NASA Turns Off Galaxy Evolution Explorer --"Discovered Giant Rings of New Stars Around Ancient, Dead Galaxies"
NASA has turned off its Galaxy Evolution Explorer (GALEX) after a decaden of operations in which the venerable space telescope used its ultraviolet vision to study hundreds of millions of galaxies across 10 billion years of cosmic time. In the space telescope's last year, it scanned across large patches of sky, including the bustling, bright center of our Milky Way. The telescope spent time staring at certain areas of the sky, findingexploded stars, called supernovae, and monitoring how objects, such as the centers of active galaxies,change over time. GALEX also scanned the sky for massive, feeding black holes and shock waves from early supernova explosions.
In 2010, NASA astronomers have found mysterious, giant loops of ultraviolet light in aged, massive galaxies, which seem to have a second lease on life. Somehow these "over-the-hill galaxies" have been infused with fresh gas to form new stars that power these truly gargantuan rings, some of which could encircle several Milky Way galaxies.
The discovery of these rings implies that bloated galaxies presumed "dead" and devoid of star-making can be reignited with star birth, and that galaxy evolution does not proceed straight from the cradle to the grave.
"In a galaxy's lifetime, it must make the transition from an active, star-forming galaxy to a quiescent galaxy that does not form stars," said Samir Salim, lead author of a recent study and a research scientist in the department of astronomy at Indiana University, Bloomington. "But it is possible this process goes the other way, too, and that old galaxies can be rejuvenated."
The findings came courtesy of the Galaxy Evolution Explorer and Hubble Space Telescope. First, the Galaxy Evolution Explorer surveyed a vast region of the sky in ultraviolet light. The satellite picked out 30 elliptical and lens-shaped "early" galaxies with puzzlingly strong ultraviolet emissions but no signs of visible star formation. Early-type galaxies, so the scientists' thinking goes, have already made their stars and now lack the cold gas necessary to build new ones.
The Galaxy Evolution Explorer could not discern the fine details of these large, rounded galaxies gleaming in the ultraviolet, so to get a closer look, researchers turned to the Hubble Space Telescope. What they saw shocked them: three-quarters of the galaxies were spanned by great, shining rings of ultraviolet light, with some ripples stretching 250,000 light-years. A few galaxies even had spiral-shaped ultraviolet features.
"We haven't seen anything quite like these rings before," said Michael Rich, co-author of the paper and a research astronomer at UCLA. "These beautiful and very unusual objects might be telling us something very important about the evolution of galaxies."
Astronomers can tell a galaxy's approximate age just by the color of its collective starlight. Lively, young galaxies look bluish to our eyes due to the energetic starlight of their new, massive stars. Elderly galaxies instead glow in the reddish hues of their ancient stars, appearing "old, red and dead," as astronomers bluntly say. Gauging by the redness of their constituent stars, the galaxies seen by the Galaxy Evolution Explorer and Hubble are geezers, with most stars around 10 billion years old.
But relying on the spectrum of light visible to the human eye can be deceiving, as some of us have found out after spending a day under the sun's invisible ultraviolet rays and getting a sunburn. Sure enough, when viewed in the ultraviolet part of the spectrum, these galaxies clearly have more going on than meets the eye.
Some ultraviolet starlight in a few of the observed galaxies might just be left over from an initial burst of star formation. But in most cases, new episodes of star birth must be behind the resplendent rings, meaning that fresh gas has somehow been introduced to these apparently ancient galaxies. Other telltale signs of ongoing star formation, such as blazing hydrogen gas clouds, might be on the scene as well, but have so far escaped detection.
Just where the gas for this galactic resurrection came from and how it has created rings remains somewhat perplexing. A merging with a smaller galaxy would bring in fresh gas to spawn hordes of new stars, and could in rare instances give rise to the ring structures as well.
But the researchers have their doubts about this origin scenario. "To create a density shock wave that forms rings like those we've seen, a small galaxy has to hit a larger galaxy pretty much straight in the center," said Salim. "You have to have a dead-on collision, and that's very uncommon."
Rather, the rejuvenating spark more likely came from a gradual sopping-up of the gas in the so-called intergalactic medium, the thin soup of material between galaxies. This external gas could generate these rings, especially in the presence of bar-like structures that span some galaxies' centers.
Ultimately, more observations will be needed to show how these galaxies began growing younger and lit up with humongous halos. Salim and Rich plan to search for more evidence of bars, as well as faint structures that might be the remnants of stellar blooms that occurred in the galaxies' pasts. Rather like recurring seasons, it may be that galaxies stirred from winter can breed stars again and then bask in another vibrant, ultraviolet-soaked summer.
Operators at Orbital Sciences Corporation in Dulles, Va., sent the signal to decommission GALEX at12:09 p.m. PDT (3:09 p.m. EDT) Friday, June 28. The spacecraft will remain in orbit for at least 65 years,then fall to Earth and burn up upon re-entering the atmosphere. GALEX met its prime objectives and themission was extended three times before being cancelled.
In addition to finding giant rings of new stars around old, dead galaxies, hghlights from the mission's decade of sky scans include: discovering a gargantuan, comet-like tail behind a speeding star called Mira; catching a black hole "red-handed" as it munched on a star; independently confirming the nature of dark energy; and discovering a missing link in galaxy evolution -- the teenage galaxies transitioning from young to old.
The mission also captured a dazzling collection of snapshots, showing everything from ghostly
nebulas to a spiral galaxy with huge, spidery arms.
In a first-of-a-kind move for NASA, the agency in May 2012 loaned GALEX to the California
Institute of Technology in Pasadena, which used private funds to continue operating the satellite
while NASA retained ownership. Since then, investigators from around the world have used GALEX
to study everything from stars in our own Milky Way galaxy to hundreds of thousands of galaxies 5
billion light-years away.
"In the last few years, GALEX studied objects we never thought we'd be able to observe, from the
Magellanic Clouds to bright nebulae and supernova remnants in the galactic plane," said David
Schiminovich of Columbia University, N.Y., N.Y, a longtime GALEX team member who led science
operations over the past year. "Some of its most beautiful and scientifically compelling images are
part of this last observation cycle."
Data from the last year of the mission will be made public in the coming year.
"GALEX, the mission, may be over, but its science discoveries will keep on going," said Kerry
Erickson, the mission's project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
A slideshow showing some of the popular GALEX images is online at: http://go.nasa.gov/17xAVDd
Graphics and additional information about the Galaxy Evolution Explorer are online at: