Magellanic Clouds in Motion
The two magellanic clouds are galaxies close to our Milky Way but their Southern position inhibits viewing by Northern telescopes.
Announced in 2006, measurements with the Hubble Space Telescope suggest the Large and Small Magellanic Clouds may be moving too fast to be orbiting the Milky Way.
In 2014, measurements from the Hubble Space Telescope made it possible to determine that the LMC has a rotation period of 250 million years.
From a space.com story about LMC:
By pointing NASA's Hubble Space Telescope toward the two clouds, scientists began to catch a glimpse of the objects' histories. "Hubble's biggest contribution is enabling us to clock how fast the Magellanic clouds are moving," said Gurtina Besla, a researcher at the University of Arizona who studies dwarf galaxies. In 2007, Besla overturned conventional wisdom when she suggested that the LMC and SMC were making their first orbit of our galaxy.
"They're moving too fast to have been long-term companions of the Milky Way," Besla said.
She used data from the European Space Agency's Gaia spacecraft to clock smaller, satellite galaxies orbiting the LMC, as well. And, understanding how these galaxies move has helped researchers better calculate the mass of the LMC. Current estimates put the LMC at about 100 billion times as massive as Earth's sun, or a quarter the mass of the Milky Way. Besla said this size means the LMC is about 10 times heavier than previously calculated.
Some other details:
LMC distance 163 Kly
LMC red shift 0.00093 z or 278 km/s
SMC distance 200 Kly
SMC red shift 0.000527 z or 158 km/s
Both LMC SMC are in a cloud of neutral hydrogen, with a bridge of hydrogen between them.
With Hubble Constant = 70 km/s / Mpc and these red shifts:
LMC distance is calculated at 13 Mly, SMC distance is 7.37 Mly
Their current accepted (closer) distances come from Cepheids not the red shift:
Using this period-luminosity relation, in 1913 the distance to the SMC was first estimated by Ejnar Hertzsprung. First he measured thirteen nearby cepheid variables to find the absolute magnitude of a variable with a period of one day. By comparing this to the periodicity of the variables as measured by Leavitt, he was able to estimate a distance of 10,000 parsecs (30,000 light years) between the Sun and the SMC. This later proved to be a gross underestimate of the true distance, but it did demonstrate the potential usefulness of this technique.
When new telescopes provide resolution to individual stars the astronomers could ignore these red shifts implying great distance. When an astronomer says 'too fast' the basis for that conclusion is not provided.Is it too fast for which distance?
The distance from red shift is much greater than via Cepheids. Is either distance absolutely certain?
Using new telescopes to capture their lateral motion, astronomers concluded they are moving too fast.
Dark matter is proposed to explain unexpected galaxy motions in clusters.
Researchers in 2006 concluded our Magellanic Clouds are probably moving past the Milky Way and are not in orbit as previously assumed. In 2014 the first measurement of the LMC rotation period was taken.
With dark matter, cosmologists claim we know exactly why galaxies move in clusters and dark matter explains why they are moving differently. This claim of certainty conflicts with unexpected discoveries with our nearby magellanic clouds with new mass and new trajectory. We cannot explain these motions in our cluster but we claim with certainty dark matter explains distant motions.
This claim of certainty cannot be justified knowing the surprises in the magellanic clouds.
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