In their paper, they explained that in order to study the weak lensing signal of the dark matter filaments, they required two sets of data: a catalog of galaxy cluster pairs that were lensed, and a catalog of background source galaxies with accurate distance measurements.
They combined lensing data from a multi-year sky survey at the Canada-France-Hawaii Telescope with information from the Sloan Digital Sky Survey that mapped luminous red galaxies (LRGs), which are massive, distant, and very old galaxies.
“LRGs are very bright galaxies,” Hudson told Seeker via email. “They tend to be more massive than the average galaxy and live in more massive dark matter ‘halos.’ It's reasonable to expect that the filament or bridge between them might also be more massive than the average.”
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Hudson and Epps combined or “stacked” more than 23,000 galaxy pairs, all located about 4.5 billion light-years away. This allowed them to create a composite image or map that shows the presence of dark matter between galaxies. Hudson told Seeker that the filament in their “image” is the average of all 23,000 pairs.
“The primary reason that we used these galaxies is that they had precise distances (as measured by another team),” Hudson explained. “These distance measurements allowed us to distinguish between pairs of galaxies that were actual pairs in 3D (meaning both are at the same distance from us) as opposed to two galaxies that appeared close on the sky but were actually at very different distances.”
3D pairs would be physically close to each other and hence, will have a bridge whereas the second group are not physically close to each other, and so would not have a bridge between them. Hudson and Epps said their results show the dark matter filament bridge is strongest between systems less than 40 million light years apart.
“By using this technique, we’re not only able to see that these dark matter filaments in the universe exist, we’re able to see the extent to which these filaments connect galaxies together,” Epps said in a statement.