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Largest-known spiral galaxy may give birth to new small galaxy, say astronomers

A multinational team of astronomers has discovered the largest-known spiral galaxy. While spiral galaxy NGC 6872 has ranked among the biggest stellar systems for years, astronomers recently labeled it the largest-known spiral galaxy after examining data from NASA’s Galaxy Evolution Explorer (GALEX) mission. NGC 6872 stretches more than 522,000 light-years, making it more than five times the size of our Milky Way galaxy.

GALEX has played an essential role in astronomers’ ability to study NGC 6872.

“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 lead scientist Rafael Eufrasio, a research assistant at NASA’s Goddard Space Flight Center in Greenbelt, Md., in a statement. The study’s findings were recently presented at the American Astronomical Society meeting in Long Beach, Calif.

The galaxy’s strange size and appearance are due to its interaction with a much smaller disk galaxy named IC 4970, which has approximately one-fifth the mass of NGC 6872. The galaxies are located 212 million light-years from Earth in the southern constellation Pavo.

Astronomers believe that large galaxies, including the Milky Way, matured through mergers and acquisitions, forming over billions of years by taking in a lot of smaller systems.

However, astronomers think that the gravitational interaction of NGC 6872 and IC 4970 may have done the opposite, creating what may turn 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, a professor of astronomy at Catholic University, in a statement.

The tidal dwarf candidate is much brighter in the ultraviolet than other regions of the galaxy, an indication that it contains a rich supply of hot young stars less than 200 million years old.

Researchers studied the galaxy using data from ESO’s Very Large Telescope, the Two Micron All Sky Survey, NASA’s Spitzer Space Telescope and the space agency’s GALEX Space Telescope.

By looking at the distribution of energy by wavelength, astronomers discovered a distinct pattern of stellar age along the galaxy’s two prominent spiral arms. While the youngest stars show up in the far end of the northwestern arm, the oldest stars appear toward the galaxy’s center. The same pattern can be found in the southwestern arm of the galaxy.

In 2007, astronomer created computer simulations of the collision that reproduced the overall appearance of the system as it appears today. IC 4970 is said to have made its closest approach approximately 130 million years ago.

According to astronomers, NGC 6872 has a stellar bar component that transitions between the spiral arms and the galaxy’s central regions. The stellar bar measures about 26,000 light-years in radius.

The team discovered no sign of recent star formation along the bar, which suggests that it formed at least a few billion years ago.

“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 team member and Goddard astrophysicist Eli Dwek in a statement.

According to the California Institute of Technology, the GALEX Space Telescope observes galaxies in ultraviolet light across 10 billion years of cosmic history. In 2003, GALEX was launched into orbit by a Pegasus rocket. While NASA originally made plans to send GALEX into orbit for twenty-nine months, a NASA review panel in 2006 recommended that the GALEX mission lifetime be extended. Seven years later, GALEX continues to provide astronomers valuable astronomical data.

In May 2012, NASA lent GALEX to Caltech. In a “first-of-a-kind” move for the space agency, a Space Act Agreement was signed on May 14 so Caltech could continue spacecraft operations and data management for the mission with private money.

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