University of Leeds:Our galaxy DOES have four arms, not two.

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MargaritaMc
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University of Leeds:Our galaxy DOES have four arms, not two.

Post by MargaritaMc » Wed Dec 18, 2013 8:13 pm

http://www.leeds.ac.uk/news/article/347 ... ssing_arms
The University of Leeds
A 12-year study of massive stars has reaffirmed that our Galaxy has four spiral arms, following years of debate sparked by images taken by NASA’s Spitzer Space Telescope that only showed two arms.

The new research, which is published online today [17 December] in the Monthly Notices of the Royal Astronomical Society, is part of the RMS Survey, which was launched by academics at the University of Leeds.

Astronomers cannot see what our Galaxy, which is called the Milky Way, looks like because we are on the inside looking out. But they can deduce its shape by careful observation of its stars and their distances from us. 

“The Milky Way is our galactic home and studying its structure gives us a unique opportunity to understand how a very typical spiral galaxy works in terms of where stars are born and why,” said Professor Melvin Hoare, a member of the RMS Survey Team in the School of Physics & Astronomy at the University of Leeds and a co-author of the research paper.

In the 1950s astronomers used radio telescopes to map our Galaxy. Their observations focussed on clouds of gas in the Milky Way in which new stars are born, revealing four major arms. NASA’s Spitzer Space Telescope, on the other hand, scoured the Galaxy for infrared light emitted by stars. It was announced in 2008 that Spitzer had found about 110 million stars, but only evidence of two spiral arms.

The astronomers behind the new study used several radio telescopes in Australia, USA and China to individually observe about 1650 massive stars that had been identified by the RMS Survey. From their observations, the distances and luminosities of the massive stars were calculated, revealing a distribution across four spiral arms.

“It isn’t a case of our results being right and those from Spitzer’s data being wrong – both surveys were looking for different things,” said Professor Hoare. “Spitzer only sees much cooler, lower mass stars – stars like our Sun – which are much more numerous than the massive stars that we were targeting.”

Massive stars are much less common than their lower mass counterparts because they only live for a short time – about 10 million years. The shorter lifetimes of massive stars means that they are only found in the arms in which they formed, which could explain the discrepancy in the number of galactic arms that different research teams have claimed.

“Lower mass stars live much longer than massive stars and rotate around our Galaxy many times, spreading out in the disc. The gravitational pull in the two stellar arms that Spitzer revealed is enough to pile up the majority of stars in those arms, but not in the other two,” explains Professor Hoare. “However, the gas is compressed enough in all four arms to lead to massive star formation.”


Dr James Urquhart from the Max Planck Institute for Radio Astronomy in Bonn, Germany, and lead author of the paper, said: “It's exciting that we are able to use the distribution of young massive stars to probe the structure of the Milky Way and match the most intense region of star formation with a model with four spiral arms.”

Professor Hoare concludes, “Star formation researchers, like me, grew up with the idea that our Galaxy has four spiral arms. It’s great that we have been able to reaffirm that picture.”
"In those rare moments of total quiet with a dark sky, I again feel the awe that struck me as a child. The feeling is utterly overwhelming as my mind races out across the stars. I feel peaceful and serene."
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Re: Our galaxy DOES have four arms, not two.

Post by neufer » Wed Dec 18, 2013 8:30 pm

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Re: University of Leeds:Our galaxy DOES have four arms, not

Post by Beyond » Thu Dec 19, 2013 12:19 am

Fat forearm training :?: Seems to me to be more like bar tendoning. :yes:
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Re: University of Leeds:Our galaxy DOES have four arms, not

Post by neufer » Thu Dec 19, 2013 1:34 am

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Re: University of Leeds:Our galaxy DOES have four arms, not

Post by Ann » Thu Dec 19, 2013 3:32 am

Thanks for posting that, Margarita, it's so interesting!

When I look at the Leeds University map of massive stars over the face of the Milky Way, I get the impression that their distribution doesn't trace any obvious spiral arms in our galaxy. It seems to me that the spiral structure of the Milky Way might be just a little underdeveloped, at least when it comes to the massive stars. I tried to find other galaxies where it is hard to really trace the spiral arms, and I was reminded of M88 and NGC 4414 (this picture by Martin Pugh is rather large). Admittedly M88 and NGC 4414 both appear to be pretty much non-barred, which makes them different from the Milky Way. Also, it is possible that the underlying structure of low-mass stars is more perfectly spiral in the Milky Way than in either M88 or NGC 4414.

For those of you who have access to Burnham's Celestial Handbook, check out page 372 in volume one. A technique used by Fritz Zwicky brings out the "red star structure" and the "blue star structure" of that iconic spiral galaxy, the Whirlpool Galaxy or M51. The "red star structure" traces a rather small, very symmetric spiral, while the blue star structure brings out the more well-known large, bright and somewhat jagged spiral.

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Re: University of Leeds:Our galaxy DOES have four arms, not

Post by geckzilla » Thu Dec 19, 2013 4:29 am

Hmm, the link at the bottom of the article to the research paper is an empty page telling me that it hasn't been released to the public.
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Re: University of Leeds:Our galaxy DOES have four arms, not

Post by rstevenson » Thu Dec 19, 2013 1:31 pm

geckzilla wrote:Hmm, the link at the bottom of the article to the research paper is an empty page telling me that it hasn't been released to the public.
Right, and without access to the paper there's no way to judge how the conclusion was drawn. That silly picture they released is worse than useless, since it suggests a kind of matching process -- see how well our red dots match up with this image of the Milky Way?, when in fact there is no such image. I'm sure the scientists involved didn't actually do any such thing, so I'd like to see the dots without the image, and the trend lines they created to illustrate the deduced spiral structure, and their error bars, and ... . Oh well, maybe someday.

