Discovery of a near-infrared bar and a pseudo-bulge in the collisional ring galaxy Cartwheel

Abstract

We report the discovery of a bar, a pseudo-bulge, and unresolved point source in the archetype collisional ring galaxy Cartwheel using careful morphological analysis of a near-infrared (NIR) K_s-band image of excellent quality (seeing = 0.42^″) at the ESO archive. The bar is oval-shaped with a semi-major axis length of 3.23^″ (~2.09 kpc), with almost a flat light distribution along it. The bulge is almost round (ellipticity = 0.21) with an effective radius of 1.62^″ (~1.05 kpc) and a Sersic index of 0.99, parameters typical of pseudo-bulges in late-type galaxies. The newly discovered bar is not recognizable as such in the optical images even with more than a factor of 2 higher spatial resolution of the Hubble Space Telescope, due to a combination of its red colour and the presence of dusty features. The observed bar and pseudo-bulge most likely belonged to the pre-collisional progenitor of the Cartwheel. The discovery of a bar in an archetype collisional ring galaxy Cartwheel is the first observational evidence to confirm the prediction that bars can survive a drop-through collision along with the morphological structures like a central bulge (pseudo).