Mysterious Gamma Rays Ballooning from Milky Way’s Core

WASHINGTON — NASA held a media teleconference on Nov. 9, to discuss a new discovery by the Fermi Gamma-ray Space Telescope. Astronomers have announced that there were two huge bubbles emitting gamma rays have been found billowing from the center of the Milky Way galaxy. These previously unseen structures was detected by NASA’s Fermi Gamma-ray Space Telescope, which extends to 25,000 light-years north and south from the galactic core.

Gamma rays are the most energetic forms of light, and in space they tend to come from violent events such as supernovae or from extreme objects such as black holes and neutron stars. A gamma ray burst from a supernova, for example, can unleash more energy in just ten seconds than the sun will over its ten-billion-year lifetime.

These are so powerful that according to a study last year, a brilliant burst of gamma rays may have caused a mass extinction event on Earth 440 million years ago—and a similar celestial catastrophe could happen again. The computer model showed that a gamma-ray burst aimed at Earth could deplete the ozone layer, cause acid rain, and initiate a round of global cooling from as far as 6,500 light-years away. Such a disaster may have been responsible for the mass die-off of 70 percent of the marine creatures that thrived during the Ordovician period (488 to 443 million years ago), which was suggested by study leader Brian Thomas, an astrophysicist at Washburn University in Kansas.

The computer simulation also showed that a significant gamma-ray burst is likely to go off within range of Earth every billion years or so, although the stream of radiation would have to be lined up just right to affect the planet. This hypothesis was first proposed in 2004 that a gamma-ray burst near Earth wiped out Ordovician life. Since then, researchers have been tackling pieces of the puzzle.

Currently Wolf-Rayet star named WR104, a massive star 8,000 light-years away in the constellation Sagittarius, is in position to be a potential threat. A Wolf-Rayet star is a highly unstable star coming to the end of its life, possibly culminating in a powerful, planet-killing gamma-ray burst. University of Sydney astronomer Peter Tuthill discovered the system ten years ago. Wolf-Rayet stars are evolved massive stars undergoing a suicidal and violent death. They are very hot (up to 50,000K) and losing mass very quickly, generating powerful stellar winds (at velocities of 2000 km/s). WR 104 was imaged using the Keck Telescope in Hawaii, and images of the pinwheel spiral star system appeared to show that we were “looking down a rifle barrel” — a potentially dangerous front-row seat when the star eventually explodes, resulting in a cataclysmic event.

If we are the unlucky species to experience such event, chemical reactions in the atmosphere would produce dark, nitrogen-based gases that would block the sun’s heat and trigger global cooling, even as the gamma rays continued to deplete ozone and let in UV rays, as the study suggested. Some of the pollution would fall as damaging acid rain, which can severely disrupt ecosystems.

The atmosphere might be able to recover within a decade, and a rise in DNA damage caused by increased UV exposure might pass after a few months or years, the researchers note. But other biological impacts—such as reduced ocean productivity—could linger for an unknown length of time

Meanwhile, these newfound bubbles this week, are made of hot, charged gas that’s releasing the same amount of energy as a hundred thousand exploding stars.

Several hypothesis have been mentioned. One is that gamma-ray bubbles are evidence of an ancient burst of star formation at the center of the galaxy. If a huge cluster of massive stars formed millions of years ago, the giants could now be dying together, creating an outbreak of supernovae. According to Finkbeiner, the bubbles could represent “the accumulated energy over many millions of years.”

The even more dramatic thought, is that the [mostly dormant] black hole at the center of the galaxy is being active for a little bit. This hypothesis is possible since scientists claim that we have a supermassive black hole  that resides at the center of our galaxy, and that it didn’t get so big by sitting quietly. Instead, the black hole must go through stages when it gobbles up massive amounts of material. When galactic black holes are actively feeding, they tend to spew high-energy jets from their poles. Astronomers have found such active galactic nuclei elsewhere in the universe, but have never before seen any convincing proof of this process happening in the Milky Way.

