Q&A: Dark Matter

Q:
According to a recent press release, Chandra provided detailed temperature maps for the gas of galaxy clusters Abell 2390 and MS2137.3-2353 and allowed astronomers to precisely determine the masses of the clusters. Most of the mass is in the form of dark matter.

How does one determine the masses from the temperature maps?

A:
The Chandra press release on dark energy:
http://chandra.harvard.edu/photo/2004/darkenergy/
states that measuring the ratio of the mass of the hot gas to the mass of the dark matter in these large galaxy clusters is key to determining whether our Universe is filled with "dark energy." The 2 measurements needed are the mass profile of the hot gas and the total mass profile of the cluster. Mass profiles can be thought of as maps of the mass distribution.

Measuring the mass profile of the hot gas is relatively straightforward since the emissivity of the x-ray emitting gas (the ratio of the radiation emitted by the gas to that emitted by a black body at the same temperature) is related to the density of the gas in a well understood way. Measuring the total mass profile is harder and requires a precise measurement of the temperature profile. Chandra is ideally suited to such precise measurements due to its high resolution.

Obtaining the total mass profile from the temperature map is technical - they use a combination of mathematical methods and computer programs. Although it is not easy to describe, I will point you to their paper in case you'd like to read through it. It is tough going but beginning on page 4, section 2.3, there is a detailed description of their steps and it might be nice to read through it and get an idea of how cutting edge science is done:
http://xxx.lanl.gov/abs/astro-ph/0405340