HILO — Dark matter is an invisible and mysterious substance that is thought to be crucial to the formation of galaxies.
And, yet, scientists using telescopes atop Maunakea have found one galaxy that has almost none at all. The discovery challenges the way scientists think about how galaxies are born.
“For decades, we thought that galaxies start their lives as blobs of dark matter,” astronomer Pieter van Dokkum of Yale University said in a press release issued by W.M. Keck Observatory. “After that everything else happens: Gas falls into the dark matter halos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way.”
Measurements made with Keck and Gemini Observatory, both on Maunakea, the Hubble Space Telescope and others showed that NGC 1052-DF2, an ultra-diffuse galaxy, followed a different path.
With spectral data from Keck, van Dokkum’s team found clusters of stars were moving much slower than expected, which indicates less mass in a system. The team determined almost all of the galaxy’s mass could be attributed to stars, which means there is almost no dark matter, the release said.
Gemini then assisted by showing the galaxy was not interacting with other galaxies.
According to the release, the results show that dark matter is separable from galaxies.
“It’s like you take a galaxy and you only have the stellar halo and globular clusters, and it somehow forgot to make everything else,” van Dokkum said. “There is no theory that predicted these types of galaxies. The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is completely unknown.”
Potential explanations, which remain speculative, are that a fledgling massive galaxy billions of years ago played a role, or that a cataclysmic event within the galaxy swept out gas and dark matter, halting star formation, the release said.
Van Dokkum plans to use Keck to search for more galaxies like NGC 1052-DF2.
“Every galaxy we know about before has dark matter, and they all fall in familiar categories like spiral or elliptical galaxies,” he said. “But what would you get if there were no dark matter at all? Maybe this is what you would get.”
Email Tom Callis at tcallis@hawaiitribune-herald.com.