* Astronomy

While analysing the complex structure of the Milky Way, an international team of astronomers from Italy and the United Kingdom has recently derived the shape of the Galactic outer stellar disc, and provided the strongest evidence that, besides being warped, it is at least 70% more extended than previously thought. Their findings will be reported in an upcoming issue of Astronomy & Astrophysics, and is a new step in understanding the large-scale structure of our Galaxy.

Using the 2MASS all-sky near infrared catalogue, Yazan Momany and his collaborators reconstructed the outer structure of the Galactic stellar disc, in particular, its warp. Their work will soon be published in Astronomy & Astrophysics. Observationally, the warp is a bending of the Galactic plane upwards in the first and second Galactic longitude quadrants (0<l<180 degrees) and downwards in the third and fourth quadrants (180<l<360 degrees). Although the origin of the warp remains unknown, this feature is seen to be a ubiquitous property of all spiral galaxies. As we are located inside the Galactic disc, it is difficult to unveil specific details of its shape. To appreciate a warped stellar disc one should, therefore, look at other galaxies. the image below shows a good example of what a warped galaxy looks like.


An edge-on view of the ESO 510-G13 warped galaxy.
Courtesy of NASA

The disc of our Galaxy is made up of three major components: the stellar, the gaseous, and the interstellar dust components. The warping of the gas and dust component has been well established and documented. In particular, the gaseous component is known to be warped and to extend out to 25,000 parsecs (pc). In contrast, the true extent of this stellar warping is still being debated. Over the past years, there has been changing evidence of a difference in the warp amplitude between stars and gas. These studies have led to the idea that the Milky Way stellar disc is truncated beyond 14,000 pc from the Galactic centre.

The new analysis by Momany and his team provides the first clear and complete view of the outer stellar disc warp. They analysed the distribution of over 115 million stars from the all-sky 2MASS catalogue that comprise the totality of the Galactic disc. Among the many different stellar types, M-giant stars were found to be the ideal stellar tracer for reconstructing the outer disc structure. They are, in fact, highly luminous but relatively cool and evolved stars, and these unique properties allow better determination of their distance. The analysis also shows that M-giants stars located at distances between 3,000 and of 17,000 pc from the Sun draw the same stellar warp signature. This means that a global and large-scale Milky Way feature has been identified to about 25,000 pc from the Galactic centre: the team thus clearly demonstrates that there is no truncation of the stellar disc beyond 14,000 pc. The figures below illustrate the shape of the Galactic outer stellar disc.


The density maps and contours of the M-giant sample at 14,000 pc from the Galactic centre. The stellar disc in the Southern hemisphere is bending downwards (right of the figure) while in the Northern hemisphere it bends upwards (left of the figure).

This image shows the density maps as derived from the 2MASS M-giant sample at 14,000 pc from the Galactic centre. The presence of the warp is quite clear at both ends of the stellar disc.


The stellar warp as derived from 2MASS M-giant stars at 14,000 pc from the Galactic centre (black filled squares) compared to the gaseous and interstellar dust warp (green triangles and asterisks).

This image quantitatively shows the amplitude and orientation of the disc’s stellar warp as a function of the Galactic longitude. It also shows the consistency of the warp signature in the three disc components (gas, dust, and stars). It is a natural consequence of the close physical correlation between these three Galactic disc components, and proves once more the existence of a global and regular warp signature for the Galactic disc.

