What is Hoag’s Object?
Hoag’s Object is one of the most spectacular ‘ring galaxies’ in the night sky. Unlike normal spiral galaxies, gas and star formation are not distributed in spiral arms throughout its disc, but instead concentrated in a ring. Ring galaxies can be formed in several ways. If one galaxy collides head on with another, shock waves can propagate out through the galaxy disc – just like those seen when you drop a pebble into a pond. This can trigger star formation in a ring as the shock wave passes.
Alternatively, rings can be formed by a stellar bar, an elongated stellar component many galaxies have in their centres. As these rotate, they can create ‘bar deserts’ devoid of gas and rings where the expelled material congregates. Finally, rings can be formed as two galaxies fly by each other.
Gas can be pulled out of the lower mass galaxy as it passes, which eventually settles into a ring. This fly-by scenario is currently the one preferred to explain Hoag’s Object. The elliptical galaxy at the core of the system interacted with another galaxy billions of years ago, accreting the gas forming the
beautiful ring we see today.