New Largest Known Spiral Galaxy Crowned

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Spiral galaxy NGC 6872 has now been found to be the largest-known spiral galaxy in the world, based on new research done by an international team of researchers.

The massive galaxy has been known as one of, but not the largest stellar systems for a long time, but now thanks to data gathered by NASA’s Galaxy Evolution Explorer (GALEX) mission, it has been found to be the largest one yet known.

When the galaxy is measured “tip-to-tip across its two outsized spiral arms, NGC 6872 spans more than 522,000 light-years, making it more than five times the size of our Milky Way galaxy.”

“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 primary researcher Rafael Eufrasio, an assistant at NASA’s Goddard Space Flight Center in Greenbelt, Md., and also a doctoral student at Catholic University of America in Washington.

The unusual scale and structure of the galaxy is thought to be caused by its interactions “with a much smaller disk galaxy named IC 4970, which has only about one-fifth the mass of NGC 6872. The odd couple is located 212 million light-years from Earth in the southern constellation Pavo.”

It’s currently accepted theory that large galaxies, such as the Milky Way, formed through recurrent collisions and mergers with other galaxies, over a period of billions of years.

“Intriguingly, the gravitational interaction of NGC 6872 and IC 4970 may have done the opposite, spawning what may develop into a new small galaxy.”

“The northeastern arm of NGC 6872 is the most disturbed and is rippling with star formation, but at its far end, visible only in the ultraviolet, is an object that appears to be a tidal dwarf galaxy similar to those seen in other interacting systems,” said researcher Duilia de Mello, a professor of astronomy at Catholic University.

“The tidal dwarf candidate is brighter in the ultraviolet than other regions of the galaxy, a sign it bears a rich supply of hot young stars less than 200 million years old.”

“The researchers studied the galaxy across the spectrum using archival data from the European Southern Observatory’s Very Large Telescope, the Two Micron All Sky Survey, and NASA’s Spitzer Space Telescope, as well as GALEX.”

“By analyzing the distribution of energy by wavelength, the team uncovered a distinct pattern of stellar age along the galaxy’s two prominent spiral arms. The youngest stars appear in the far end of the northwestern arm, within the tidal dwarf candidate, and stellar ages skew progressively older toward the galaxy’s center.”

“The southwestern arm displays the same pattern, which is likely connected to waves of star formation triggered by the galactic encounter.”

“A 2007 study by Cathy Horellou at Onsala Space Observatory in Sweden and Baerbel Koribalski of the Australia National Telescope Facility developed computer simulations of the collision that reproduced the overall appearance of the system as we see it today. According to the closest match, IC 4970 made its closest approach about 130 million years ago and followed a path that took it nearly along the plane of the spiral’s disk in the same direction it rotates. The current study is consistent with this picture.”

“As in all barred spirals, NGC 6872 contains a stellar bar component that transitions between the spiral arms and the galaxy’s central regions. Measuring about 26,000 light-years in radius, or about twice the average length found in nearby barred spirals, it is a bar that befits a giant galaxy.”

“The team found no sign of recent star formation along the bar, which indicates it formed at least a few billion years ago. Its aged stars provide a fossil record of the galaxy’s stellar population before the encounter with IC 4970 stirred things up.”

“Understanding the structure and dynamics of nearby interacting systems like this one brings us a step closer to placing these events into their proper cosmological context, paving the way to decoding what we find in younger, more distant systems,” said researcher and Goddard astrophysicist Eli Dwek.

The science of galaxy formation is unsettled though, for obvious reasons, we don’t have the capacity to truly observe these processes occurring. We only have the ability to see the different ‘states’ that ‘galaxies’ appear to be in, and infer the transformations that took place between these states. But this assumes that galaxy formation follows regular and constant principles, which it very well may not. At any rate there remains much that is unknown about the process, and a lot of intriguing research on the subject currently being done.

Source: NASA/Goddard Space Flight Center

Image Credits: NASA’s Goddard Space Flight Center/ESO/JPL-Caltech/DSS

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