Giant structure in our galaxy

By NASA Headquarters, Washington, D.C.
Published: November 10, 2010

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 it may be the remnant of an eruption from a super-sized 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 from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, 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.

Finkbeiner, along with Meng Su and Tracy Slatyer, both from Harvard, 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 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 had one 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 from 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 the instrument's best picture of the gamma-ray sky, the result of 2 years of data collection.

"Fermi scans the entire sky every 3 hours, and as the mission continues and our exposure deepens, we see the extreme universe in progressively greater detail," said Julie McEnery from NASA's Goddard Space Flight Center in Greenbelt, Maryland. 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 from NASA Headquarters, Washington, D.C. "These latest discoveries continue to demonstrate Fermi's outstanding performance."

Milky Way mystery possibly solved

Integral mapped the glow of 511 keV gamma rays from electron-positron annihilation. The map shows the whole sky, with the galactic center in the middle. The emission extends to the right. ESA/Integral/MPE/G. Weidenspointner

January 24, 2008

Provided by NASA's Goddard Space Flight Center

Observations from the last 4 years from the European Space Agency's Integral (INTErnational Gamma-Ray Astrophysics Laboratory) satellite may have cleared up one of the most vexing mysteries in our Milky Way: the origin of a giant cloud of antimatter surrounding the galactic center.

Integral found that the cloud extends farther on the western side of the galactic center than it does on the eastern side. This imbalance matches the distribution of a population of binary star systems that contain black holes or neutron stars, strongly suggesting that these binaries are churning out at least half of the antimatter, and perhaps all of it.

"The reported Integral detection of an asymmetry represents a significant step forward toward a solution of one of the major outstanding problems in high-energy astrophysics. I think I can hear a collective sigh of relief emanating from the community," says Marvin Leventhal, a University of Maryland professor emeritus and a pioneer in this field.

The cloud itself is roughly 10,000 light-years across, and generates the energy of about 10,000 Suns. The cloud shines brightly in gamma rays due to a reaction governed by Einstein's famous equation E=mc^2. Negatively charged subatomic particles known as electrons collide with their antimatter counterparts, positively charged positrons. When electrons and positrons meet, they can annihilate one another and convert all of their mass into gamma rays with energies of 511,000 electron-volts (511 keV).The antimatter cloud was discovered in the 1970s by gamma-ray detectors flown on balloons. Scientists have proposed a wide range of explanations for the origin of the antimatter, which is exceedingly rare in the cosmos. For years, many theories centered around radioactive elements produced in supernovae, prodigious stellar explosions. Others suggested that the positrons come from neutron stars, novae, or colliding stellar winds.

In recent years, some theorists championed the idea that particles of dark matter were annihilating one another, or with atomic matter, producing electrons and positrons that annihilate into 511-keV gamma rays. But other scientists remained skeptical, noting that the dark matter particles had to be significantly lighter than most theories predicted.

"The Integral results seem to rule out dark matter as the major source of the gamma rays," says the paper's second author Gerry Skinner, who currently works at NASA's Goddard Space Flight Center, Skinner is a co-investigator of Integral's SPI (SPectrometer for Integral) instrument, which made this discovery.

Integral found certain types of binary systems near the galactic center are also skewed to the west. These systems are known as hard low-mass X-ray binaries, since they light up in high-energy (hard) X-rays as gas from a low-mass star spirals into a companion black hole or neutron star. Because the two "pictures" of antimatter and hard low-mass X-ray binaries line up strongly suggests the binaries are producing significant amounts of positrons."Simple estimates suggest that about half and possibly all the antimatter is coming from X-ray binaries," says Georg Weidenspointer of the Max Planck Institute for Extraterrestrial Physics in Germany, lead author of the paper.

While Integral's discovery clears up one mystery, it raises a new one. Scientists don't understand how low-mass X-ray binaries could produce enough positrons to explain the cloud, and they also don't know how they escape from these systems. "We expected something unexpected, but we did not expect this," says Skinner. The antimatter is probably produced in a region near the neutron stars and black holes, where powerful magnetic fields launch jets of particles that rip through space at near-light speed.


