A New, Distant Arm of the Milky Way Galaxy

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.

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.
http://www.cfa.harvard.edu/news/2011/su201121.html

Nearby Galaxy Boasts Two Monster Black Holes, Both Active

Viewed in visible light, Markarian 739 resembles a smiling face, with a pair of bright cores underscored by an arcing spiral arm. The object is really a pair of merging galaxies. Data from Swift and Chandra reveal the western core (right) to be a previously unknown AGN; past studies already had identified an AGN in the eastern core. The two supermassive black holes are separated by about 11,000 light-years. The galaxy is 425 million light-years away.

Read more: http://www.nasa.gov/mission_pages/swift/bursts/monster-black-holes.html

Milky Way Is Warped, Like a Beer Bottle Cap

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

The Milky Way may appear warped as seen from the outside

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.
Ref: arxiv.org/abs/1105.2523: A Molecular Spiral Arm in the Far Outer Galaxy
http://www.technologyreview.com/blog/arxiv/26763/

Test of the Einstein Equivalence Principle

The S Stars in the Galactic-center region are found to be on near-perfect Keplerian orbits around presumably a supermassive black hole, with periods of 15–50 yr. Since these stars reach a few percent of light speed at pericenter, various relativistic effects are expected, and have been discussed in the literature. We argue that an elegant test of the Einstein equivalence principle should be possible with existing instruments, through spectroscopic monitoring of an S star concentrated during the months around pericenter, supplemented with an already-adequate astrometric determination of the inclination. In essence, the spectrum of an S star can be considered a heterogeneous ensemble of clocks in a freely-falling frame, which near pericenter is moving at relativistic speeds…. Read more: http://arxiv.org/PS_cache/arxiv/pdf/1105/1105.0918v1.pdf