SN: Milky Way may get an extension

[bystander]

Milky Way may get an extension
Science News | Ron Cowen | 2011 May 20

A newly discovered star-forming arm at the fringes of the Milky Way may be a vast, outer extension of the arm Scutum-Centaurus. [i](Credit: T. Dame, Robert Hurt)[/i]

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The Milky Way's basic structure involves two large spiral arms believed to originate at either end of an elongated central bar, but only parts of the arms can be seen — gray segments indicate portions not yet detected. [i](Credit: Thomas Dame)[/i]

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Discovery of new structure suggests galaxy has a rare symmetry

A new study suggests the Milky Way doesn’t need a makeover: It’s already just about perfect.

Astronomers base that assertion on their discovery of a vast section of a spiral, star-forming arm at the Milky Way’s outskirts. The finding suggests that the galaxy is a rare beauty with an uncommon symmetry — one half of the Milky Way is essentially the mirror image of the other half.

Thomas Dame and Patrick Thaddeus of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., say the structure they’ve discovered is most likely the outer extension of the Scutum-Centaurus arm from the inner galaxy. The finding suggests that Scutum-Centaurus wraps all the way around the Milky Way, making it a symmetric counterpart to the galaxy’s other major star-forming arm, Perseus.

The two arms appear to extend from opposite ends of the galaxy’s central, bar-shaped cluster of stars, each winding around the galaxy, the researchers note in an upcoming issue of Astrophysical Journal Letters.

Dame found evidence for the new structure while reviewing galactic data on atomic hydrogen gas, which radiates at a radio wavelength of 21 centimeters. After tracing the extension of the arm in the 21-centimeter radio emission, “I was in the unique position of being able to walk up two flights of stairs to the roof of my building [at Harvard] and search for carbon monoxide emissions from molecular clouds using the CfA 1.2-meter radio telescope,” says Dame. Molecular gas clouds contain the raw material for making stars.

“This is a major new discovery,” comments Robert Benjamin of the University of Wisconsin-Whitewater. “Dame and Thaddeus have found evidence for a large-scale coherent structure, spanning 60 degrees in the sky … which contains giant molecular gas clouds very far from the galactic center.” The newfound structure lies about 49,000 light-years from the galaxy’s center, and one of the arm’s many large molecular clouds contains an amount of molecular hydrogen equivalent to that of 50,000 suns.

Virtually every spiral arm in the Milky Way has been found in sections, Dame notes. When astronomers realized that the Sagittarius arm, found in the northern sky, and the Carina Arm, in the south, were part of a single, larger structure, they became known as the Sagittarius-Carina Arm. Similarly, since Dame and Thaddeus believe the new arm is an extension of Scutum-Centaurus, “we suggested ‘Outer Scutum-Centaurus’ as a more logical name,” Dame says. The structure is longer than the known parts of the Scutum-Centaurus arm, he adds.

The new feature was previously overlooked because it tilts out of the plane of the galaxy, following the outer galaxy’s warp. Most studies examining spiral arms focus on the galaxy’s plane, says Dame.

The team’s “identification of the feature as a discrete structure is new, and the discovery that it contains molecular gas makes a very strong case for this being a spiral arm,” Benjamin notes.

Mapping all the carbon monoxide in the newfound feature will take several years, says Dame. Nonetheless, the galactic symmetry revealed by the new observations, along with previous evidence, suggests that the Milky Way’s spiral structure is both simpler and easier to study than had long been assumed, he and Thaddeus note. “We've not proven that, but it's evidence in that direction,” says Dame.

A Molecular Spiral Arm in the Far Outer Galaxy - TM Dame, P Thaddeus

• Astrophysical Journal Letters 734(1) L24 (2011 Jun 10) DOI: 10.1088/2041-8205/734/1/L24
  arXiv.org > astro-ph > arXiv:1105.2523 > 12 May 2011

Arm's Trace: Astronomers Spot a Newfound Piece of the Milky Way Galaxy
Scientific American | John Matson | 2011 May 23

New arm discovered in outer edge of the Milky Way Galaxy
PhysOrg | Bob Yirka | 2011 May 23

The Milky Way's New Arm
Sky & Telescope | Kelly Beatty | 2011 May 25

New Arm Embraces Milky Way
Universe Today | Tammy Plotner | 2011 May 25

[bystander]

Milky Way Galaxy Is Bent Like a Sombrero
Space.com | Charles Q. Choi | 2011 May 24

[url=http://home.arcor.de/axel.mellinger/][b]Axel Mellinger's Milky Way Panorama 2.0[/b][/url]

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A newfound stretch of the Milky Way reveals that our galaxy's distant rim is warped like the edge of a sombrero, and that our galaxy may be far more symmetrical than previously thought, scientists say.

Astronomers know the Milky Way is shaped like a spiral, but how many arms this spiral possesses has long been a matter of debate. Recently, scientists have suggested that our galaxy has only two major arms, the Perseus Arm and the Scutum-Centaurus Arm, stretching out from a central bar, with our sun lying relatively near the Perseus Arm.

Now scientists have more evidence supporting this two-armed structure, but they add a twist to this idea, literally. They suggest the Scutum-Centaurus Arm has a bend to it, warping the edges of the Milky Way.

