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Dark dwarf galaxy found at a cosmological distance

Submitter: John McKean
Description: We believe that galaxies like our own formed over cosmic time through the mergers of many smaller galaxies. We can test this theory by searching for the tell-tale signatures of relics that survive as small dwarf galaxies located around the edges of more massive galaxies. Around our own galaxy, astronomers have failed to find these dwarf galaxies in the number that is expected. Therefore, it has been suggested that the Milky Way could be a strange case, galaxies form in a different way than expected, or that these dwarf galaxies are made up almost exclusively of dark matter.

In a paper published in Nature (19 January 2012) a team of researchers led by Simona Vegetti (Helena Kluyver visitor at ASTRON; MIT) and including ASTRON researcher John McKean reported the discovery of a low-mass dark dwarf galaxy at a look back time of about 10 billion years (redshift 0.881). The dwarf galaxy was found by studying the distorting effect its gravity has on background light.

The image on the left shows an almost complete Einstein ring from the gravitational lens B1938+666 (the light from the main lensing galaxy has been removed). Through modelling the light, the team were able to make a mass model for the lens, but found that an extra mass clump was needed to fit the data, which is evidence of the dark dwarf galaxy. This is shown in the convergence map on the right. The mass of the dwarf galaxy is about 200 million times that of our Sun, similar to the Sagittarius dwarf galaxy of our own Milky Way.

Using this and another previously published discovery, the team find that the mass-fraction of dark matter haloes made up of dwarf galaxies and the number of dwarf galaxies as a function of mass is just consistent with the hierarchical view of galaxy formation. More dwarf galaxies will need to be found using the gravitational lensing technique to confirm this result - so watch this space!!!
Copyright: Simona Vegetti / Nature
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