Astronomers have long known that a spectacular barred spiral galaxy named NGC 6872 is a behemoth, but by compiling data from several space- and ground-based observatories and running a few computer simulations, they have now determined this is the largest spiral galaxy we know of.
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, from the Goddard Space Flight Center the Catholic University of America in Washington. He presented the findings Thursday at the American Astronomical Society meeting in Long Beach, California.
But this galaxy didn’t get so gargantuan all on its own. Astronomers think large galaxies, including our own, grew through mergers and acquisitions — assembling over billions of years by absorbing numerous smaller systems.
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.
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 researchers used archived data from the Galaxy Evolution Explorer (GALEX) mission, and studied the galaxy across the spectrum using data from the European Southern Observatory’s Very Large Telescope, the Two Micron All Sky Survey, and NASA’s Spitzer Space Telescope.
A combined image of the Helix Nebula from the Spitzer Space Telescope,the Galaxy Evolution Explorer (GALEX) and the Wide-field Infrared Survey Explorer (WISE).. Credit: NASA/Caltech
The Helix Nebula has been called the “Eye of God,” or the “Eye of Sauron,” and there’s no denying this object appears to be a cosmic eye looking down on us all. And this new image – a combined view from Spitzer and GALEX — gives a blue tint to the eye that we’ve seen previously in gold, green and turquoise hues from other telescopes. But really, this eye is just a dying star. And it is not going down without a fight. The Helix Nebula continues to glow from the intense ultraviolet radiation being pumped out by the hot stellar core from the white dwarf star, which, by the way, is just a tiny white pinprick right at the center of the nebula.
The Helix nebula, or NGC 7293, lies 650 light-years away in the constellation of Aquarius. Planetary nebulae are the remains of Sun-like stars, and so one day – in about five billion years – our own Sun may look something like this — from a distance. Earth will be toast.
The team from the Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX) that cooperated to create this image describe what is going on:
When the hydrogen fuel for the fusion reaction runs out, the star turns to helium for a fuel source, burning it into an even heavier mix of carbon, nitrogen and oxygen. Eventually, the helium will also be exhausted, and the star dies, puffing off its outer gaseous layers and leaving behind the tiny, hot, dense core, called a white dwarf. The white dwarf is about the size of Earth, but has a mass very close to that of the original star; in fact, a teaspoon of a white dwarf would weigh as much as a few elephants!
The intense ultraviolet radiation from the white dwarf heats up the expelled layers of gas, which shine brightly in the infrared. GALEX has picked out the ultraviolet light pouring out of this system, shown throughout the nebula in blue, while Spitzer has snagged the detailed infrared signature of the dust and gas in red, yellow and green. Where red Spitzer and blue GALEX data combine in the middle, the nebula appears pink. A portion of the extended field beyond the nebula, which was not observed by Spitzer, is from NASA’s all-sky Wide-field Infrared Survey Explorer (WISE).
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I’m going to try and say this before the Bad Astronomer does: Holy Haleakala! A team of astronomers using the Pan-STARRS1 telescope on Mount Haleakala in Hawaii have found evidence of a black hole ripping a star to shreds. While this isn’t the first time this type of activity has been detected, these new observations are the best views so far of what happens to objects that are consumed by a black hole. Plus, astronomers, for the first time, know what kind of star was destroyed and watched as it happened. This all helps in providing more insight into how black holes behave: They aren’t enormous vacuum cleaners that suck up and destroy everything around them, or sharks that seek out and consume their victims. Instead, like Venus Fly Traps, they wait for objects to come to them.
“Black holes, like sharks, suffer from a popular misconception that they are perpetual killing machines,” said Ryan Chornock of the Harvard-Smithsonian Center for Astrophysics (CfA). “Actually, they’re quiet for most of their lives. Occasionally a star wanders too close, and that’s when a feeding frenzy begins.”
If a star passes too close to a black hole, tidal forces can rip it apart. The remaining gases then swirl in toward the black hole. But just a small fraction of the material near a black hole falls in, while most of it just circles for a while – sometimes forever. The material close the black hole gets superheated, causing it to glow. By searching for newly glowing supermassive black holes, astronomers can spot them in the midst of a feast.
