The First Galaxy Without Dark Matter Is About To Be Torn Apart
A cosmic puzzle is finally solved, as new observations answer the question of why this galaxy exists at all.
For the last couple of years, astronomy has had a tremendous puzzle to reckon with. When you look at all the large-scale structures out there in the Universe — large galaxies, galaxy groups and clusters, the vast cosmic web and even the all-sky radiation left over from the Big Bang — the same universal picture emerges. In addition to all the normal matter made of Standard Model particles in all their forms, an additional source of invisible mass is required: dark matter. Everywhere we look, on all these large scales, the same 5-to-1 ratio of dark-to-normal matter adequately explains every one of our observations.
But on small scales, the story should be quite different. All the different forces and effects should create two populations of small galaxies: ones with enormous amounts of dark matter relative to their normal matter, which should persist for long periods of time, and ones with very little relative dark matter, which should be destroyed on short cosmic timescales. Yet one galaxy, NGC 1052-DF4 (called DF4 for short), has complicated matters tremendously, as it appears to have no dark matter but hasn’t formed new stars in some 7 billion years. In a brilliant new study led by Mireia Montes, that mystery has at last been solved, as an otherwise common galaxy is in the last stages of being ripped apart. Here’s the science of how we figured it out.
The theory. In theory, dark matter and normal matter both permeate the Universe, but respond differently from one another. If you have a gravitational field, such as a region where the density of matter is greater than the surrounding regions, both normal and dark matter will experience equal attractive forces. But normal matter will:
- collide, clump, and bind together,
- experience inelastic collisions,
- shed both linear momentum and angular momentum,
- and can be pushed around by…