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Bullet Galaxy

The Bullet Galaxy (RXC J2359.3-6042 CC) is a galaxy in the galaxy cluster RXC J2359.3-6042 (Abell 4067 or ACO 4067). The Bullet Galaxy is the sole component of one half of a cluster merger between the bulk of the cluster and this galaxy, which is plowing through the cluster, similar to how merging clusters Bullet Cluster and Bullet Group have merged. Unlike the previously mentioned mergers, the Bullet Galaxy's isn't even because it's between one galaxy and a galaxy cluster. The cluster merger is happening at lower speed than the Bullet Cluster, thus allowing the core of the Bullet Galaxy to retain cool gas and remain relatively undisturbed by its passage through the larger cluster. This cluster merger is the first one observed between a single galaxy and a cluster. The galaxy and cluster lies at redshift z=0.0992, some 1.4×10⁹ ly (4.3×10⁸ pc) away. The galaxy is traveling through the cluster at a speed of 1,310 km/s (2,900,000 mph).

By studying this unique merging, researchers can gain insight on dark matter, and how it interacts with other objects in space. According to astrophysicists James Bullock, "Galaxy clusters that are merging with each other represent interesting laboratories for this kind of question,” when he was speaking of dark matter and the Bullet cluster.

Bullet Cluster

Bullet Cluster

One of the biggest successes appeared to be a collision of galaxy clusters called the Bullet Cluster. It provided one of the most spectacular and intuitive indications that seemed to show that dark matter was real. Our own report on the first evidence of the Bullet Cluster, written more than a decade ago, was pretty excited. And in the stories that followed about the existence of dark matter, we've tended to treat the Bullet Cluster as a gold standard. If you can't explain the Bullet Cluster, then your theory is probably a bit useless really.

The image above shows the remnant of two galaxy clusters that have collided, with a smaller "bullet" that has passed through the larger cluster. The energy of the collision is such that regular matter has been heated to very high temperatures, causing it to glow like crazy in the X-ray regime (which is shown in red). So, an X-ray telescope can produce a clear image of the matter distribution of both the bullet and the larger cluster. Even better, this collision appears to be almost side-on to us, so we have the best seat in the house to observe it.

In addition, both clusters have significant mass and act like gravitational lenses. By imaging objects that are behind the clusters and understanding how the images are distorted by the intervening lens, we can map out the Bullet Cluster's mass. This is shown in blue.

Overlaying the two images shows that the mass is not where the matter is—hence, dark matter. This is only one of several collisions between clusters that show similar features—gravity without apparent matter—but the Bullet Cluster is, without doubt, the cleanest example of them all.

However, the Bullet Cluster shows something that is, arguably, more important: science works. Although the initial publication was touted as evidence for dark matter, it was quickly realized that the story may be more complicated than that. In fact, the story even started to shade toward the Bullet Cluster being evidence against dark matter. Theoretical physicists let their imaginations loose, bringing dark energy and modified theories of gravity to the table. But eventually, as the dust settled, thinking came back around to the original interpretation being correct.

Looking back at the Bullet Cluster today—how we got from here to there and back again—highlights how science works in that same clean manner. Data is king, but theory is the kingdom; you need both, and neither is set in stone