Understanding the unusual life of the Cartwheel galaxy using stellar populations

Abstract

RiGs are the result of the impact between two galaxies, with one of them passing close to the centre of the other, piercing its gaseous and stellar disc. In this framework, the impact generates a shock wave front that propagates within the disc of the target galaxy soon after the encounter, producing a characteristic expanding ring-shaped structure. RiGs represent one of the most extreme environments in which we can study the physical properties of galaxies and the transformations they undergo during collisions. The paradigm RiG is the Cartwheel galaxy at z = 0.03. This galaxy has been the object of both theoretical and observational studies, but the details of the mechanisms that lead to its peculiar morphology and physical properties are still far from clear. We performed a spatially resolved analysis as a function of galactocentric distance, exploiting spectroscopic data from MUSE observations combined with photometric data covering a large wavelength range, from GALEX to JWST/MIRI. Using FIF of the stellar spectra, an analysis of the nebular emission, and joint full spectral and photometry fitting, we derived physical parameters and SFHs in four spatially distinct regions of the galaxy. We find that, apart from the peculiar morphology, a large fraction of the Cartwheel galaxy is not affected by the recent impact from the companion bullet, and retains the characteristics of a typical spiral galaxy. The outer ring is strongly affected by the recent impact, and is completely dominated by stars formed not earlier than 400 Myr ago. Our picture suggests that the collision shock wave, while moving forward to the external region of the galaxy, drags the already formed stars, sweeping the inner areas outwards, as proposed by recent collision models. At the same time, the ages found in the external ring are older than the predicted timescale of the ring expansion after the collision.