When galaxies form, they leave a lot of debris strewn around and this might reveal vital clues about the nature of dark matter, think scientists. Galaxies are believed to form by collision of smaller bodies of stars, which then equilibriate due to gravitational interactions. Even the Milky Way is believed to be surrounded by smaller galaxies, called satellites. A group of scientists looking at the Local Group, or a group of a few galaxies, including our Milky Way and the neighboring Andromeda, have found that the number of observed satellite galaxies is much smaller than expected from cold dark matter simulations.
These satellite galaxies have unusually low number of stars and can be made up of mostly dark matter. These dark satellites could only be seen by the way they bend light due to their gravitation.
Searching for Dark Satellites
Attempts to find dark satellite galaxies in the Local Group had yielded a candidate at a redshift of z= 0.222, which works out to about 3 billion light years, which is quite close by cosmological standards. However, the number was just not tallying up; out of a predicted number of 10,000 only 30 showed up. Scientists decided that the Local Group case might be an anomaly and looked further out.
In a paper published in Nature, by the team of Vegetti (MIT), Lagattuta (University of California, Davis), McKean (Netherlands Institute of Radio Astronomy), Auger(University of California, Santa Barbara), Fassnacht (University of California, Davis) and Koopmans (University of Groningen, Netherlands) report to have observed a dark satellite galaxy at a redshift of 0.881, which puts its distance at about 10 billion light years.
Using General Relativity to See!
The method is a beautiful consequence of Einstein’s General Theory of Relativity, the modern theory of gravity. It says that mass bends space-time and light rays will follow this bent path as they travel. This allows us to use a technique called gravitational lensing. The technique is quite simple in principle. A dark matter object will act as a lens, bending light as it travels through it. If you have a light-emitting galaxy behind a dark matter galaxy, the light will get distorted and we will be seeing an image of the galaxy at a position where it is not really there. A beautiful demonstration of this is manifested as an Einstein’s Ring.
Using the Keck-II telescope, one of these Einstein Rings was observed and a satellite galaxy was noticed at about 10 billion light years away, which has about the same mass as the Sagittarius galaxy. This is the first gravitational detection of a low-mass dark satellite galaxy at cosmological distances!
Exactly as predicted!!
The most exciting feature about this is that the mass calculations yield just the right mass for the galaxy to be consistent with cold dark matter simulations. It gives more credence to the hierarchical formation of stellar structures like galaxies as described by cold dark matter models.
The existence of this low-mass dark galaxy is just within the bounds we expect if the universe is composed of dark matter that has a cold temperature. However, further dark satellites will need to be found to confirm this conclusion
Ignorance may not always be blissful, but it is certainly exciting most of the time!