The study team says that they have ruled out another theory that the bubbles could be proof of the mysterious substance known as dark matter.

According to theory, dark matter particles annihilate when they collide, releasing showers of new particles along with huge amounts of energy. It’s thought that dense clumps of dark matter exist at the cores of galaxies, so looking for the results of collisions is one way astronomers hope to prove the substance exists.

“What bothers me about that explanation is those sharp edges that we see on the bubbles,” Finkbeiner said, referring to the fact that the structures are well-defined domes.

Dark matter would have existed at the galaxy’s core from the start, and the particles would have been continuously interacting.

“If something has been going on for billions of years and it is in a steady state, I would not expect to see a sharp-edged structure like this,” Finkbeiner said.

Finkbeiner and his team found the gamma-ray bubbles using data from Fermi’s Large Area Telescope, the most sensitive gamma-ray detector yet launched.

The scientists then had to process the raw data so they could see through the “fog” of gamma rays that’s made as high-energy electrons moving near the speed of light interact with light and interstellar gas in the Milky Way.

Through the Fermi Gamma-ray Space Telescope, scientists now have a way to measure the contribution of black hole jets directly and found earlier this year that the source of the high-energy radiation is even more of a mystery than anticipated. No one knows where the rest of the fog’s gamma rays are coming from, and for now there’s no obvious candidate in sight. Taking a cue from medieval mapmakers, the Fermi team has dubbed the unknown gamma ray sources dragons.

In general, studying gamma rays and their sources can help scientists understand the high-energy events involved in the births and deaths of stars and galaxies, which may ultimately aid studies of the origins of the universe.

Further studies will be required to get at the true nature of the energy source blowing the bubbles, Princeton’s Spergel said.

“But it is a striking image,” he said, “and I think one that will be challenging astronomers over the coming years to do both future observational work and theoretical work to understand what’s going on here and to make connections to other areas of galactic and extragalactic astronomy.”

NOTES:

The NASA Teleconference panelists were:
– Jon Morse, director, Astrophysics Division, NASA Headquarters in Washington
– Julie McEnery, Fermi project scientist, NASA’s Goddard Space Flight Center in Greenbelt, Md.
– Doug Finkbeiner, associate professor of astronomy, Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
– Simona Murgia, Fermi research associate, SLAC National Accelerator Laboratory in Menlo Park, Calif.
– David Spergel, astrophysicist, Princeton University, Princeton, N.J.

David Spergel is a theoretical astrophysicist. His interests range from the search for planets around nearby stars to the shape of the universe. He is part of a group of scientists and engineers at Princeton University who are developing new technologies that should hopefully enable the direct imaging of earth-like planets. He recently led a study of an 4-meter class telescope/occulter system that we called THEIA (Telescope for Habitable Exoplanets and Interstellar/Intergalactic Astronomy). With a wide field imager, an ultraviolet spectrograph, a planet imager/spectrograph and a companion occulter,THEIA is capable of addressing many of the most important questions in astronomy: Are we alone? Are there other habitable planets? How frequently do solar systems form and survive? How do stars and galaxies form and evolve? How is dark matter distributed in galaxies and in the filaments? Where are most of the atoms in the universe? How were the heavy elements necessary for life created and distributed through cosmic time?

Douglas Finkbeiner is the Associate Professor of Astronomy and Physics at Harvard University. As a graduate student in the UC Berkeley Physics Department, Finkbeiner worked with Marc Davis & David Schlegel to produce a dust map, useful for estimating Galactic extinction and Galactic microwave emission, which is a significant foreground for CMB anisotropy experiments. While at Princeton, Finkbeiner also studied the Galactic microwave emission in the WMAP data, finding signs of the long-suspected spinning dust emission, work since followed up in detail with postdoc Greg Dobler. Another curious (and controversial) result from WMAP is the unexpected excess in the inner Milky Way, the “haze”, which may be synchrotron emission from cosmic-ray electrons and positrons produced by dark matter annihilation. This possibility is now being investigated at CfA by Finkbeiner, along with grad student Tracy Slatyer. Together with Nima Arkani-Hamed and Neal Weiner, they wrote a paper entitled “A Theory of Dark Matter” tying together many observation results in high-energy astrophysics with a unified model of dark matter.

