ESO 498-G5, a spiral within a spiral

 
ESO 498-G5, a spiral within a spiral

ESO 498-G5 (AM 0922-245) is a spiral galaxy located 100 million light-years away in the constellation of Pyxis.

Its luminous spiral arms with dark filaments wind all the way into the centre, so that ESO 498-G5’s core looks a bit like a miniature spiral galaxy. Astronomers refer to the distinctive spiral-like bulges as disc-type bulges, or pseudobulges. The bright elliptical centres of many spiral galaxies, which appear as glowing masses, are called classical bulges.

Star formation is still going on in disc-type bulges and has ceased in classical bulges. Classical bulges look much like a miniature version of an elliptical galaxy, embedded in the centre of a spiral, while disc-type bulges look like a second, smaller spiral galaxy located at the heart of the first — a spiral within a spiral.

The similarities between types of galaxy bulge and types of galaxy go beyond their appearance. Just like giant elliptical galaxies, the classical bulges consist of great swarms of stars moving about in random orbits. Conversely, the structure and movement of stars within disc-type bulges mirror the spiral arms arrayed in a galaxy’s disc. These differences suggest different origins for the two types of bulges: while classical bulges are thought to develop through major events, such as mergers with other galaxies, disc-type bulges evolve gradually, developing their spiral pattern as stars and gas migrate to the galaxy’s centre.

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