Radio astronomers Thomas Dame and Patrick Thaddeus at the Harvard-Smithsonian Center for Astrophysics first used existing maps of microwave radiation from the sky to follow the Scutum-Centaurus Arm past the point it had previously been analyzed. They next confirmed evidence of a stretch of gas linked with this arm on the other side of the Milky Way using the 1.2-meter telescope at their center.

"This was quite an unanticipated new feature of the galaxy," Thaddeus told SPACE.com. "No one predicted that it existed. One can now trace the Scutum-Centaurus Arm nearly 360 degrees around the galactic system."

This newfound extension reaches an average of about 68,500 light-years away, with the molecular gas there the most distant yet detected in the Milky Way. The fact this new stretch of gas lies slightly outside the plane of the rest of the galaxy explains why astronomers didn't see it clearly before.

"The galaxy is very flat, but like a Mexican hat, at the edges, it's a bit warped by a few degrees," Thaddeus said.

This stretch of gas makes the Scutum-Centaurus Arm a symmetrical counterpart of the Perseus Arm, meaning the Milky Way is similar to galaxies such as the Great Barred Spiral about 61 million light-years away.

"This tells us the Milky Way is much more symmetrical than we had any reason to believe," Thaddeus said.

Milky Way Is Warped, Like a Beer Bottle Cap
Technology Review | The Physic arXiv Blog | kfc | 2011 May 16

The discovery of a new arm in the Milky Way suggests that our galaxy is warped, say astronomers

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In 1852, Stephen Alexander, an astronomer at the College of New Jersey, put forward the radical suggestion that the Milky Way galaxy is a spiral.

But while today's astronomers agree on this general shape, they disagree over the precise structure of the spiral and in particular on the number of arms.

Astronomers have named at least 6 arms and in the 1990s, evidence emerged that the galaxy had a central bar. The uncertainty is easy to understand. Our view of the galaxy shows the nearer stars superimposed on the ones that are further away. And much of the opposite side of the Milky Way galaxy is obscured entirely by the central mass of stars at the centre.

Recently, however, a clearer picture has begun to emerge. The growing consensus is that the Milky Way has a central bar with two main arms, called the Perseus Arm, which passes with a few kiloparsecs of the Sun, and the Scutum-Centaurus Arm. (The other arms are now thought to be minor structures made up largely of gas.)

Today, Thomas Dame and Patrick Thaddeus at the Harvard-Smithsonian Center for Astrophysics in Cambridge provide further evidence of this 2-arm structure but with a twist that explains why astronomers have previously been unable to see it clearly.

Astronomers traditionally study the Milky Way's structure by measuring the movement of large clouds of hydrogen and carbon monoxide gas within it (the velocity of distant stars being too difficult to pin down).

The new evidence that Dame and Thaddeus have accumulated shows the existence of a new arm on the other side of the Milky Way, but further from the centre than we are. The new arm is 18 kpc long and so stretches some 50 degrees across the sky.

Dame and Thaddeus conclude that this arm is an extension of the Scutum-Centaurus Arm, the rest of which is obscured behind the galactic middle.

That makes sense. The Perseus arm, which we can see more clearly, wraps 300 degrees around the galactic centre. If Dame and Thaddeus are correct, the Scutum-Centaurus Arm must be exactly symmetrical with this. That makes the Milky Way similar to the Great Barred Spiral, a barred, twin spiral some 56 million light years from here.

But why has it taken astronomers so long to find the end of this arm? The reason, says Dame and Thaddeus, is that this arm is bent. Sot it's not in the galactic plane but slightly above it.

Which means the Milky Way is warped, like the cap from a freshly-opened beer bottle.

[bystander]

A New, Distant Arm of the Milky Way Galaxy
Smithsonian Astrophysical Observatory
Weekly Science Update | 2011 Jun 10

[size=85]The Milky Way's basic structure is believed to involve two main spiral arms emanating from opposite ends of an elongated central bar. But only parts of the arms can be seen - gray segments indicate portions not yet detected. Other known spiral arm segments--including the Sun's own spur--are omitted for clarity. [i](Credit: T. Dame)[/i][/size]

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Our Milky Way galaxy, like other spiral galaxies, has a disk with sweeping arms of stars, gas, and dust that curve around the galaxy like the arms of a huge pinwheel. The Sun, Earth, and solar system are located in a spur of material that lies between two of the spiral arms, collectively orbiting around the galaxy about 25,000 light-years from its center. Because the Milky Way contains copious amounts of dust that blocks our optical views, it is extremely difficult to study the galaxy from our vantage point within the disk. Thus the details of the spiral arms in our own galaxy are much less certain than is the structure of external spirals such as Andromeda, which is a few million light-years away but sits well above the plane of obscuring dust. Radio wavelengths can peer through the dust, however, and molecules like carbon monoxide that emit in the radio and concentrate in the galaxy's spiral arms are particularly good tracers of their structure.

Using a small 1.2-meter radio telescope on the roof of their science building in Cambridge, CfA astronomers Tom Dame and Pat Thaddeus used carbon monoxide emission to search for evidence of spiral arms in the most distant parts of the galaxy, and discovered a large new spiral arm peppered with dense concentrations of molecular gas. The CfA scientists suggest that the new spiral is the far end of the Scutum-Centaurus Arm, one of the two main spiral arms thought to originate from opposite ends of our galaxy's central bar (see figure). If their proposal is confirmed, it will demonstrate that the Milky Way has a striking symmetry, with the new arm being the symmetric counterpart of the nearby Perseus Arm.

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