Jan. 10, 2013
J. D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov
Lynn Chandler
Goddard Space Flight Center, Greenbelt, Md.
301-286-2806
lynn.chandler-1@nasa.gov
RELEASE: 13-014
NASA'S GALEX REVEALS THE LARGEST-KNOWN SPIRAL GALAXY
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.
The GALEX mission is led by the California Institute of Technology in
Pasadena, which is responsible for science operations and data
analysis. NASA's Jet Propulsion Laboratory, also in Pasadena, manages
the mission and built the science instrument. GALEX was developed
under NASA's Explorers Program managed by NASA's Goddard Space Flight
Center. In May 2012, In May 2012, NASA announced it was loaning GALEX
to Caltech, which continues spacecraft operations and data management
using private funds.
For images related to this finding, visit:
http://go.nasa.gov/WpGpDJ
-end-
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