Astronomers find a galaxy that had its dark matter siphoned away

The galaxy NGC 1052-DF4 surprised scientists by having almost no dark matter to complement its stellar population. Recently a team of astronomers has provided an explanation: a nearby galaxy has stripped NGC 1052-DF4 of its dark matter, and is currently in the process of destroying the rest of it too.

Despite its complete invisibility, dark matter is the fundamental building block of just about everything big in the universe. When you look at a galaxy, you see all the stars and blobs of glowing gas, but all that visible matter makes up less than 15% of the total mass of the galaxy. The rest is distributed in a large “halo” of dark matter. We don’t know what the dark matter is, but it’s a big deal.

This had been the normal pattern until the discovery of the galaxy NGC 1052-DF4 in 2015. That galaxy, sitting about 45 million light-years away, has almost no dark matter.

This was a big puzzle, because galaxies need halos of dark matter to even begin forming, so how could all those stars and gas bind together without a dark side?

A team of astronomers have used a combination of ground-based observations and the Hubble Space Telescope to examine NGC 1052-DF4 in more detail, especially looking at globular clusters. Globular clusters are small, dense clumps of that orbit a galaxy (the Milky Way has over 150 of them).

The astronomers found that the globular clusters orbiting NGC 1052-DF4 weren’t in all sorts of random orbits, but tended to be aligned with each other. Additionally, closer examination of the stars in NGC 1052-DF4 revealed the presence of tidal tails, which are long streams of stars extending along opposite directions of a galaxy.

Tidal tails only appear when a galaxy gets too close to a neighbor, and the globular clusters showed who was responsible: NGC 1035.

It looks like NGC 1052-DF4 got into a tussle with NGC 1035, and NGC 1035 won. Through the interaction, NGC 1035 stripped away the loose dark matter of the halo, leaving behind only the dense core of stars in NGC 1052-DF4.

But that too is on the way out. NGC 1035 isn’t done, and will soon rip apart the entire galaxy. We just happened to catch it in the middle of the act.