The spectacular barred spiral galaxy NGC
6872 has ranked among the biggest stellar systems for decades. A team
of astronomers from the United States, Chile and Brazil have crowned it
the largest-known spiral, based on archival data from NASA's Galaxy
Evolution Explorer (GALEX) mission, which has since been loaned to the
California Institute of Technology. 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.
"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.
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.
Computer simulations of the collision between NGC 6872 and IC 4970
reproduce the basic features of the galaxies as we see them today. They
indicate that IC 4970's closest encounter occurred 130 million years ago
and that the smaller galaxy followed a path (dashed curve) close to the
plane of the spiral's disk and in the same direction it rotates. 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.
Source: The Daily Galaxy via www.nasa.gov
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