Hoag’s object, remnant of a vanished bar?

Abstract

The beautiful ringed Hoag’s object, named after its discoverer, is an interesting galaxy. Because of the roundness of its ring-like structure, it has been proposed to be a collisional ring galaxy; however, there is no obvious nearby culprit galaxy that could have collided with it. Considering an alternative, much gentler hypothesis, we study the development of the observed structure via a turning, bar perturbation in the disk potential. However, there is currently no obvious bar present, and rings produced by bars are typically oval. On the basis of much recent work improving our understanding of bar evolution, we assume the bar grows and then vanishes. In simulations of a disk of particles, under such a bar turning in the disk plane, we obtain a bulge core, empty void, and circular ring in the disk that mimic the observations of Hoag’s object. We conclude the inner edge of the ring is just beyond the outer Lindblad resonance (OLR) with the bar pattern speed. We estimate the amount of gas mass in the bulge core to be twice that of the ring. Our simulations indicate that the Hoag Object ring could survive at least 6 billion years after the bar vanishes.