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PostPosted: Wed, 14-01-15, 2:13 GMT 
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One of the Milky Way’s arms might extend further than previously thought:

http://www.universetoday.com/118153/one-of-the-milky-ways-arms-might-encircle-the-entire-galaxy/
http://iopscience.iop.org/2041-8205/798/2/L27/
http://arxiv.org/abs/1412.2425

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PostPosted: Wed, 14-01-15, 11:19 GMT 
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DW,

thanks for quoting this Chinese paper about a possibly larger extention of the Scutum-Centaurus arm of the iMW.

Yet given the lack of most relevant references (the RMS survey, a 12-year study!),

http://arxiv.org/pdf/1310.4758v1 (published in MNRAS 437, 1791–1807 (2014), link http://mnras.oxfordjournals.org/content ... l.pdf+html)

http://www.ast.leeds.ac.uk/RMS/
http://www.sci-news.com/astronomy/scien ... 01649.html
...
I still have some reservations about the present paper. The missing RMS reference about Massive Young Stellar Objects (MYSOs) and compact/ultra-compact HII regions represents the cleanest recent evidence for a four principal arm structure of the MW (Norma, Sagittarius, Perseus and Scutum-Centaurus arms). Moreover, it provides lots of data tracing the so-called Outer arm in the region of present interest (II nd quadrant) (You remember that the Spitzer telescope claimed in 2008 to see only 2 principal arms...)

Here is a summary image from the above RMS paper:
[click on image by all means for a much bigger size!]
Attachment:
rms_MW.jpg
rms_MW.jpg [ 102.47 KiB | Viewed 2386 times ]


During the past year I have done a large amount of most interesting studies about the arm structure of the MW, in part also with the help of Christophe.
I actually communicated with the RMS lead scientist Prof. J.S. Urquhart, who was so kind as to also send me their "hot" numerical data!

+++++++++++++++++++++++++++++
The point was to combine the new MYSO data with a complete set of published ATNF pulsar data which largely increases the evidence for the arm structure from the RMS data alone.
+++++++++++++++++++++++++++++

So I actually have the contents of an interesting respective publication in the draw. If you are interested in some respective work, let me know. Of course we also have already a MW template for celestia.Sci, incorporating the latest solid evidences...

Unfortunately, I am still VERY busy with some unforseen fulltime research matters (in my lab) for a number of further days. I try to contact you as soon as time allows.

Fridger

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PostPosted: Thu, 15-01-15, 1:54 GMT 
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Yes, this is very fascinating. In fact now that you mention MYSOs, my recent Disk Detective internship at NASA was also concerned with searching for YSOs (and debris disks) using mid-IR data. Several teams are doing this, using different methods (we were using crowdsourcing).

In that case, we were using data from WISE (Wide-field Infrared Survey Explorer), which is not only a much more recent mission (2009-2013) than MSX (1996-1997), but also has full-sky coverage and higher resolution.

So I think that YSO discoveries from WISE data (as well as ATNF pulsar data) could further improve understanding of the Milky Way structure.


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PostPosted: Thu, 15-01-15, 17:21 GMT 
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DW,
dirkpitt wrote:
In that case, we were using data from WISE (Wide-field Infrared Survey Explorer), which is not only a much more recent mission (2009-2013) than MSX (1996-1997), but also has full-sky coverage and higher resolution.

It is true that the MSX satellite was only operational between 1996 and 1997. Yet I strongly suggest you consult the official paper about the Red MSX Source survey from Sep 2013:
http://iopscience.iop.org/0067-0049/208 ... 8_1_11.pdf

The Red MSX Source data base has evolved over >12 years and all relevant upcoming information about MYSOs has been continuously implemented!
dirkpitt wrote:

So I think that YSO discoveries from WISE data (as well as ATNF pulsar data) could further improve understanding of the Milky Way structure.

@YSOs: It is crucial that the extracted YSO's are shown to be MASSIVE, i.e. MYSOs are required for tracing the MW arms near the galactic plane! The latter are VERY bright, embedded infrared sources that have yet to begin ionising their surroundings to form an ultra-compact H II region. They are likely to be already burning hydrogen in their cores, whilst still accreting at the surface.

@WISE data: Of course, the Red MSX Source data base has also been extensively cross-correlated with the much more recent WISE data!

+++++++++++++++++++++++++++++
As a crucial check the collaboration repeated e.g. the same task of extracting ALL MYSOs and compact HII regions near the galactic plane with the WISE data alone under precisely identical conditions. Since WISE and MSX have complementary advantages and disadvantages, the results were, however, in no way an improvement.
+++++++++++++++++++++++++++++

Results:

Of the 72 WISE objects (i.e. a few percent) that are not detected by MSX, a visual inspection reveals that only 13 are definitely real, with another 7 possibly so. The others are a mixture of extended objects, regions around saturated objects, and a few very confused regions. => hence NO improvement.

Surely, the WISE data have better spatial resolution at 10μm than do the MSX
data by a factor of three and they are considerably deeper in limiting flux.
. However, unfortunately, WISE suffers from saturation for bright sources, and the published WISE catalog is not a complete source catalog so much as a flux-detection catalog

Too bad, since MYSOs are luminous (L > 10^4 Lsun)...

So just comparing the operational dates of the MSX and WISE satellites is not a good guide in this highly specialized and demanding project of tracing the Milkyway arms via MYSOs and pointlike HII regions!

Fridger

PS: Note: the above paragraphs in italic are literal quotes from the above Red MSX Source paper.

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PostPosted: Thu, 15-01-15, 19:01 GMT 
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Perhaps these excerpts from a recent interview of Prof. Melvin Hoare (Univ. Leeds/UK) make the importance of MYSOs versus YSOs in the tracing task of the MW arms more transparent. Moreover, arguments are given below why Spitzer was missing two MW arms in 2008!

Prof. M. Hoare wrote:
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.
...
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.

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“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.
+++++++++++++++++++++++

Since lower mass stars live much longer than massive stars and thus rotate around our Galaxy many times, they have time enough for spreading out in the MW 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. However, the gas is compressed enough in all four arms to lead to massive star (MYSO) formation.


Fridger

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PostPosted: Sun, 18-01-15, 6:55 GMT 
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Thanks Fridger, the MSX paper presents a strong case for the MSX data, and now I know why Massive YSOs are important not only in terms of telling us more about massive star formation, but also in the context of "tracing" the arms of our galaxy.


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