Using the Keck Telescope to measure the motion and speed of DF4’s diffuse gas, as well as seven of its globular clusters, van Dokkum’s team calculated that DF4’s distance is similar to DF2’s — hovering around 65 million light-years away, give or take about 9 million light-years.
“We conclude that NGC 1052-DF2 is not an isolated case, but that a class of such objects exists,” van Dokkum’s team wrote in their DF4 discovery paper. “The origin of these large, faint galaxies with an excess of luminous globular clusters and an apparent lack of dark matter is, at present, not understood.”
But, yet again, Trujillo and his team calculated their own distance to DF4. Based on Hubble data available at the time, they identified what they think is DF4’s TRGB. This led them to conclude that DF4 is about 46 million light-years away, which would mean its globular clusters aren’t actually that strange, and instead are pretty similar to those found in the Milky Way and elsewhere.
“All in all,” Trujillo’s team concluded in their response paper, “the proposition that both NGC 1052-DF2 and NGC 1052-DF4 are ‘missing dark matter’ is still far from being placed on sure footing.”
Hubble takes another look
In the summer of 2019, in order to determine whether Trujillo’s team had identified DF4’s true TRGB, van Dokkum’s group used Hubble’s keen eye to collect new, deep images of DF4. On October 16, they posted another paper, which has been submitted to The Astrophysical Journal Letters, on the preprint site arXiv. Based on the fresh Hubble data, which picked up many more, much fainter stars, the paper claims the short-distance camp again misidentified DF4’s brightest red giant stars, leading to a closer derived distance.
“In the new data, there really is no ambiguity,” says study author Shany Danieli of Yale, who is van Dokkum’s colleague. “We think the new data really rule out the [shorter-distance] option.”