Last but not least, this new evidence of an extended and warped Milky Way stellar disc allows the team to solve a heated debate among astronomers. In the past years, astronomers have identified over-densities in the opposite direction to the Galactic centre. Located in the Galactic plane, they stretch over 100 degrees in Monoceros constellation. Known as the Monoceros Ring, this over-density was believed to be the remnant of a dwarf satellite galaxy cannibalised by the Milky Way. Another well-known example exists in the Sagittarius constellation of how the Milky Way halo is continuously building up by means of cannibalised smaller galaxies. Recently, an over-density located in Canis Major was associated to the Monoceros Ring and identified as the core of a satellite galaxy currently being accreted into the Galactic plane. Momany and colleagues’ work, however, casts serious doubt on this scenario. They show that the Canis Major over-density is easily explained by the imprint of the Galactic warp. They may also be able to explain the Monoceros Ring by the complex structure of the outer disc, but they cannot offer a definite conclusion about this issue yet, as very little is known about the Monoceros Ring. It seems, however, that the Sagittarius dwarf remains the only example we have for the moment of how our Milky Way is still growing by cannibalising smaller galaxies.

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Astronomers have discovered a narrow stream of stars extending at least 45 degrees across the northern sky. The stream is about 76,000 light-years distant from Earth and forms a giant arc over the disk of the Milky Way galaxy.

In the March issue of the Astrophysical Journal Letters, Carl Grillmair, an associate research scientist at the California Institute of Technology's Spitzer Science Centre, and Roberta Johnson, a graduate student at California State University Long Beach, report on the discovery.

"We were blown away by just how long this thing is. As one end of the stream clears the horizon this evening, the other will already be halfway up the sky" - Carl Grillmair.

The stream begins just south of the bowl of the Big Dipper and continues in an almost straight line to a point about 12 degrees east of the bright star Arcturus in the constellation Bootes. The stream emanates from a cluster of about 50,000 stars known as NGC 5466.
The newly discovered stream extends both ahead and behind NGC 5466 in its orbit around the galaxy. This is due to a process called tidal stripping, which results when the force of the Milky Way's gravity is markedly different from one side of the cluster to the other. This tends to stretch the cluster, which is normally almost spherical, along a line pointing towards the galactic centre.
At some point, particularly when its orbit takes it close to the galactic centre, the cluster can no longer hang onto its most outlying stars, and these stars drift off into orbits of their own. The lost stars that find themselves between the cluster and the galactic centre begin to move slowly ahead of the cluster in its orbit, while the stars that drift outwards, away from the galactic centre, fall slowly behind.
Ocean tides are caused by exactly the same phenomenon, though in this case it's the difference in the moon's gravity from one side of Earth to the other that stretches the oceans. If the gravity at the surface of Earth were very much weaker, then the oceans would be pulled from the planet, just like the stars in NGC 5466's stream.


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Despite its size, the stream has never previously been seen because it is so completely overwhelmed by the vast sea of foreground stars that make up the disk of the Milky Way. Grillmair and Johnson found the stream by examining the colours and brightness's of more than nine million stars in the Sloan Digital Sky Survey public database.

"It turns out that, because they were all born at the same time and are situated at roughly the same distance, the stars in globular clusters have a fairly unique signature when you look at how their colours and brightness's are distributed" - Carl Grillmair.

Using a technique called matched filtering, Grillmair and Johnson assigned to each star a probability that it might once have belonged to NGC 5466. By looking at the distribution of these probabilities across the sky, "the stream just sort of reached out and smacked us.

"The new stream may be even longer than we know, as we are limited at the southern end by the extent of the currently available data. Larger surveys in the future should be able to extend the known length of the stream substantially, possibly even right around the whole sky"- Carl Grillmair.

The stars that make up the stream are much too faint to be seen by the unaided human eye. Owing to the vast distances involved, they are about three million times fainter than even the faintest stars that we can see on a clear night.
Such discoveries are important for our understanding of what makes up the Milky Way galaxy. Like earthbound rivers, such tidal streams can tell us which way is "down," how steep is the slope, and where the mountains and valleys are located.
By measuring the positions and velocities of the stars in these streams, astronomers hope to determine how much Dark Matter the Milky Way contains, and whether the dark matter is distributed smoothly, or in enormous orbiting chunks.

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Nearly 400 years after Galileo determined the wispy Milky Way actually comprises myriad individual stars, scientists using NASA's Rossi X-ray Timing Explorer have done the same for the "X-ray Milky Way."


