Utilizing the latest ultraviolet technology for observing the heavens, scientists have identified the largest known spiral galaxy– a swirling beauty with an unusual backstory.
The poetically named NGC 6872 is the new title-holder, at roughly 5 times the size of Milky Way. The barred spiral galaxy lies 212 million light-years from Earth in the southern constellation Pavo, and spreads two huge spiral arms 522,000 light-years across the cosmos. Our Milky Way has a diameter of merely 100,000 light-years by comparison.
The new finding was made quite by accident after a detailed study of data from several instruments, including NASA’s state-of-the-art Galaxy Evolution Explorer spacecraft, or GALEX.
“I was not looking for the largest spiral – it just came as a gift,” Rafael Eufrasio of the Catholic University of America and Nasa’s Goddard Space Flight Center told BBC News. For decades NGC 6872 has held a proud position among the biggest known spiral galaxies, “but it’s much larger than we thought,” explained Eufrasio.
Eufrasio and his colleagues were busy searching through data from GALEX for star-forming regions around NGC 6872, when they noticed vast swathes of ultraviolet light emitted from young stars at the galaxy’s fringes.
“Without GALEX’s ability to detect the ultraviolet light of the youngest, hottest stars, we would never have recognized the full extent of this intriguing system,” said Eufrasio.
Both the tremendous size and unusual shape of NCG 6872 are a direct result of crashing into a neighboring galaxy called IC 4970, which has only one fifth the mass of its larger sister. “The galaxy that collided with the [central disc of NGC 6872] splashed stars all over the place – 500,000 light-years away,” said Eufrasio.
Computer simulations based on rigorous observations suggest that the two galaxies collided about 130 million years ago. “We’re just seeing one example of two interacting galaxies but in the past that happened much more often – that’s how the big [spiral galaxy] discs we have were probably formed,” Eufrasio noted. “Putting that in a larger context, it’s a very cool system.”
“It shows the evolution of galaxies in the larger context of the Universe – how the large galaxies we had before were accreted from small clumps in the early Universe,” he added.
The bar that links NGC 6872’s galactic arms to its central regions is also massive, with a radius of 26,000 light years. This makes it about two times larger than the bars of nearby spirals. The team estimates that the bar formed several billion years ago or more, given the lack of evidence for any recent star formation in the section.
While excited by their discovery, Eufrasio and his colleagues emphasized that bigger spirals than NGC 6872 may well exist, just waiting to be found as our instruments and techniques for studying celestial bodies in the far reaches of space become increasingly sophisticated.
A prime example of such technological sophistication, the GALEX spacecraft, has been operational since 2003, seeking to better understand the history of star formation in the universe. After spending $150 million on the project, NASA cut funding in February 2011. In May 2012 the California Institute of Technology took control, and is leveraging private funding to continue the mission and enable future discoveries.