USydney: Dwarf galaxy rotation - cosmic rethink?

[MargaritaMc]

University of Sydney

Mysterious dance of dwarfs may force a cosmic rethink

18 July 2014

The discovery that many small galaxies throughout the universe do not 'swarm' around larger ones like bees do but 'dance' in orderly disc-shaped orbits is a challenge to our understanding of how the universe formed and evolved.

The finding, by an international team of astronomers, including Professor Geraint Lewis from the University of Sydney's School of Physics, is announced today in Nature.*

"Early in 2013 we announced our startling discovery that half of the dwarf galaxies surrounding the Andromeda Galaxy are orbiting it in an immense plane" said Professor Lewis. "This plane is more than a million light years in diameter, but is very thin, with a width of only 300 000 light years."

The universe contains billions of galaxies. Some, such as the Milky Way, are immense, containing hundreds of billions of stars. Most galaxies, however, are dwarfs, much smaller and with only a few billion stars.

For decades astronomers have used computer models to predict how these dwarf galaxies should orbit large galaxies. They had always found that they should be scattered randomly.
... Professor Geraint Lewis. [said] "This is a big problem that contradicts our standard cosmological models."

• *Nature article (paywalled) Velocity anti-correlation of diametrically opposed galaxy satellites in the low-redshift Universe

• Discussed at Universe Today

• The 2013 paper mentioned in the University of Sydney news release is at arXiv A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy

• Rochester Institute of Technology. "Map of universe questioned: Dwarf galaxies don't fit standard model." ScienceDaily. ScienceDaily, 11 June 2014. http://www.sciencedaily.com/releases/20 ... 093627.htm Which refers to http://arxiv.org/abs/1406.1799

[MargaritaMc]

An interesting related paper at the Astrophysical Journal Letters
http://iopscience.iop.org/2041-8205/789/1/L24
The arXiv version is here http://arxiv.org/abs/1406.1799

[rstevenson]

University of Sydney
... said Professor Lewis. "This plane is more than a million light years in diameter, but is very thin, with a width of only 300 000 light years."

The PDF I downloaded from arxiv says...

a coherent planar structure ... with a root-mean-square (rms) thickness of 12.6 ± 0.6 kpc (< 14.1 kpc at
99% confidence)

Taking 14 kpc (14,000 parsecs) times 3.26 pc per light year, I get just under 46,000 light years, not the 300,000 mentioned in the article, which certainly fits better with the description of the plane as "thin."

Rob

[geckzilla]

Why I am bad at math: Because computers do it all for me. Google calculator concurs with your calculation, Rob.

[rstevenson]

Why I am bad at math: Because computers do it all for me. Google calculator concurs with your calculation, Rob.

I did it first in my head (I'm one of those geeks), then again on my handheld, and then checked it on [a drum roll please] Google calculator.

Rob

[Ann]

Good to see you here again, Rob. (And you too, Margarita, although you come here more frequently than Rob.)

Anyway, how interesting! The Andromeda galaxy and its satellites are one humongous version of Saturn and its rings!

Ann

[Chris Peterson]

The PDF I downloaded from arxiv says...

a coherent planar structure ... with a root-mean-square (rms) thickness of 12.6 ± 0.6 kpc (< 14.1 kpc at
99% confidence)

Taking 14 kpc (14,000 parsecs) times 3.26 pc per light year, I get just under 46,000 light years, not the 300,000 mentioned in the article, which certainly fits better with the description of the plane as "thin."

I didn't read the paper. But you could easily have a full thickness of 300,000 ly which has an RMS thickness of 46,000 ly.

[rstevenson]

I just double-checked the paper. They don't mention thickness in any context other than rms. Perhaps Prof. Lewis, (the source of the quote) who is, after all, one of the authors of the paper, was speaking about full thickness rather than rms thickness, but why he would do so I can't imagine. The paper depends in several ways on their calculations of a suitable rms thickness, and the way that fits so well with a certain set of the dwarf galaxies.

Rob

[MargaritaMc]

Why I am bad at math: Because computers do it all for me. Google calculator concurs with your calculation, Rob.

I did it first in my head (I'm one of those geeks), then again on my handheld, and then checked it on [a drum roll please] Google calculator.

Rob

I'm very impressed. One day, if my studies continue, I might understand what it means...

M

PS. Thanks Ann.

[MargaritaMc]

Observatoire Astronomique de Strasbourg (CNRS/Université de Strasbourg)
CNRS. "Satellite galaxies put astronomers in a spin." ScienceDaily, 23 July 2014. http://www.sciencedaily.com/releases/20 ... 110835.htm

... [A] study carried out in Strasbourg and Sydney based on the Sloan Digital Sky Survey, a survey covering a third of the sky that makes it possible to explore the properties of distant galaxies, has shown that, in 380 galaxies observed, located between 30 and 700 million light years away and having at least two visible satellite galaxies, the small satellite galaxies also appear to orbit around their hosts. ...
These findings call into question the predictions of the standard model at galactic scales. This is because, if this phenomenon were linked to the accretion of satellite galaxies along filaments of dark matter in the Universe, it would be necessary to explain why these rotating structures are much thinner than the filaments that gave rise to them, and also why the two brightest satellite galaxies, which are the two that can be seen, systematically always come from the same filament.
Alternatively, the discovery may mean that our current models need to be completely revised. Today, everything appears to indicate that the standard model provides a faithful representation of observations at the largest scales of the Universe3, but that, for the moment, we are overlooking something fundamental at smaller scales.