Most dark matter dominated galaxy in the Universe
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One of the Milky Way’s satellite galaxies - Segue 1 - has turned out to be the most dark matter dominated galaxy ever discovered. We ask research astronomer Marla Geha how this discovery came to light.
How might a single galaxy become so dominated with dark matter, and what does this mean for the ‘clumpiness’ of dark matter in the Universe?
Our current galaxy formation theory predicts that dark matter comes is a wide variety of different sized clumps. More massive clumps are rare, while smaller clumps are more numerous. The Milky Way galaxy resides in one of these more massive clumps and is expected to have several hundred smaller dark matter clumps surrounding it. Although theory can robustly predict the number of dark matter clumps around the Milky Way, it is much less able to predict how many of these will host stars. Until a few years ago, the answer was very few -- the Milky Way had only 11 small dwarf galaxy satellites in orbit. The idea was that there were many 'empty' dark matter clumps around the Milky Way. The recent discoveries of many new Milky Way satellite galaxies, including Segue 1, suggest that perhaps many of these dark matter clumps do indeed host stars -- just not that many stars.
Is there anything else that could account for the galaxies' heavy masses?
We estimated the mass of Segue 1 by measuring how fast stars stars in this galaxy are moving. Given the mass of the stars we observe in Segue 1, we expected speeds of 0.5 km/s. Instead, we observed velocities ten times higher of about 5 km/s. This difference means that the mass of Segue 1 is dominated by dark matter (that is, mass we can actually see). However, this conclusion rests on an assumption that these stars are moving under the influence of gravity from the mass of Segue1 only. If outside influences are important, such as the gravitational influence of the Milky Way, then it is possible we have measured the mass incorrectly. While we don't have any evidence
that Milky Way is affecting the motion of stars in Segue 1, we can't yet prove this is the case.
How does the discovery help in our understanding of both galaxy formation and the nature of dark matter?
In terms of galaxy formation, Segue 1 and the other very faint galaxies suggests that making a small number of stars in dark matter clumps may be easier than we previously thought.
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