NS: Giant glowing bubbles found around Milky Way

[bystander]

Giant glowing bubbles found around Milky Way
New Scientist - 03 June 2010

IS THE Milky Way blowing giant bubbles? A pair of gamma ray bubbles, shaped like an hourglass, seem to be spewing from the black hole we think lies at the centre of our galaxy. That is according to the latest maps from the Fermi Gamma-ray Space Telescope. Its large area telescope has been scanning the whole sky every three hours since June 2008.

Two giant bubbles, forming an hourglass shape, emanate from the centre of the Milky Way (dark band is the galaxy's disc), according to a new analysis of data taken by the Fermi Gamma-ray Space Telescope [i](Meng Su et al.)[/i]

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The source of the bubbles is a mystery but it seems unlikely that dark matter is responsible. This was what Douglas Finkbeiner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, first suspected when he looked at the maps with his colleagues last year².

But a new analysis with more Fermi data suggests that the gamma radiation traces out a pair of distinct bubbles that span some 65,000 light years from end to end - towering above the 2000-light-year-thick disc of the galaxy.

Such a well-defined shape is inconsistent with dark matter, which you would expect to be smoothly distributed and produce a diffuse glow, from gamma rays produced after dark matter particles meet and annihilate each other. "We are pretty sure the majority of emissions are not from dark matter," says Finkbeiner's student Meng Su.

Instead, they think the bubbles may have been blown out by the explosion of short-lived, massive stars born in a burst of new star formation about 10 million years ago. Alternatively, the bubbles may have been forged about 100,000 years ago by high-speed jets of matter created when roughly 100 suns' worth of material fell into the black hole at the centre of our galaxy¹.

1. Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind? - Meng Su et al
   • arXiv.org > astro-ph > arXiv:1005.5480 > 29 May 2010 (v1), 18 Oct 2010 (v3)
2. The Fermi Haze: A Gamma-Ray Counterpart to the Microwave Haze - Gregory Dobler et al
   • arXiv.org > astro-ph > arXiv:0910.4583 > 26 Oct 2009 (v1), 07 May 2010 (v2)

[Ann]

Well, we know that there are a lot of supernova remnants near the center of the Milky Way. There are also two very massive clusters there, the Quintuplet and Arches clusters. And if M 82 can blow twin bubbles this size from supernovae alone, is it so surprising that our own galaxy can produce enough supernovae to blow two much smaller bubbles?



Ann

[rstevenson]

I'm Forever Blowing Bubbles

Rob

[bystander]

Fermi Telescope Finds Giant Structure in our Galaxy
NASA Fermi Gamma-ray Space Telescope | 09 Nov 2010

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[attachment=1]498881main_DF1_Fermi_all-sky_1-10_GeV.25.jpg[/attachment]

[attachment=0]498886main_DF4_bubbles_graphs.25.jpg[/attachment]

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NASA's Fermi Gamma-ray Space Telescope has unveiled a previously unseen structure centered in the Milky Way. The feature spans 50,000 light-years and may be the remnant of an eruption from a supersized black hole at the center of our galaxy.

"What we see are two gamma-ray-emitting bubbles that extend 25,000 light-years north and south of the galactic center," said Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who first recognized the feature. "We don't fully understand their nature or origin."

The structure spans more than half of the visible sky, from the constellation Virgo to the constellation Grus, and it may be millions of years old. A paper about the findings has been accepted for publication in The Astrophysical Journal.

Finkbeiner and his team discovered the bubbles by processing publicly available data from Fermi's Large Area Telescope (LAT). The LAT is the most sensitive and highest-resolution gamma-ray detector ever launched. Gamma rays are the highest-energy form of light.

Other astronomers studying gamma rays hadn't detected the bubbles partly because of a fog of gamma rays that appears throughout the sky. The fog happens when particles moving near the speed of light interact with light and interstellar gas in the Milky Way. The LAT team constantly refines models to uncover new gamma-ray sources obscured by this so-called diffuse emission. By using various estimates of the fog, Finkbeiner and his colleagues were able to isolate it from the LAT data and unveil the giant bubbles.

Scientists now are conducting more analyses to better understand how the never-before-seen structure was formed. The bubble emissions are much more energetic than the gamma-ray fog seen elsewhere in the Milky Way. The bubbles also appear to have well-defined edges. The structure's shape and emissions suggest it was formed as a result of a large and relatively rapid energy release - the source of which remains a mystery.

One possibility includes a particle jet from the supermassive black hole at the galactic center. In many other galaxies, astronomers see fast particle jets powered by matter falling toward a central black hole. While there is no evidence the Milky Way's black hole has such a jet today, it may have in the past. The bubbles also may have formed as a result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way's center several million years ago.

"In other galaxies, we see that starbursts can drive enormous gas outflows," said David Spergel, a scientist at Princeton University in New Jersey. "Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics."

