Rare ring galaxies in Serpens and Sculptor

Image: APOD R. Lucas (STScI/AURA), Hubble Heritage Team, NASA
Hoag’s object is an example of a rare ring galaxy, located at RA = 15h 17m 14.4s, dec = +21d 35m 08s, within the boundaries of the constellation of Serpens. These galaxies are thought to be either an end point of the evolution of the structure of a barred spiral galaxy under dynamical, gravitational interaction or the result of a collision, in which galaxies would “pass through” each other with respect to individual stars, but which would nevertheless become structurally disrupted under the influence of tidal forces. The Cartwheel galaxy in Sculptor, below, is likely an example of the latter. In either case, one result seems to be the triggering of one or more waves of star formation in a halo of gas and dust which itself surrounds a core of older stars, the halo being pressurised and locally compressed also by the same gravitational forces, and likely threaded by magnetic fields. Whether or not this might be related to possible past energetic events in the central core is unknown.
Look closely: there is another ring galaxy seen through and beyond Hoag’s object! One would think the likelihood of such a chance alignment on the sky would be infinitesimal, but perhaps these snapshots in time of ongoing interactions between galaxies show collisions or other large-scale effects are more common in the local Universe than might at first be supposed. Hoag’s object lies at a redshift z = 0.04. A full treatment of the origin of unusual galaxy morphologies can be found here.


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