A galactic bullseye
Located approximately 300 million light-years away in the southern constellation of Volans, the galaxy AM 0644-741 is a peculiar galaxy. Also known as the Lindsay-Shapley Ring, it consists of a yellowish core surrounded by a brilliant, detached ring 150,000 light-years in diameter. This structure is the aftermath of a colossal galactic collision. The currently accepted model suggests that a smaller intruder galaxy passed directly through the center of what was once a normal spiral galaxy.
This cosmic impact generated a gravitational disruption, creating a density wave that expands outwards through interstellar gas and dust, much as a ripple spreads from a stone dropped in a pond. This wave compresses the gas, triggering a massive wave of star formation. The expanding ring is intensely blue, caused by of the formation of huge, hot, young stars which burn with a blue light. The ring is not uniform; it is dotted with pink regions, which are clouds of glowing hydrogen gas, ionized by the intense ultraviolet radiation from the newborn stars.
The double-ring puzzle
The most perplexing feature of AM 0644-741 is the presence of a second, fainter inner ring. While collisional models readily account for a single, expanding ring of star birth, the formation of two distinct, stable rings presents a significant challenge to astrophysicists. Observations show that massive star formation bifurcates into two closely spaced rings north of the nucleus, with particularly intense activity where the rings appear to intersect. One hypothesis suggests that the unique double-ring structure reflects strong caustics—areas where light rays converge—along the inner and outer edges of a single, powerful density wave.
The central "yolk" of the galaxy is the original core of the target galaxy, composed of older, yellower stars. Data from the Chandra X-ray Observatory reveals that the main ring is a site of star birth, and of stellar death. It contains a striking number of very bright X-ray sources, believed to be binary systems where a black hole or neutron star is pulling matter from a companion star. These are the remnants of the most massive stars that formed in the ring and have already ended their lives in supernova explosions. Galactic simulation models predict the main ring will continue to expand for another 300 million years before it begins to disintegrate.