Archive for February 27th, 2012

February 27, 2012

The dwarf satellite galaxy problem

Simulations of galaxy formation based on the Lambda-Cold Dark Matter (ΛCDM) cosmological model predict that a large galaxy such as the Milky Way should have many dwarf satellite galaxies, perhaps thousands. However, only about 20 or 30 have been identified. Where are the rest? Are they really there? That question alludes to the “dwarf galaxy problem“.

Astrophysicists suspect that most satellite galaxies are much smaller than the galaxies they orbit. And, in addition, such dwarf galaxies may consist mainly of dark matter, with few visible stars, so they should be very difficult to detect, even if there are a lot of them. Since dwarf galaxies consisting mainly of dark matter are so difficult to find by visible light, there could be enough of them to reconcile the large number of dwarf galaxies that simulations predict to exist with the small number actually observed.

Surprisingly, recent research (Vegetti, et al) has been able to detect a very distant dwarf satellite galaxy by gravitational lensing effects – and from that it is possible to infer that a large number should exist.

The image shows an Einstein ring, which consists of a foreground galaxy (JVAS B1938+666) in the middle, and the distorted image of a more distant galaxy making up most of the ring. A detailed mathematical analysis of the image has confirmed that a minor irregularity in the ring is caused by the presence of a dwarf satellite galaxy of the lens galaxy.

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