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| [Image Credit: NASA's Goddard Space Flight Center/ESO/JPL-Caltech/DSS] |
Washington: 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.
GALEX has since been loaned to the California Institute of Technology in
Pasadena, Calif.
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., and a doctoral student at Catholic
University of America in Washington. He presented the findings Thursday at the
American Astronomical Society meeting in Long Beach, Calif.
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.
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 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 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.