Scientists have discovered that a barred spiral galaxy located 212 million light years from the earth is the largest-known spiral, measuring five times the size of our Milky Way galaxy.

Using archival data from Nasa’s Galaxy Evolution Explorer (GALEX) mission, scientists found that measuring 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 recognised the full extent of this intriguing system,” lead scientist Rafael Eufrasio, a research assistant at Nasa’s Goddard Space Flight Center in Greenbelt said in a statement.

The galaxy’s unusual size and appearance stem from its interaction 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.
Astronomers think large galaxies, including our own, grew through mergers and acquisitions. 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 team member Duilia de Mello.

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 analysing 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.

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.