This Integral image shows the distribution of hard low-mass X-ray binaries. This stellar population has a distribution that matches the extent of the 511 keV map. ESA/Integral/MPE/G. Weidenspointner

NASA's Gamma-ray Large Area Space Telescope (GLAST), scheduled to launch in 2008, may help clarify how objects such as black holes launch particle jets. Conceivably, it could even detect higher-energy gamma rays from heavier types of dark matter particles annihilating one another.

Our Galaxy's Central Molecular Zone

Credit: A. Ginsburg (U. Colorado - Boulder) et al.,
BGPS Team, GLIMPSE II Team

Monday, November 10, 2008

The central region of our Milky Way Galaxy is a mysterious and complex place. Pictured here in radio and infrared light, the galaxy's central square degree is highlighted in fine detail. The region is known as the Central Molecular Zone.

While much of the extended emission is due to dense gas laced with molecules, also seen are emission nebulas lit up by massive young stars, glowing supernova remnants, and the curving Galactic Center Radio Arc in purple. T

he identity and root cause for many other features remains unknown. Besides a massive black hole named Sgr A*, the Galactic Center houses the galaxy's most active star forming region. This image is not just interesting scientifically. It's esthetic beauty won first prize this year in the AUI/NRAO Image Contest.

The Winding Milky Way

photo: A theoretical model of a galaxy like the Milky Way, showing trails of stars torn from disrupted satellite galaxies that have merged with the central galaxy. The region shown is about 1 million light-years on a side; the Sun is just 25,000 light-years from the center of the galaxy and would appear close to the center of this picture. Credit: K. Johnston/J. Bullock

Date: Friday, August 22, 2008


A new map reveals a complicated outer halo in our galaxy (Provided by Sloan Digital Sky Survey)

The halo of stars that envelops the Milky Way is like a river delta crisscrossed by stellar streams large and small, according to new data from the Sloan Digital Sky Survey (SDSS-II). While the largest rivers of this delta have been mapped out over the last decade, analysis of the new SDSS-II map shows that smaller streams can be found throughout the stellar halo, says Kevin Schlaufman, a graduate student at the University of California at Santa Cruz.

Schlaufman reported his results Saturday at an international symposium in Chicago, titled "The Sloan Digital Sky Survey: Asteroids to Cosmology." Over the last 3 years, Schlaufmann explains, the SEGUE survey of SDSS-II has measured the motions of nearly a quarter million stars in selected areas of the sky. A careful search for groups of stars at the same velocity turned up 14 distinct structures, 11 of them previously unknown.

"Even with SEGUE, we are still only mapping a small fraction of the galaxy," says Schlaufman, "so 14 streams in our data implies a huge number when we extrapolate to the rest of the Milky Way." If each velocity structure were a separate stream, Schlaufman explains, there would be close to 1,000 in the inner 75,000 light-years of the galaxy. However, these structures could arise from a smaller number of streams that are seen many times in different places.

"A jumble of pasta" is the way Columbia University researcher Kathryn Johnston described her theoretical models of the Milky Way's stellar halo. In a review talk at the symposium, Johnston explained how dwarf galaxies that pass close to the Milky Way can be stretched by gravitational tides into spaghetti-like strands, which wind around the galaxy as stars trace out the same orbital paths at different rates.

"In the center of the galaxy, these stellar strands crowd together and you just see a smooth mix of stars," says Johnston. "But as you look further away you can start to pick out individual strands, as well as features more akin to pasta shells that come from dwarfs that were on more elongated orbits. By looking at faint features, Kevin may be finding some of the 'angel hair' that came from smaller dwarfs, or ones that were destroyed longer ago."

Heidi Newberg of Rensselaer Polytechnic Institute and her thesis student Nathan Cole have been trying to follow some of the larger strands as they weave across the sky. "It's a big challenge to piece things together," says Cole, "because the stream from one dwarf galaxy can wrap around the Milky Way and pass through streams of stars ripped from other dwarf galaxies."

