17 Feb. 2011

 

How much dark matter to make a galaxy?

 

Thanks to the Herschel Space Observatory, it has been possible to determine how much dark matter must be present at the time of formation of a galaxy, so that it can contract to form stars and begin its multi-billion year evolution.
As our knowledge of the distribution of galaxy clusters at various epochs of the Universe increases, it is becoming ever clearer that dark matter has a fundamental role in the formation of galaxies. Although we know very little about what dark matter might actually be made of, all the evidence indicates that a multitude of dark matter concentrations in the primordial Universe caused the gravitational collapse of gas, to form the first stars and galaxies.
By using Herschel to study the cosmic infrared background, produced by innumerable galaxies 10-11 billion light years away, a group of researchers (amongst them Alexandre Amblard, UC Irvine, first author of the paper in "Nature"; Asantha Cooray, University of California, Irvine; William Danchi, Herschel program scientist, NASA Headquarters, Washington; Jamie Bock, JPL, Pasadena) have determined how much dark matter is needed to maintain the process of galaxy formation.
By reproducing the observed large scale structure in the distribution of the galaxies with computer simulations, they determined, that, on average, 300 billion solar masses of dark matter are needed to form a galaxy. If there is less than this, then the formation process halts and the gas doesn't manage to collapse to produce stars. If there is too much dark matter, then the gas cannot cool efficiently and the forming "proto-galaxy" fragments, producing many dwarf galaxies, rather than a single large one.
In the above image we see a portion of the sky examined by the Herschel team. This area of sky, lying in the constellation of Ursa Major, is called the "Lockman Hole", and was chosen because in this direction there is very little dust from our own galaxy, something that represents "contamination" in an infrared cosmological study. Therefore, the structures that can be seen in the image are due entirely to the distribution of very distant galaxies. These actually appear to be somewhat more numerous than previously thought.

 

by Michele Ferrara & Marcel Clemens

credit: ESA/Herschel/SPIRE/HerMES