Hints of the bubbles appear in earlier spacecraft data. X-ray observations from the German-led Roentgen Satellite suggested subtle evidence for bubble edges close to the galactic center, or in the same orientation as the Milky Way. NASA's Wilkinson Microwave Anisotropy Probe detected an excess of radio signals at the position of the gamma-ray bubbles.

The Fermi LAT team also revealed Tuesday the instrument's best picture of the gamma-ray sky, the result of two years of data collection.

"Fermi scans the entire sky every three hours, and as the mission continues and our exposure deepens, we see the extreme universe in progressively greater detail," said Julie McEnery, Fermi project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.

NASA's Fermi is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy, with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

"Since its launch in June 2008, Fermi repeatedly has proven itself to be a frontier facility, giving us new insights ranging from the nature of space-time to the first observations of a gamma-ray nova," said Jon Morse, Astrophysics Division director at NASA Headquarters in Washington. “These latest discoveries continue to demonstrate Fermi's outstanding performance.”

[bystander]

APOD: Huge Gamma Ray Bubbles Found Around Milky Way (2010 Nov 10)
HEAPOW: Temporarily Blowing Bubbles? (2010 Nov 22)

Milky Way's Gamma-ray Bubbles Shaped by Dark Matter?
Discovery News | Amy Shira Teitel | 2012 May 08

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Dark matter, the elusive stuff that makes up a substantial portion of all the mass in the universe, is largely a mystery to astronomers. They've tried finding it and creating it, but so far have unearthed no conclusive proof as to what exactly it is, though most theories state that we interact with it through gravity.

But Christoph Weniger, of the Max Planck Institute for Physics in Munich, has a different theory to explain new possible evidence for dark matter. By carrying out statistical analysis of publicly available data from NASA’s Fermi Gamma-ray Space Telescope, he's found a gamma-ray line across the sky that he says is a clear signature of dark matter.

Astrophysicists generally think that supermassive black holes, like the one at the center of the Milky Way, release jets that interact with surrounding dark matter. This interaction is thought to be the source of high-energy gamma rays that satellites like Fermi can detect. What satellites can see are the photons produced when these jets interact with dark matter.

Weniger looked for signs of such an interaction in about three and a half years' worth of gamma-ray observations carried out by the Fermi satellite's Large Area Telescope.

To increase his chances of success, he only considered data from those regions of the Milky Way that should generate the highest ratios of dark-matter photons to photons from background sources. He was looking specifically for a peak in energy, a sign that a photon was produced by the collision between and annihilation of two particles; the photon left over should have the same mass as one dark-matter particle. This energy would theoretically appear as a very narrow peak, a line in gamma-ray spectra, distinct from the broad energy distribution seen across the visible universe.

That’s just what he found -- a line in the gamma-ray spectrum.

But he’s quick to admit it’s a provisional result. His data points come from about 50 photons, and he’ll need a lot more to prove conclusively that his line is related to dark matter. It’s possible the line he observed is from a known, though no less mysterious, astronomical phenomenon: the pair of enormous gamma ray-emitting bubbles extending outward from the plane of the Milky Way.

In December 2010, scientists working with the Fermi telescope found two giant lobes extending from the black hole at the center of our galaxy.

Twenty-five thousand light-years high, each bubble spans more than half of the visible sky, reaching from the constellation Virgo to the constellation Grus, and may be relatively young at just a million or so years old.

The bubbles are a recent find, normally masked by the fog of gamma rays that appears throughout the sky as a result of particles moving near the speed of light interacting with light and interstellar gas in the Milky Way. Scientists only found the bubbles by manipulating the data from the telescope to draw out the striking feature.

The manipulated images show the bubbles have well-defined edges, suggesting they were formed as a result of a large and relatively rapid energy release -- the source of which is still unknown. Interestingly, the energy cutoff of the bubbles corresponds to the gamma-ray line Weniger found, the one he’s associating with a dark-matter signature.

It’s possible the bubbles and the line have the same origin. Or, dark matter might be the cause of the bubbles' defined endpoint.

Whether or not the two observations turn out to be linked -- which of course hinges on conclusive proof of Weniger’s gamma-ray line -- both are very cool and part of the fascinating and mystery nature of our corner of the universe.

