One ring to encompass them all: a giant stellar structure that surrounds the Galaxy

We present evidence that the curious stellar population found by the Sloan Digital Sky Survey in the Galactic anticentre direction extends to other distant fields that skirt the plane of the Milky Way. New data, taken with the Isaac Newton Telescope Wide Field Camera, show a similar population, narrowly aligned along the line of sight, but with a galactocentric distance that changes from ~15 to ~20 kpc (over ~100° on the sky). Despite being narrowly concentrated along the line of sight, the structure is fairly extended vertically out of the plane of the disc, with a vertical scaleheight of 0.75 +/- 0.04 kpc. This finding suggests that the outer rim of the Galaxy ends in a low surface brightness stellar ring. Presently available data do not allow us to ascertain the origin of the structure. One possibility is that it is the wraith of a satellite galaxy devoured long ago by the Milky Way, although our favoured interpretation is that it is a perturbation of the disc, possibly the result of ancient warps. Assuming that the ring is smooth and axisymmetric, the total stellar mass in the structure may amount to ~2 × 10⁸ M_solar up to~10⁹ M_solar.