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Posts Tagged ‘Fermi Space Telescope

Giant Milky Way bubbles blown by black hole merger

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Did an insurgent dwarf galaxy create these colossal, high-energy bubbles (shown in false colour)? (Image: NASA/GSFC)

Did an insurgent dwarf galaxy create these colossal, high-energy bubbles (shown in false colour)? (Image: NASA/GSFC)

A tiny galaxy that collided with the Milky Way spawned two huge bubbles of high-energy particles that now tower over the centre of our galaxy. This new model for the birth of the mysterious bubbles also explains discrepancies in the ages of stars at the galactic middle.

In 2010, sky maps made by NASA’s Fermi Gamma-ray Space Telescope revealed two lobes of particles billowing out from the heart of the Milky Way, each one stretching 25,000 light years beyond the galactic plane.

Astronomers suspected the bubbles were inflated by a period of violence in the galactic centre about 10 million years ago, but no one could say what had triggered the outburst.

Earlier this year, Kelly Holley-Bockelmann from Vanderbilt University in Nashville, Tennessee, was discussing the problem with Tamara Bogdanović from the Georgia Institute of Technology in Atlanta.

“We pieced together all the evidence and realised they could be explained by a single catastrophic event – the collision between two black holes,” recalls Holley-Bockelmann.

Tango and crash

We know that a supermassive black hole weighing as much as 4 million suns lurks at the core of the Milky Way. We also have an array of dwarf galaxies orbiting our much larger spiral galaxy, as well as hints that past satellite dwarfs have collided with us.

According to the new theory, a small galaxy with its own central black hole dove into the Milky Way and began spiralling through our galaxy. After billions of years, the stripped-down dwarf’s black hole made it to the galactic centre.

The two black holes then performed a tight gravitational tango before finally merging. This final act produced violent forces that flung out many of the stars that were born in the Milky Way’s middle, explaining why astronomers now find far fewer old stars there than they have every right to expect.

The whirling black holes also disrupted giant clouds of gas, some of which got squeezed so much that they collapsed to form clusters of bright new stars. Much of the rest of the gas swirled into the merged black holes, getting so hot from compression that it radiated huge amounts of energy.

“We think it’s both the energy from this ‘burp’ near the black hole and the winds of gas from the starburst that inflated the Fermi bubbles,” says Holley-Bockelmann.

Round up the runaways

“This hypothesis is probably worth considering,” says Mark Morris, an authority on the galactic centre at the University of California, Los Angeles. But he cautions against making it a leading explanation without more evidence.

Holley-Bockelmann and colleagues think that the colliding dwarf galaxy was formed early in the history of the universe and consisted mainly of dark matter and the central black hole, without many stars. That would account for why we see no tell-tale trail of stars left behind as the dwarf galaxy fragmented.

Instead, the team proposes another test of their model: hunting for the old stars catapulted outwards during the black hole merger.

“It should have carved nearly 1000 stars out of the galactic centre,” says Bogdanović. “These stars should still be racing through space, about 10,000 light years from their original orbits.” The team is now searching for these runaway stars in data amassed by the Sloan Digital Sky Survey, which has observed the properties of hundreds of millions of stars in our galaxy.

Journal reference: Monthly Notices of the Royal Astronomical Society, doi.org/kqh

Read more: www.newscientist.com

Written by physicsgg

March 8, 2013 at 1:34 pm

Fermi Uses Gamma Rays to Unearth Clues About “Empty” Space

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All-sky map of gamma rays observed in our galaxy by the Fermi Large Area Telescope. Found mostly in the plane of our galaxy (horizontal band) and toward the galactic center, gamma rays are present in... (Image by NASA/DOE/Fermi LAT Collaboration)

by David Reffkin

The SLAC-built Large Area Telescope (LAT), the main instrument of the Fermi Gamma-ray Space Telescope, has been studying the gamma-ray sky for almost four years. During that time, the LAT has identified hundreds of gamma-ray sources, including pulsars and active galactic nuclei. It has shown that the Crab Nebula isn’t the steady emitter of gamma rays it’s long been thought to be. The LAT has catalogued lightning in the Earth’s atmosphere and flares on the sun.

But, as reported in a paper soon to appear in The Astrophysical Journal, most of the gamma rays detected by the LAT cannot be attributed to individual point sources.

The team discovered that the pulsars, active galactic nuclei, and the all the rest of the gamma-ray sources pinpointed by the LAT account for only about 10 percent of the gamma-ray photons that have been detected. Extragalactic diffuse emission, a glow that pervades the universe and originates in distant, indistinct sources, comprises approximately 15 percent of the total.

Most of the gamma rays detected by the LAT are “diffuse” emissions originating from our own Milky Way galaxy.

The main source of these pervasive, diffuse gamma-ray emissions are high-energy charged particles known as cosmic rays that interact with the gas and radiation fields between the stars, which is called the interstellar medium. The interstellar medium is far from empty space; scientists are busy gathering crucial information about the gas, dust and high-energy charged particles that fill it.

These cosmic particles can come from pulsars, supernova explosions and other powerful astrophysical sources. There may be an additional contribution from annihilating dark-matter particles or the black hole at the center of our galaxy.

The study was led by Gudlaugar Johannesson, a former postdoctoral researcher member and current affiliate of the SLAC- and Stanford-based Kavli Institute for Particle Astrophysics and Cosmology who is now at the University of Iceland; Andrew Strong of the Max Planck Institute in Garching, Germany; and KIPAC and Stanford scientist Troy Porter.

According to Porter, the new analysis leads to several conclusions. For example, it shows that the density of cosmic rays is higher than anticipated in the outer regions of the galaxy and beyond the central galactic plane. In addition, the total amount of gamma radiation from cosmic ray electrons due to interactions with infrared and visible light – which consist of photons of much lower energy than gamma rays – is larger than previously thought.

One of the major goals of the LAT collaboration is to better define these origins and the composition of the interstellar medium that permeates our galaxy, and this research has provided crucial input for their work.

Read more: news.slac.stanford.edu

Written by physicsgg

April 20, 2012 at 4:18 pm

Fermi Space Telescope Fails to See Evidence Of Dark Matter

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If dark matter fills the universe, astronomers should see the gamma rays it produces. That evidence has so far failed to materialise
Among the most dramatic events in the universe are the death of stars as they collapse into black holes and the collision of black holes themselves. These events are so violent that they shake the firmament, generating gravity waves that ripple across the cosmos. They also generate huge blasts of neutrinos that can sometimes be picked up by giant neutrino telescopes on Earth.
But while these events are fascinating, not least because they almost certainly involve physics beyond our ken, they are hugely difficult to observe. That’s because neutrinos and gravity waves are notoriously shy.
Neutrinos usually pass straight through the Earth. In fact, astronomers have only once detected neutrinos from beyond the Solar System and that was almost 25 years ago during a supernova called SN1987A….. Read the rest of this entry »

Written by physicsgg

June 21, 2011 at 2:30 pm