Most galaxies are found by their light — their stars, their gas, their dust glowing across the electromagnetic spectrum. CDG-2 was found by its absence. Located 300 million light-years away in the Perseus cluster, it shines with the light of only about one million Suns, dimmer than most individual star clusters. Ninety-nine percent of its mass appears to be dark matter.
Published in The Astrophysical Journal Letters, Li and colleagues at the University of Toronto identified CDG-2 not by looking for faint starlight but by searching for tight groupings of globular clusters — dense, spherical collections of old stars that typically orbit galaxies. They found four globular clusters suspiciously close together, then used Hubble to confirm that an ultra-faint galaxy connected them. The Milky Way has over 150 globular clusters. CDG-2 has four. Its hydrogen gas — the fuel for star formation — was likely stripped away by gravitational interactions with other galaxies inside the Perseus cluster.
The structural insight is about detection by proxy. The galaxy itself is nearly invisible. The globular clusters are visible. The spatial correlation between four otherwise unremarkable star clusters pointed to the existence of something that cannot be directly seen. The method inverts the usual relationship between a galaxy and its satellites: normally, we find the galaxy first and its clusters second. Here, the clusters are the evidence and the galaxy is the inference. CDG-2 is a gravitational structure that has been stripped of almost everything that makes galaxies detectable, except four gravitationally bound star clusters that refuse to disperse. The ghost is real because its ornaments are still hanging.