with the age of the dominant older population systematically increasing as its distance from the outer ring increases. The presence of populations of a wide range of ages in the ring suggests that the stars formed in the wave in the past were dragged along it to the current position of the ring. We derive an average steady star formation rate, SFR = 5 M⊙ yr−1, over the past 150 Myr, with an increase to ∼18 M⊙ yr−1 in the recent 10 Myr.