This dazzling Chandra X-ray image shows a multitude of X-ray sources in our galaxy's center. While most of these sources are from black holes and neutron stars, scientists have found that most of the X-rays in our galaxy come from millions on relatively dim sources that cannot be imaged, namely white dwarfs and stars with active coronas.
Credit: NASA

The origin of this X-ray counterpart to the Milky Way, known to scientists as the galactic X-ray background, has been a long-standing mystery. Scientists have determined the background is not diffuse, as many have thought. Rather, it emanates from untold hundreds of millions of individual sources dominated by a type of dead star called a white dwarf, along with stars with unusually strong coronas.

If confirmed, this new finding would have a profound impact on our understanding of the history of our galaxy, from star-formation and supernova rates to stellar evolution. The result solves major theoretical problems yet points to a surprising undercounting of stellar objects, perhaps by a hundredfold.

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Astronomers using the National Science Foundation's Robert C. Byrd Green Bank Telescope (GBT) have discovered a huge "superbubble" of hydrogen gas rising nearly 10,000 light-years above the plane of our Milky Way Galaxy. They believe the gas may be driven by supernova explosions and the intense stellar winds from an unseen cluster of young stars in one of our Galaxy's spiral arms.

"This giant gas bubble contains about a million times more mass than the Sun and the energy powering its outflow is equal to about 100 supernova explosions" - Yurii Pidopryhora, National Radio Astronomy Observatory (NRAO) and Ohio University.

Pidopryhora, along with Jay Lockman of NRAO, and Joseph Shields of Ohio University, presented their results to the American Astronomical Society's meeting in Washington, DC.
The superbubble is nearly 23,000 light-years from Earth. The astronomers discovered it by combining numerous smaller images made with the GBT into one large image. In addition, they added images of ionised hydrogen in the region that were made by a University of Wisconsin optical telescope on Kitt Peak in Arizona.

"We see that all the hydrogen gas in this region of the Galaxy is disturbed, with many smaller outflows closer to the plane of the Galaxy and then a giant plume of gas that forms a sort of cap on the whole thing" - Yurii Pidopryhora.

The ionised hydrogen, with atoms violently stripped of their electrons, seems to fill the interior of the superbubble while the neutral hydrogen forms its walls and cap.
Our Milky Way Galaxy, about 100,000 light-years across, somewhat resembles a giant dinner plate, with most of its stars and gas residing in a flat disk.

"Gas driven outward from the plane of the Galaxy's disk has been seen many times before, but this superbubble is particularly large. The eruption that drove this much mass so far out of the plane has to have been unusually violent" - Jay Lockman .

The scientists speculate that the gas may be blown outward by the strong stellar winds and supernova explosions from numerous massive young stars in a cluster.

"One theoretical model shows that young stars could power an outflow that matches what we see very closely" - Yurii Pidopryhora.

According to that model, the superbubble probably is 10-30 million years old.

"Finding this superbubble practically in our back yard is quite exciting, because these superbubbles are very important factors in how galaxies evolve" - Jay Lockman .

Superbubbles, powered by supernova explosions and young stellar winds, control the way heavy elements, produced only in the cores of stars, are distributed throughout the galaxy, the scientists said. Those heavy elements are then incorporated into the next generation of stars -- and planets -- to form.

"The formation of our own Sun and planets probably was heavily influenced, if not triggered, by a nearby supernova explosion" - Jay Lockman .

In addition, if the outflow from superbubbles is energetic enough, it could blow the gas into intergalactic space, never to return to the galaxy.

"This would shut down the formation of new stars in the galaxy" - Yurii Pidopryhora.

The Green Bank Telescope, dedicated in 2000, is the largest fully-steerable radio telescope in the world, with more than two acres of collecting area in its giant dish. Located within the National Radio Quiet Zone in West Virginia, the GBT provides extraordinary sensitivity for observing faint radio-emitting objects in the distant Universe.

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