GALEX Mission Reveals Largest Known Spiral Galaxy

By  //  March 4, 2014

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System Five Times Size Of Milky Way

This composite of the giant barred spiral galaxy NGC 6872 combines visible light images from the European Southern Observatory’s Very Large Telescope with far-ultraviolet (1,528 angstroms) data from NASA’s GALEX and 3.6-micron infrared data acquired by NASA’s Spitzer Space Telescope. A previously unsuspected tidal dwarf galaxy candidate (circled) appears only in the ultraviolet, indicating the presence of many hot young stars. IC 4970, the small disk galaxy interacting with NGC 6872, is located above the spiral’s central region. (Image coutresy of NASA’s Goddard Space Flight Center/ESO/JPL-Caltech/DSS)

BREVARD COUNTY • KENNEDY SPACE CENTER, FLORIDA –The spectacular barred spiral galaxy NGC 6872 has ranked among the biggest stellar systems for decades.

Now a team of astronomers from the United States, Chile and Brazil has crowned it the largest known spiral, based on archival data from NASA’s Galaxy Evolution Explorer (GALEX) mission, which has been loaned to the California Institute of Technology.

NASA’s Galaxy Evolution Explorer (GALEX) is an orbiting space telescope observing galaxies in ultraviolet light across 10 billion years of cosmic history. It was launched in 2003 from a Pegasus rocket. (Image courtesy of NASA)

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 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., who is a doctoral student at Catholic University of America in Washington.

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 — assembling over billions of years by absorbing numerous smaller systems.

“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.” Duilia de Mello, professor of astronomy at Catholic University

NEW GALAXY

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, a professor of astronomy at Catholic University.

The tidal dwarf candidate is brighter in 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.

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.

Computer simulations of the collision between NGC 6872 and IC 4970 reproduce the basic features of the galaxies as we see them today. (Image courtesy of NASA)

BILLIONS OF YEARS OLD

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 team member and Goddard astrophysicist Eli Dwek.

The study also included Fernanda Urrutia-Viscarra and Claudia Mendes de Oliveira at the University of Sao Paulo in Brazil and Dimitri Gadotti at the European Southern Observatory in Santiago, Chile.

GALEX was developed under NASA’s Explorers Program. Last May NASA loaned GALEX to Caltech, which continues spacecraft operations and data management using private funds.

The Galaxy Evolution Explorer (GALEX) is an orbiting space telescope observing galaxies in ultraviolet light across 10 billion years of cosmic history.

A Pegasus rocket launched GALEX into orbit on April 28, 2003. Although originally planned as a 29-month mission, the NASA Senior Review Panel in 2006 recommended that the mission lifetime be extended.

GALEX’s observations are telling scientists how galaxies, the basic structures of our universe, evolve and change. Additionally, GALEX observations are investigating the causes of star formation during a period when most of the stars and elements we see today had their origins.

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