Astronomers find 2nd galaxy lacking dark matter (earthsky.org)

By Deborah Byrd in Space | April 2, 2019

A year ago, astronomers announced they’d found the first known galaxy without dark matter. Called NGC 1052-DF2 – or just DF2 for short – this object is 6.5 million light-years away and roughly the same size as our Milky Way galaxy, but with 200 times fewer stars. Now the same team of astronomers is back with what they say is “stronger evidence” about DF2’s bizarre nature. Plus the team says it has found a second galaxy without dark matter. The astronomers have published their studies in two separate papers in the peer-reviewed Astrophysical Journal Letters. One study was published in the March 20, 2019, issue, and the other was published on March 27.

Shany Danieli, an astronomy graduate student at Yale University and lead author of one of the studies, commented in a statement:

When astronomers think about dark matter, the observed motions of space objects often come to mind. In the 1970s, Vera Rubin and Kent Ford of the Carnegie Institution of Washington realized that stars at the outskirts of the large spiral galaxy next to ours, the Andromeda galaxy, were moving just as fast as the stars near the center. This observation apparently violated Newton’s Laws of Motion, which explain, for example, why Mars moves more slowly in orbit than Earth (because it is farther from the sun). And so the idea was born that “something” unseen must exist beyond the visible boundaries of the Andromeda galaxy, and, by extension, all galaxies. That something is what we call dark matter.

The Yale astronomers are studying the motions of space objects, too. In the first of the two new studies, they used the W. M. Keck Observatory’s Keck Cosmic Web Imager (KCWI) to gather high-precision measurements of globular star clusters inside the galaxy DF2. They said they found that – unlike the stars in Rubin and Ford’s 1970s study – these clusters are moving at a speed consistent with the mass of the galaxy’s normal matter. Their statement explained:
*** more: https://earthsky.org/space/galaxies-lacking-dark-matter-df2-udg