How does the mass-to-light ratio of galaxies change over time?

Post by Tom Kitching, MSSL

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In a recent paper, led by collaborator Dr Mike Hudson the CFHTLenS survey was used to investigate how the relationship between the total amount of matter surrounding galaxies, compared to the amount of luminous (stellar) matter changes as a the Universe ages.

There is a vast and growing amount of evidence that the stars we see in galaxies are just the tip of the ice-berg when it comes to the total amount of matter. In this recent paper the gravitational lensing signal around individual galaxies was used to measure the amount of total matter present; which would include any unseen or dark matter. It was confirmed that there is much more dark matter than star-matter, and that in fact there is roughly 30 times more dark matter than star-matter!

There is much more than meets the eye. The stars present in galaxies account for approximately 3% of the total matter present in most galaxies. From http://upload.wikimedia.org/wikipedia/commons/c/c3/NGC_4414_%28NASA-med%29.jpg

What this study found for the first time was that this ratio of dark matter to star-matter is not a constant as the Universe ages but is actually changing. It was found that the peak ratio falls as a function of cosmic time from 3.8±0.3 percent when the Universe was 7.4 billion years old to only 3.0±0.2 percent at when the Universe was 10.3 billion years old. This is a very precise measurement, measuring a change of only 1 percent over a timescale of nearly 3 billion years!

Why is this change happening? The paper shows that the change is actually dominated by changes in galaxies that are “red”, these are large and old galaxies where the production-rate of stars is slowing down as they run out of available gas, as the Universe ages. Interestingly if this change is dominated by the red galaxies then it implies that in the other galaxies, so-called “blue” galaxies, that are young and star-forming, the amount of stars that are made is balanced by the amount of dark matter attracted to those galaxies.

  • The full article can be found at this link : http://arxiv.org/abs/1310.6784
  • The CFHTLenS data can be accessed here : http://www.cfhtlens.org
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