Image credit: NASA-Goddard

A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope - Christoph Weniger

• arXiv.org > hep-ph > arXiv:1204.2797 > 12 Apr 2012

[bystander]

Ghostly Gamma-Ray Beams Blast from Milky Way's Center
Harvard Smithsonian Center for Astrophysics | 2012 May 29

[i]This artist's conception shows an edge-on view of the Milky Way galaxy. Newly discovered gamma-ray jets (pink) extend for 27,000 light-years above and below the galactic plane, and are tilted at an angle of 15 degrees. Previously known gamma-ray bubbles are shown in purple. The bubbles and jets suggest that our galactic center was much more active in the past than it is today. (Credit: David A. Aguilar (CfA))[/i]

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As galaxies go, our Milky Way is pretty quiet. Active galaxies have cores that glow brightly, powered by supermassive black holes swallowing material, and often spit twin jets in opposite directions. In contrast, the Milky Way's center shows little activity. But it wasn't always so peaceful. New evidence of ghostly gamma-ray beams suggests that the Milky Way's central black hole was much more active in the past.

"These faint jets are a ghost or after-image of what existed a million years ago," said Meng Su, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), and lead author of a new paper in the Astrophysical Journal.

"They strengthen the case for an active galactic nucleus in the Milky Way's relatively recent past," he added.

The two beams, or jets, were revealed by NASA's Fermi space telescope. They extend from the galactic center to a distance of 27,000 light-years above and below the galactic plane. They are the first such gamma-ray jets ever found, and the only ones close enough to resolve with Fermi.

The newfound jets may be related to mysterious gamma-ray bubbles that Fermi detected in 2010. Those bubbles also stretch 27,000 light-years from the center of the Milky Way. However, where the bubbles are perpendicular to the galactic plane, the gamma-ray jets are tilted at an angle of 15 degrees. This may reflect a tilt of the accretion disk surrounding the supermassive black hole.

"The central accretion disk can warp as it spirals in toward the black hole, under the influence of the black hole's spin," explained co-author Douglas Finkbeiner of the CfA. "The magnetic field embedded in the disk therefore accelerates the jet material along the spin axis of the black hole, which may not be aligned with the Milky Way."

The two structures also formed differently. The jets were produced when plasma squirted out from the galactic center, following a corkscrew-like magnetic field that kept it tightly focused. The gamma-ray bubbles likely were created by a "wind" of hot matter blowing outward from the black hole's accretion disk. As a result, they are much broader than the narrow jets.

Both the jets and bubbles are powered by inverse Compton scattering. In that process, electrons moving near the speed of light collide with low-energy light, such as radio or infrared photons. The collision increases the energy of the photons into the gamma-ray part of the electromagnetic spectrum.

The discovery leaves open the question of when the Milky Way was last active. A minimum age can be calculated by dividing the jet's 27,000-light-year length by its approximate speed. However, it may have persisted for much longer.

"These jets probably flickered on and off as the supermassive black hole alternately gulped and sipped material," said Finkbeiner.

It would take a tremendous influx of matter for the galactic core to fire up again. Finkbeiner estimates that a molecular cloud weighing about 10,000 times as much as the Sun would be required.

"Shoving 10,000 suns into the black hole at once would do the trick. Black holes are messy eaters, so some of that material would spew out and power the jets," he said.

Evidence for Gamma-ray Jets in the Milky Way - Meng Su, Douglas P. Finkbeiner

• arXiv.org > astro-ph > arXiv:1205.5852 > 26 May 2012

[bystander]

Gamma-Ray Jets from the Milky Way
Smithsonian Astrophysical Observatory
Weekly Science Update | 2012 June 01

Click to play embedded YouTube video.

Supermassive black holes, some as massive as ten billion solar masses, are believed to lie at the nuclei of most galaxies. Radio observations have found jets of fast-moving charged particles emanating from the nuclei of many such galaxies and extending across tens of thousands of light-years. They are thought to be the result of accreting matter onto the black hole. These jets inject significant amounts of energy into the medium, sometimes creating a hot cocoon. The mechanism by which these jets turn on and off is one of the major puzzles in high energy astrophysics, but in some cases the jets may have been produced continuously for tens of millions of years.

The black hole at the center of the Milky Way is, at least for the moment, in a quiescent state. It has no known jets and shows no signs of dramatic activity -- but it may not have been so passive in the past. In fact, it would be strange if the Milky Way's black hole did not in general behave like the black holes in other galaxies and have episodes of activity.

CfA astronomers Meng Su and Doug Finkbeiner combed through gamma-ray images obtained recently by the Fermi Space Telescope, looking for evidence of past activity from the Milky Way's black hole. Two years ago the scientists discovered two superbubbles of hot gas extending outward from the nucleus, presumably the result of some kind of previous black hole activity. By very carefully subtracting the gamma-ray images of these bubbles from the faint, diffuse gamma ray sky emission, the astronomers discovered residual evidence of a narrow but gigantically long structure - about 27,000 light-years - extending in both directions from the plane of the galaxy. Moreover, the scientists suggest a plausible but unconfirmed scenario in which both the superbubbles and the jets were produced by accretion around the black hole during a more active epoch. If these results are confirmed, the new paper not only sheds important light on the history of our galaxy's activity, it is the first time images of jets have ever been seen in the gamma-rays.

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