Free Astronomy Magazine May-June 2018

an ultra-diffuse gal- axy. A 2015 survey of the Coma gal- axy cluster showed these large, faint objects to be sur- prisingly common. But none of the ultra-diffuse galax- ies discovered so far have been found to be lacking in dark matter. So even among this unusual class of galaxy, NGC 1052- DF2 is an oddball. Van Dokkum and his team spotted the galaxy with the Dragonfly Tele- photo Array, a cus- tom-built telescope in New Mexico they designed to find these ghostly galax- ies. They then used the W.M. Keck Ob- servatory in Hawaii to measure the mo- tions of 10 giant groupings of stars called globular clus- ters in the galaxy. Keck revealed that the globular clusters were moving at rel- atively low speeds, less than 23,000 miles per hour. Stars and clusters in the outskirts of galaxies containing dark matter move at least three times faster. From those measurements, the team calculated the galaxy’s mass. “If there is any dark matter at all, it’s very little,” van Dokkum explained. “The stars in the galaxy can account for all the mass, and there doesn’t seem to be any room for dark matter.” The researchers next used NASA’s SPACE CHRONICLES 53 MAY-JUNE 2018 a galaxy.” The unique galaxy, called NGC 1052-DF2, contains at most 1/400th the amount of dark matter that astronomers had expected. The galaxy is as large as our Milky Way, but it had escaped attention be- cause it contains only 1/200 th the number of stars. Given the object’s large size and faint appearance, as- tronomers classify NGC 1052-DF2 as Hubble Space Telescope and the Gemini Observatory in Hawaii to uncover more details about the unique galaxy. Gemini revealed that the galaxy does not show signs of an interac- tion with another galaxy. Hubble helped them better identify the globular clusters and measure an accurate distance to the galaxy. The Hubble images also revealed the galaxy’s unusual appearance. “I spent an hour just staring at the Hubble image,” van Dokkum re- called. “It’s so rare, particularly these days after so many years of Hubble, that you get an image of something and you say, ‘I’ve never seen that before.’ This thing is as- tonishing: a gigantic blob that you can look through. It’s so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy.” The ghostly galaxy doesn’t have a noticeable central region, or even spiral arms and a disk, typical fea- tures of a spiral galaxy. But it doesn’t look like an elliptical galaxy, either. The galaxy also shows no evidence that it houses a central black hole. Based on the colors of its globular clusters, the galaxy is about 10 bil- lion years old. Even the globular clusters are oddballs: they are twice as large as typical stellar groupings seen in other galaxies. “It’s like you take a galaxy and you only have the stellar halo and glob- ular clusters, and it somehow forgot to make everything else, ” van Dokkum said. “There is no theory that predicted these types of gal- axies. The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is com- pletely unknown.” But the researchers do have some ideas. NGC 1052-DF2 resides about 65 million light-years away in a col- lection of galaxies that is domi- N GC 1052-DF2 is a ghostly galaxy lacking dark mat- ter that resides about 65 million light-years away in the NGC 1052 Group. Hubble took this image on 16 November 2017 using its Advanced Camera for Surveys. [NASA, ESA, and P. van Dokkum (Yale University)]