Bruce Lieberman, a paleontologist at the University of Kansas, helped develop the initial theory about the Ordovician die-off but did not co-author the study papers that was produced. The prevailing idea is that an ice age caused the extinction event, but he questions the completeness of that hypothesis for there were other times there have been ice ages without mass extinctions. Furthermore, the ice age during the Ordovician was comparatively short, lasting only about 500,000 years before the climate cycled back to a warm spell—almost as if something unusual set the icy period in motion. So far Thomas and Melott have uncovered a pattern of higher UV radiation during the Ordovician extinction that would match cosmic bombardment over the South Pole. Lieberman believes the disappearance of trilobites, extinct arthropods related to horseshoe crabs, could be tied to the Ordovician event. Although most trilobites are mud-scurrying bottom dwellers, the juveniles of some species have a life stage that sends them floating in the shallow water column, making them vulnerable to higher UV radiation.

NOTES ON MASS EXTINCTIONS:

More than 90 percent of all organisms that have ever lived on Earth are extinct. As new species evolve to fit ever changing ecological niches, older species fade away. But the rate of extinction is far from constant. At least a handful of times in the last 500 million years, 50 to more than 90 percent of all species on Earth have disappeared in a geological blink of the eye.

Though these mass extinctions are deadly events, they open up the planet for new life-forms to emerge. Dinosaurs appeared after one of the biggest mass extinction events on Earth, the Permian-Triassic extinction about 250 million years ago. The most studied mass extinction, between the Cretaceous and Paleogene periods about 65 million years ago, killed off the dinosaurs and made room for mammals to rapidly diversify and evolve.

The causes of these mass extinction events are unsolved mysteries, though volcanic eruptions and the impacts of large asteroids or comets are prime suspects in many of the cases. Both would eject tons of debris into the atmosphere, darkening the skies for at least months on end. Starved of sunlight, plants and plant-eating creatures would quickly die. Space rocks and volcanoes could also unleash toxic and heat-trapping gases that—once the dust settled—enable runaway global warming.

An extraterrestrial impact is most closely linked to the Cretaceous extinction event. A huge crater off Mexico’s Yucatán Peninsula is dated to about 65 million years ago, coinciding with the extinction. Global warming fueled by volcanic eruptions at the Deccan Flats in India may also have aggravated the event. Whatever the cause, dinosaurs, as well as about half of all species on the planet, went extinct.

Massive floods of lava erupting from the central Atlantic magmatic province about 200 million years ago may explain the Triassic-Jurassic extinction. About 20 percent of all marine families went extinct, as well as most mammal-like creatures, many large amphibians, and all non-dinosaur archosaurs. An asteroid impact is another possible cause of the extinction, though a telltale crater has yet to be found.

Largest Ever Die-Off

The Permian-Triassic extinction event about 250 million years ago was the deadliest: More than 90 percent of all species perished. Many scientists believe an asteroid or comet triggered the massive die-off, but, again, no crater has been found. Another strong contender is flood volcanism from the Siberian Traps, a large igneous province in Russia. Impact-triggered volcanism is yet another possibility.

Starting about 360 million years ago, a drawn-out event eliminated about 70 percent of all marine species from Earth over a span of perhaps 20 million years. Pulses, each lasting 100,000 to 300,000 years, are noted within the larger late Devonian extinction. Insects, plants, and the first proto-amphibians were on land by then, though the extinctions dealt landlubbers a severe setback.

The Ordovician-Silurian extinction, about 440 million years ago, involved massive glaciations that locked up much of the world’s water as ice and caused sea levels to drop precipitously. The event took its hardest toll on marine organisms such as shelled brachiopods, eel-like conodonts, and the trilobites.

Happening Now?

Today, many scientists think the evidence indicates a sixth mass extinction is under way. (SOURCE: National Geographic)

by MJ Santos, founder & Publisher