Clusters Background Information: Dark Matter | ![]() |
Okay, so we have the mass of a cluster. What has this told us? Let's look at an example, the Coma cluster. If we measure the dynamical mass of the Coma cluster, using the techniques we've described here, we get something like Mdyn ~ 3-4 x 1015 Msun So how much of this mass in in galaxies? The total luminosity of the Coma cluster galaxies is Ltot ~ 5 x 1012 Lsun And if we say that each galaxy has a mass-to-light ratio (including both stars and any dark matter within the galaxy) of 10, this says that the galaxies have a total mass of Mgal ~ 10 x 5x1012 = 5x1013 Msun That's only a few percent of the total mass! But there's also hot gas throughout the cluster which shines not in visible light, but X-rays. Using X-ray satellites, we can observe and estimate the mass of this gas; it turns out to be about Mx ~ 3x1014 Msun That's 6 times more mass than is in the galaxies, but still an order of magnitude less than the dynamical mass of the cluster. Based on arguments like this, astronomers have been led to the conclusion that 90% of the mass in galaxy clusters -- and in the universe in general -- remains unobserved, the strange "dark matter" that we still don't understand...
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