2 arms or 4 ?‏

13 Nov 2009

Does our Milky Way galaxy have 2 spiral arms or 4 spiral arms?

There is an ongoing argument in the scientific journals now, that may have more to do with Global Warming advocacy than science. Great! 😉

… another chance to test our critical thinking skills by checking for logical argumentation and the correct use of the scientific method 🙂

 Here are the papers I am talking about:
“Testing the link between terrestrial climate change and Galactic spiral structure”
or the newer version

Here are two criticisms of that paper and FOR the 4 arm theory
First, by a short statement Henrik Svensmark ….

“Svensmark, however, believes that the spiral–arm correlation itself still stands. He says the analysis by Melott and co-workers has problems in determining the location of spiral arms and also wrongly assumes that the whole spiral-arm pattern moves with a single speed.”




…. then (with more detailed with refs) by Nir Shaviv:


sylvain (02:06:43) :
I’ve contacted Nir Shaviv about the paper that RC introduce at the end of the post:
Here is what he said:
—–Message d’origine—–
De : Nir Shaviv
Envoyé : 4 août 2009 01:52
À : Sylvain
Objet : Re: New paper promoted by realclimate
Hi Sylvain,
Here is what I think of melott et al.: (I will add it to sciencebits at some point…)
– Nir
1 – Melott assumes only one pattern speed for the spiral structure, and therefore do not consider the dynamics which shows that it is a pattern composed of 2 spiral arms with one pattern speed + 4 arms with another. It so happens that they coincide at this point in time. e.g., look at this paper: http://www.phys.huji.ac.il/%7Eshaviv/articles/NaozShaviv.pdf (full ref: http://adsabs.harvard.edu/abs/2007NewA…12..410N ) It also happens that the 2-armed structure is almost co-rotating with the solar system.
In fact, assuming that the milky way has a very complicated pattern (different number of arms, very antisymmetric, etc., and assuming that it can rotate like a rigid pattern with one pattern speed, for many 10^8 years, is unrealistic.

2 – The Melott analysis is not consistent with the Spitzer reconstruction, nor it is consistent with the CRF variations observed in Iron meteorites. (Spitzer: http://adsabs.harvard.edu/abs/2009PASP..121..213C Iron meteorites: http://adsabs.harvard.edu/abs/2003NewA….8…39S )

3 – The Melott analysis is based on the spiral arm reconstruction of Englmaier et al. However, there are a few critical problems with the way they do it. In particular, they assume there is an arm in a region in which they see no arm in the density plot, and they ignore an arm passage through a region where the gas map shows a clear elongated concentration. This can be seen in the attached figure (it is composed of the original density plot + arms denoted by Englmaier et al., plus the solar system trajectory according to Melott et al.. Note the location of the solar system is not exactly the same!!! The yellow dots denote passages according to Melott et al., including a passage through something which was dented as an arm but without the density concentration to support it. The red dot denotes a passage through an arm like condensation but not denoted by Melott.

4 – Melott et al. don’t inlcude additional effects on the trajectory, that the potential is not cylindrically symmetric (the arms introduce something of order a 10% correction. They also don’t include the effect of orbital parameter diffusion.

Here are some popular articles AGAINST the 4 arms theory:
Galactic link to climate change in doubt
Now, physicists Adrian Melott and Andrew Overholt of the University of Kansas and Martin Pohl of Iowa State University in the US have carried out a new study of the supposed link between the solar system’s galactic motion and climate change and have found that the correlation does not exist.

They base their work on a new model of the galaxy produced last year by astronomer Peter Englmaier, who used the distribution of carbon monoxide molecules throughout the Milky Way, as revealed in infrared data collected by NASA’s Spitzer mission, to trace out the structure of the spiral arms.

… and even an Uppsala PRO catastrophic man-made global warming site arguing over the paper:
Stjärnorna kvittar det lika
“Så hur stämmer Svensmarks hypotes med den nya astronomiska kunskapen? Detta har en grupp astronomer nu undersökt2 och svaret är, inte alls.”
May the best evidence win!
So … what do you think ?…

 … or 4? 
… you be the judge.
Best regards,
Galaxy loving Brady

This entry was posted in Uncategorized. Bookmark the permalink.