So, kind of like with Junior, the giant Venus Fly Trap in the movie “Little Shop of Horrors,” the feast is evident from what doesn’t get eaten.
This computer simulation shows a star being shredded by the gravity of a massive black hole. Some of the stellar debris falls into the black hole and some of it is ejected into space at high speeds. The areas in white are regions of highest density, with progressively redder colors corresponding to lower-density regions. The blue dot pinpoints the black hole’s location. The elapsed time corresponds to the amount of time it takes for a Sun-like star to be ripped apart by a black hole a million times more massive than the Sun.
The team discovered this type of glow on May 31, 2010, with Pan-STARRS1 and also with NASA’s Galaxy Evolution Explorer (GALEX). The flare brightened to a peak on July 12th before fading away over the course of a year. The event took place in a galaxy 2.7 billion light-years away, and the black hole contains as much mass as 3 million Suns, making it about the same size as the Milky Way’s central black hole.
“We observed the demise of a star and its digestion by the black hole in real time,” said Harvard co-author Edo Berger.
“We’re also witnessing the spectral signature of the ejected gas, said Suvi Gezari of The Johns Hopkins University who lead the research, “which we find to be mostly helium. It is like we are gathering evidence from a crime scene. Because there is very little hydrogen and mostly helium in the gas we detect from the carnage, we know that the slaughtered star had to have been the helium-rich core of a stripped star.”
Follow-up observations with the MMT Observatory in Arizona showed that the black hole was consuming large amounts of helium. Therefore, the shredded star likely was the core of a red giant star. The lack of hydrogen showed this is likely not the first time the star had encountered the same black hole, and that it lost its outer atmosphere on a previous pass.
The star may have been near the end of its life, the astronomers say. After consuming most of its hydrogen fuel, it had probably ballooned in size, becoming a red giant. The astronomers think the bloated star was looping around the black hole in a highly elliptical orbit, similar to a comet’s elongated orbit around the Sun.
This computer-simulated image shows gas from a tidally shredded star falling into a black hole. Some of the gas also is being ejected at high speeds into space. Astronomers observed a flare in ultraviolet and optical light from the gas falling into the black hole and glowing helium from the stars's helium-rich gas expelled from the system. Credit: NASA, S. Gezari (The Johns Hopkins University), and J. Guillochon (University of California, Santa Cruz)
“This is the first time where we have so many pieces of evidence, and now we can put them all together to weigh the perpetrator (the black hole) and determine the identity of the unlucky star that fell victim to it,” Gezari said. “These observations also give us clues to what evidence to look for in the future to find this type of event.”
The team’s results were published today in the online edition of the journal Nature.
Supernova explosions and the jets of a monstrous black hole are scattering one galaxy’s star-making gas, driving a dramatic transformation from spiral galactic youth to elderly elliptical, according to a new study of a recently merged galaxy. Cool gas, the fuel from which new stars form, is essential to the youth and vigor of a galaxy. But supernova explosions can start the decline in star formation, and then shock waves from the supermassive black hole finish the job, turning spiral galaxies to “red and dead” ellipticals.
Astronomers think they have identified a recently merged galaxy, NGC 3801, where this gas loss has just gotten underway. Using ultraviolet observations from NASA’s Galaxy Evolution Explorer (GALEX) and a host of other instruments, the new findings fill an important gap in the current understanding of galactic evolution.
“We have caught a galaxy in the act of destroying its gaseous fuel for new stars and marching toward being a red-and-dead type of galaxy,” said Ananda Hota, lead author of a new paper in the Monthly Notices of the Royal Astronomical Society. “We have found a crucial missing piece to connect and solve the puzzle of this phase of galaxy evolution.”
It has long been known that gas-rich spiral galaxies like our Milky Way smash together to create elliptical galaxies such as the one observed in the study. These big, round galaxies have very little star formation.
The supermassive black holes that reside in the centers of galaxies can flare up when engorged by gas during galactic mergers. As a giant black hole feeds, colossal jets of matter shoot out from it, giving rise to what is known as an active galactic nucleus. According to theory, shock waves from these jets heat up and disperse the reservoirs of cold gas in elliptical galaxies, thus preventing new stars from taking shape.
