The nature of a dusty ring in Virgo

We detected a ring-like distribution of far-infrared (FIR) emission in the direction of the centre of the Virgo cluster (VC). We studied this feature in the FIR, radio and optical domains, and deduced that the dust within the feature reddens the galaxies in the direction of the VC but does not affect stars within the Milky Way. This is likely to be a dusty feature in the foreground of the VC, presumably in the Galactic halo. The HI distribution follows the morphology of the FIR emission and shows peculiar kinematic behaviour. We propose that a highly supersonic past collision between an HI cloud and the Galactic HI formed a shock that heated the interface gas to soft X-ray temperatures. HI remnants from the projectile and from the shocked Galactic HI rain down on to the disc as intermediate-velocity gas. Our finding emphasizes that extragalactic astronomy must consider the possibility of extinction by dust at high galactic latitude and far from the Galactic plane, which may show structure on 1° and smaller scales. This is particularly important for studies of the VC, e.g. in the determination of the Hubble constant from Cepheids in cluster galaxies.