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Re: University of Leeds:Our galaxy DOES have four arms, not

Post by Chris Peterson » Thu Dec 19, 2013 2:40 pm

rstevenson wrote:Right, and without access to the paper there's no way to judge how the conclusion was drawn.
Of course, that's often the case when a press release is the only source of information.
That silly picture they released is worse than useless, since it suggests a kind of matching process -- see how well our red dots match up with this image of the Milky Way?, when in fact there is no such image.
I think the image is superb- exactly the way this research should be presented to the mainstream press. That one image very clearly shows the concept behind the research- how the distribution of measured stellar locations can be used to understand the spiral arm structure of our galaxy.
I'm sure the scientists involved didn't actually do any such thing, so I'd like to see the dots without the image, and the trend lines they created to illustrate the deduced spiral structure, and their error bars, and ... . Oh well, maybe someday.
Of course... but sometimes papers lag press releases. And sometimes they're hard to get if you're not a subscriber.

If I were doing this work, and had access to the stellar coordinates, I'd probably analyze the data in at least two ways: by fitting the coordinates to synthesized 2-arm and 4-arm structures, and by employing radial frequency analysis- FFT or wavelet. But if the paper (or at least, the paper abstract) shows up, we should know more.
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Re: University of Leeds:Our galaxy DOES have four arms, not

Post by neufer » Thu Dec 19, 2013 3:14 pm

http://en.wikipedia.org/wiki/Milky_Way#Spiral_arms wrote: <<Outside the gravitational influence of the Galactic bars, astronomers generally organize the interstellar medium and stars in the disk of the Milky Way into four spiral arms which all start near the Galaxy's center. Spiral arms typically contain more interstellar gas and dust than the Galactic average as well as a high concentration of star formation, traced by H II regions and molecular clouds. As in most spiral galaxies, each spiral arm can be described as a logarithmic spiral. Estimates of the pitch angle of the arms range from ≈7° to ≈25°.

Maps of the Milky Way's spiral structure are notoriously uncertain and exhibit striking differences. Some 150 years after Alexander [Alexander, S. (1852). "On the origin of the forms and the present condition of some of the clusters of stars, and several of the nebulae". The Astronomical Journal 2: 97.] first suggested that the Milky Way was a spiral, there is currently no consensus on the nature of the Galaxy's spiral arms. Perfect logarithmic spiral patterns ineptly describe features near the Sun, namely since galaxies commonly exhibit arms that branch, merge, twist unexpectedly, and feature a degree of irregularity. The possible scenario of the Sun within a spur / Local arm emphasizes that point and indicates that such features are probably not unique, and exist elsewhere in the Galaxy.

Two spiral arms, the Scutum–Centaurus arm and the Carina–Sagittarius arm, have tangent points inside the Sun's orbit about the center of the Milky Way. If these arms contain an overdensity of stars compared to the average density of stars in the Galactic disk, it would be detectable by counting the stars near the tangent point. Two surveys of near-infrared light, which is sensitive primarily to red giant stars and not affected by dust extinction, detected the predicted overabundance in the Scutum–Centaurus arm but not in the Carina–Sagittarius arm: the Scutum-Centaurus Arm contains approximately 30% more red giant stars than would be expected in the absence of a spiral arm. In 2008, Robert Benjamin of the University of Wisconsin–Whitewater used this observation to suggest that the Milky Way possesses only two major stellar arms: the Perseus arm and the Scutum–Centaurus arm. The rest of the arms contain excess gas but not excess old stars. In December 2013, astronomers found that the distribution of young stars and star-forming regions matches the four-arm spiral description of the Milky Way. Thus, the Galaxy appears to have two spiral arms as traced by old stars and four spiral arms as traced by gas and young stars. The explanation for this apparent discrepancy is unclear.

A simulation published in 2011 suggested that the Milky Way may have obtained its spiral arm structure as a result of repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.

Another aspect is the so-called "wind-up problem" of the spiral arms. If the inner parts of the arms rotate faster than the outer part, then the galaxy will wind up so much that the spiral structure will be thinned out. But this is not what is observed in spiral galaxies; instead, astronomers propose that the spiral pattern is a density wave emanating from the Galactic Center. This can be likened to a moving traffic jam on a highway—the cars are all moving, but there is always a region of slow-moving cars. This model also agrees with enhanced star formation in or near spiral arms; the compressional waves increase the density of molecular hydrogen and protostars form as a result.

It has been suggested that the Milky Way contains two different spiral patterns: an inner one, formed by the Sagittarius arm, that rotates fast and an outer one, formed by the Carina and Perseus arms, whose rotation velocity is slower and whose arms are tightly wound. In this scenario, suggested by numerical simulations of the dynamics of the different spiral arms, the outer pattern would form an outer pseudoring and the two patterns would be connected by the Cygnus arm.

Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago.>>
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Re: University of Leeds:Our galaxy DOES have four arms, not

Post by MargaritaMc » Fri Dec 20, 2013 7:02 pm

An earlier draft of the paper (not the finalised version that will appear in MNRAS) is available on Arxiv:

http://arxiv.org/abs/1310.4758

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Re: University of Leeds:Our galaxy DOES have four arms, not

Post by Ann » Sat Dec 21, 2013 10:16 am

Thanks, Margarita. I only read the abstract, but I found it quite interesting. And it was so much more convincing than the map (with the red dots) that accompanied the first article.

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