NGC 3801 shows signs of such a process. This galaxy is unique in that evidence of a past merger is clearly seen, and the shock waves from the central black hole’s jets have started to spread out very recently. The researchers used the Galaxy Evolution Explorer to determine the age of the galaxy’s stars and decipher its evolutionary history. The ultraviolet observations show that NGC 3801’s star formation has petered out over the last 100 to 500 million years, demonstrating that the galaxy has indeed begun to leave behind its youthful years. The lack of many big, new, blue stars makes NGC 3801 look yellowish and reddish in visible light, and thus middle-aged.
What’s causing the galaxy to age and make fewer stars? The short-lived blue stars that formed right after it merged with another galaxy have already blown up as supernovae. Data from NASA’s Hubble Space Telescope revealed that those stellar explosions have triggered a fast outflow of heated gas from NGC 3801’s central regions. That outflow has begun to banish the reserves of cold gas, and thus cut into NGC 3801’s overall star making.
Some star formation is still happening in NGC 3801, as shown in ultraviolet wavelengths observed by the Galaxy Evolution Explorer, and in infrared wavelengths detected by NASA’s Spitzer Space Telescope. But that last flicker of youth will soon be extinguished by colossal shock waves from the black hole’s jets, seen in X-ray light by NASA’s Chandra X-ray Observatory. These blast waves are rushing outward from the galactic center at a velocity of nearly two million miles per hour (nearly 900 kilometers per second). The waves will reach the outer portions of NGC 3801 in about 10 million years, scattering any remaining cool hydrogen gas and rendering the galaxy truly red and dead.
Astronomers think the transition captured early-on in the case of NGC 3801 — from the merger of gas-rich galaxies to the rise of an old-looking elliptical — happens very quickly on cosmic time scales.
“The quenching of star formation by feedback from the active galactic nucleus probably occurs in just a billion years. That’s not very long compared to the 10-billion-year age of a typical big galaxy,” said Hota. “The explosive shock wave event caused by the central black hole is so powerful that it can dramatically change the future course of the evolution of an entire galaxy.”
Additional observations for the study in optical light come from the Sloan Digital Sky Survey and in radio using the Very Large Array in New Mexico.
Hota is an astronomer in Pune, India, conducted the study as a post-doctoral research fellow at the Institute of Astronomy & Astrophysics at Academia Sinica in Taipei, Taiwan.
A mission which helped map the ultraviolet sky and worked to confirm the nature of dark energy is coming to an end. Galaxy Evolution Explorer, or GALEX, was placed in standby mode today after nearly nine years of service and will be decommissioned later this year. With data from the mission, scientists were able to catalog millions of galaxies spanning 10 billion years of cosmic time.
The Galaxy Evolution Explorer launched in April of 2003 on board a Pegasus XL rocket. It completed its prime mission in the fall of 2007, but the mission was extended to continue its census of stars and galaxies. The variable star Mira. Image credit: Galex
Other mission highlights include the discovery of a gigantic comet-like tail behind a speeding star, finding rings of new stars around old galaxies, exploring “teenager” galaxies, which help to explain how galaxies evolve, and catching a black hole devouring a star.
The mission was part of NASA’s Explorer’s program and was built and managed by the Jet Propulsion Laboratory. Scientists from around the world participated in GALEX studies.
A five-year survey of 200,000 galaxies, stretching back seven billion years in cosmic time, has led to one of the best independent confirmations that dark energy is driving our universe apart at accelerating speeds. The survey used data from NASA’s space-based Galaxy Evolution Explorer and the Anglo-Australian Telescope on Siding Spring Mountain in Australia.
The findings offer new support for the favored theory of how dark energy works — as a constant force, uniformly affecting the universe and propelling its runaway expansion. They contradict an alternate theory, where gravity, not dark energy, is the force pushing space apart. According to this alternate theory, with which the new survey results are not consistent, Albert Einstein’s concept of gravity is wrong, and gravity becomes repulsive instead of attractive when acting at great distances.
“The action of dark energy is as if you threw a ball up in the air, and it kept speeding upward into the sky faster and faster,” said Chris Blake of the Swinburne University of Technology in Melbourne, Australia. Blake is lead author of two papers describing the results that appeared in recent issues of the Monthly Notices of the Royal Astronomical Society. “The results tell us that dark energy is a cosmological constant, as Einstein proposed. If gravity were the culprit, then we wouldn’t be seeing these constant effects of dark energy throughout time.”
