A dark matter bridge detected between pairs of galaxies

Example of galaxy pair connected by dark matter filaments (Image courtesy S. Epps & M. Hudson / University of Waterloo)
Example of galaxy pair connected by dark matter filaments (Image courtesy S. Epps & M. Hudson / University of Waterloo)

An article published in the journal “Monthly Notices of the Royal Astronomical Society” describes a research on dark matter filaments that connect two galaxies. A team of astronomers led by Mike Hudson of the University of Waterloo in Canada exploited a weak gravitational lensing effect to create an image that shows even if indirectly a kind of dark matter bridge between two galaxies.

Discussions on dark matter and its nature have been going on ever since its existence was proposed to explain certain gravitational effects noted at cosmic level. Not everyone agrees about its existence and attempts to locate it seem really hard despite being estimated that it forms 25% of the universe because it generally doesn’t interact with ordinary matter. If it really exists, among other things it connects galaxies with filaments that form a kind of web and this research tried to identify it in some way.

The researchers used a technique called weak gravitational lensing, an effect that distorts the images of distant galaxies due to a mass that has enough force of gravity to bend light in a measurable manner. This effect predicted by Einstein’s general theory of relativity is exploited for example to observe distant galaxies whose images are magnified by a nearer galaxy just as if it were an optical lens. In this case, however, the idea was to try to identify the distortions caused by dark matter, which are much weaker.

To carry out this research, Mike Hudson and his colleagues used more than 23,000 images of pairs of galaxies collected over some years in a survey conducted using the Canada-France-Hawaii Telescope. The images were combined to create a map that showed the distortion caused by the weak gravitational lens created by dark matter connecting two galaxies.

The false-color image shows an example of two galaxies, in the white areas, connected by a bridge of dark matter, in the red areas. The results also show to what extent a bridge of dark matter filaments combines two galaxies: for example, the researchers noted that it’s stronger between galaxies less than 40 million light years apart.

Once again the effects of dark matter are identified but it’s not seen directly. It’s still something measurable in practice, not a theoretical model and the fact that the results are in agreement with the statistical predictions is a plus. For skeptics it’s a chance to test any alternative theories to the existence of dark matter.

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