Astronomers have long questioned Yale professor Pieter van Dokkum’s team’s discovery of NGC1052-DF2, a galaxy that lacked dark matter entirely. After all, dark matter — the elusive, invisible substance that explains galactic phenomena that Newtonian physics cannot — is thought to be the most common type of matter in the known universe and is found in nearly every celestial object.
But just as NGC1052-DF2 is no ordinary galaxy, the scientists in van Dokkum’s group are no ordinary astronomers. Instead of using typical refractory telescopes to see the sky, his team instead chose to tie together several telephoto lenses, like the ones used in sports photography, to discover new objects that may be harder to see using traditional methods. Van Dokkum and other researchers then surveyed the night sky from an observatory in New Mexico.
It was there that his research assistant Shany Danieli GRD ’20 spotted a faint, sparse smudge of a star cluster that seemed to move more slowly than other galaxies: NGC1052-DF2.
Later that year, van Dokkum mentioned Danieli’s discovery to San Jose State University professor Aaron Romanowsky over a conference dinner in Baltimore. Romanowsky happened to have pictures of the galaxy on his laptop, and after agreeing with van Dokkum’s group’s conclusions, he joined the team.
“That’s when we said we could either ignore this because it doesn’t fit with our expectations or go out on a limb and publish it,” van Dokkum said.
The group confirmed the discovery by viewing the same object through the Keck Observatory telescopes in Hawaii and the Hubble Space Telescope. They attributed the galaxy’s odd properties to a lack of dark matter and published the study in Nature last year. The article is now the third most popular out of the 1,036 articles of a similar age in the entire journal. Still, their results were met with responses that put their calculations into question, according to van Dokkum.
“We got a lot of interest from our colleagues who wanted to make sure that we had done things right,” he added.
In a new study published on March 27 in The Astrophysical Journal Letters, second author van Dokkum reconfirmed the discovery. For that article, the researchers traveled back to Keck Observatory and observed the galaxy’s sluggishness once again.
While at Keck, not only did van Dokkum’s team validate their findings, but also they found a second galaxy that seemed to exhibit the same behavior as the first. According to another study published by van Dokkum in the same journal a week earlier, the new galaxy — NGC1052-DF4 — is almost identical to the first in terms of size, velocity and brightness. Van Dokkum, the first author of this study, said that these two studies confirm the existence of dark matter.
“Paradoxically, the fact that we see two galaxies that have little to no dark matter actually shows that dark matter is a real thing — that some galaxies have it and some don’t,” he said. “This shows that it’s not a manifestation of normal matter but is actually something separable from normal matter.”
Since the velocity of a star is directly correlated with how much mass a star’s galaxy has, most celestial objects have unnaturally fast stars that, according to Newton’s laws of motion, should not be moving that fast given the comparatively low amounts of visible matter. To account for the missing matter, scientists hypothesize that invisible dark matter is present.
Van Dokkum said NGC1052-DF2 and its counterpart NGC1052-DF4 are special because their stars move slowly enough that no additional matter is needed to explain its speed.
“In these two galaxies that we found, the motions are exactly what we would expect if there was no dark matter at all,” he said.
Van Dokkum said there is still a chance that both objects were slanted in a way that prevents accurately measuring their motion. But now that NGC1052-DF4 has been discovered, this worry is fleeting.
“The probability that we’re looking at two such things exactly from that one angle where you wouldn’t see the motion is vanishingly small,” he said.
Romanowsky said the next step for him and his fellow researchers is to determine the galaxies’ distances from Earth using the Hubble Space Telescope. The team also plans to survey more of the night sky to hopefully find other examples of dark matter-free galaxies.
For now, Romanowsky said that the team’s discoveries are worth celebrating.
“What makes working in astronomy fun is that the Universe is constantly full of surprises,” he said.
Matt Kristoffersen | firstname.lastname@example.org