Dark energy is thought to dominate our universe, making up about 74 percent of it. Dark matter, a slightly less mysterious substance, accounts for 22 percent. So-called normal matter, anything with atoms, or the stuff that makes up living creatures, planets and stars, is only approximately four percent of the cosmos.
The idea of dark energy was proposed during the previous decade, based on studies of distant exploding stars called supernovae. Supernovae emit constant, measurable light, making them so-called “standard candles,” which allows calculation of their distance from Earth. Observations revealed dark energy was flinging the objects out at accelerating speeds.
his diagram illustrates two ways to measure how fast the universe is expanding -- the "standard candle" method, which involves exploded stars in galaxies, and the "standard ruler" method, which involves pairs of galaxies. Image credit: NASA/JPL-Caltech
Dark energy is in a tug-of-war contest with gravity. In the early universe, gravity took the lead, dominating dark energy. At about 8 billion years after the Big Bang, as space expanded and matter became diluted, gravitational attractions weakened and dark energy gained the upper hand. Billions of years from now, dark energy will be even more dominant. Astronomers predict our universe will be a cosmic wasteland, with galaxies spread apart so far that any intelligent beings living inside them wouldn’t be able to see other galaxies.
The new survey provides two separate methods for independently checking the supernovae results. This is the first time astronomers performed these checks across the whole cosmic timespan dominated by dark energy. The team began by assembling the largest three-dimensional map of galaxies in the distant universe, spotted by the Galaxy Evolution Explorer. The ultraviolet-sensing telescope has scanned about three-quarters of the sky, observing hundreds of millions of galaxies.
“The Galaxy Evolution Explorer helped identify bright, young galaxies, which are ideal for this type of study,” said Christopher Martin, principal investigator for the mission at the California Institute of Technology in Pasadena. “It provided the scaffolding for this enormous 3-D map.”
The astronomers acquired detailed information about the light for each galaxy using the Anglo-Australian Telescope and studied the pattern of distance between them. Sound waves from the very early universe left imprints in the patterns of galaxies, causing pairs of galaxies to be separated by approximately 500 million light-years.
This “standard ruler” was used to determine the distance from the galaxy pairs to Earth — the closer a galaxy pair is to us, the farther apart the galaxies will appear from each other on the sky. As with the supernovae studies, this distance data were combined with information about the speeds at which the pairs are moving away from us, revealing, yet again, the fabric of space is stretching apart faster and faster.
The team also used the galaxy map to study how clusters of galaxies grow over time like cities, eventually containing many thousands of galaxies. The clusters attract new galaxies through gravity, but dark energy tugs the clusters apart. It slows down the process, allowing scientists to measure dark energy’s repulsive force.
“Observations by astronomers over the last 15 years have produced one of the most startling discoveries in physical science; the expansion of the universe, triggered by the Big Bang, is speeding up,” said Jon Morse, astrophysics division director at NASA Headquarters in Washington. “Using entirely independent methods, data from the Galaxy Evolution Explorer have helped increase our confidence in the existence of dark energy.”
For more information see the Australian Astronomical Observatory
Title this ‘Zombie Galaxies’ or ‘Night of the Living Galaxies.’ Astronomers have found mysterious, giant loops of ultraviolet light around old, massive galaxies, which were presumed to be “dead,” and these galaxies seem to have come back to 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 old bloated galaxies that were once 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.”
Using two orbiting observatories, NASA’s Galaxy Evolution Explorer and Hubble Space Telescope, the astronomers surveyed a vast region of the sky in ultraviolet light. GALEX picked out 30 elliptical and lens-shaped “early” galaxies with puzzlingly strong ultraviolet emissions but no signs of visible star formation, and Hubble was used to take a closer look.
What Hubble showed shocked the astronomers. 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.”
But astronomers are unsure where the gas for this galactic resurrection came from and how it has created rings. One possibility is that a smaller galaxy merged with a big, old one, bringing 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.”
Another option that the astronomers like better is that the rejuvenating spark could have come 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.
The study detailing the findings appeared in the April 21 issue of the Astrophysical Journal.