Toward the constellation Virgo, where SDSS images revealed an excess of stars covering a huge area of sky, Newberg finds that there are at least two superposed structures, and possibly three or more. The SEGUE velocity measurements can separate systems that overlap in sky maps, Newberg explained in her symposium talk. "Part of what we see toward Virgo is a tidal arm of the Sagittarius dwarf galaxy, whose main body lies on the opposite side of the Milky Way, but we don't know the origin of the other structures. There really aren't enough pasta varieties to describe all the structures we find."

In addition to stellar streams, astronomers searching the SDSS data have found 14 surviving dwarf companions of the Milky Way, including two new discoveries announced Saturday at the symposium by Gerard Gilmore of Cambridge University. These satellite galaxies are orbiting within the halo of invisible dark matter whose gravity holds the Milky Way itself together. Most of them are much fainter than the ten satellites known prior to the SDSS.

Because even the SDSS can only detect these ultra-faint dwarfs if they are relatively nearby, there could be several hundred more of them further out in the Milky Way's dark halo, according to independent analyses by graduate students Sergey Koposov, of the Max Planck Institute for Astronomy in Heidelberg, Germany, and Eric Tollerud, of the University of California at Irvine. "Even so," says Koposov, "we expect that the number of dark matter clumps is much larger than that, so something must prevent the smaller clumps from gathering gas and forming stars."

The SDSS dwarfs have far fewer stars than the previously known satellites, notes Gilmore, but they have similar spatial extents, and the stars within them move at similar speeds. "I think the internal dynamics of these tiny galaxies may be hard to explain with our conventional ideas about dark matter," says Gilmore.

"The SDSS has taught us a huge amount about the Milky Way and its neighbors," says Johnston, who is pleased to see some of the predictions of her models confirmed by the new data. "But we're still just beginning to map the galaxy in a comprehensive way, and there's a trove of discoveries out there for the next generation of surveys, including the two new Milky Way surveys that will be carried out in SDSS-III."

Pinning down the Milky Way's spin

Date: Monday, September 22, 2008

Pulsating stars enable a new precise determination of the rotation of our Galaxy

New, very precise measurements have shown that the rotation of the Milky Way is simpler than previously thought. A remarkable result from the most successful ESO instrument HARPS, shows that a much debated, apparent 'fall' of neighbourhood Cepheid stars towards our Sun stems from an intrinsic property of the Cepheids themselves.

The result, obtained by a group of astrophysicists led by Nicolas Nardetto, will soon appear in the journal Astronomy & Astrophysics.

Since Henrietta Leavitt's discovery of their unique properties in 1912, the class of bright, pulsating stars known as Cepheids has been used as a distance indicator. Combined with velocity measurements, the properties of Cepheids are also an extremely valuable tool in investigations of how our galaxy, the Milky Way, rotates.

"The motion of Milky Way Cepheids is confusing and has led to disagreement among researchers," says Nardetto. "If the rotation of the Galaxy is taken into account, the Cepheids appear to 'fall' towards the Sun with a mean velocity of about 2 km/s."

A debate has raged for decades as to whether this phenomenon was truly related to the actual motion of the Cepheids and, consequently, to a complicated rotating pattern of our galaxy, or if it was the result of effects within the atmospheres of the Cepheids.

Nardetto and his colleagues observed eight Cepheids with the high precision HARPS spectrograph, attached to the 3.6-m ESO telescope at La Silla, 2400 m up in the mountains of the Chilean Atacama Desert. HARPS, or the High Accuracy Radial Velocity Planetary Searcher, is best known as a very successful planet hunter, but it can also be used to resolve other complicated cases, where its ability to determine radial velocities - the speed with which something is moving towards or away from us - with phenomenally high accuracy is invaluable. "Our observations show that this apparent motion towards us almost certainly stems from an intrinsic property of Cepheids," says Nardetto.

The astronomers found that the deviations in the measured velocity of Cepheids were linked to the chemical elements in the atmospheres of the Cepheids considered. "This result, if generalised to all Cepheids, implies that the rotation of the Milky Way is simpler than previously thought, and is certainly symmetrical about an axis," concludes Nardetto.