A tightly bunched collection of galaxies has been observed entering a galactic cluster at a speed of more than 4.7 million kilometers per hour (2.9 million miles/hour). Only the second time such an event has been observed, astronomers hope it will prove as rich a guide to dark matter as the first.
The small group of galaxies is passing through the middle of RXCJ2359.3-6042, a galaxy cluster also known as Abell 4067, which is located 1.4 billion light-years away. Dr. Gayoung Chon of the Max Plank Institute for Extraterrestrial Physics observed the galactic collision with the X-ray Multi-Mirror Newton space telescope. His paper, to be published in Astronomy and Astrophysics, is available in draft form on arXiv.
The sheer scale of the interactions occurring here are mind-bending. Chon estimates Abell 4067 to be 2.1 x 1014 times the mass of the sun, or almost a 100 times that of the local group in which the Milky Way resides.
As the paper notes, “Merging galaxy clusters have been found to be interesting laboratories for the study of a variety of astrophysical processes.” Such mergers occur in many ways depending on the mass ratios and shapes of the galaxies involved. A study of 108 clusters revealed what the paper describes as “a special merger configuration that has never been observed in this form before.”
The incoming galaxies are described as a “compact bullet” because of their density and speed. Meanwhile, Abell 4067 is a diffuse cluster without much density at its core. The only comparable known case is 1E 0657-558, also known as the Bullet Cluster. 1E 0657-558 has been a hugely valuable find for astronomers, providing confirmation for the theorized, but previously unwitnessed, existence of dark matter. Moreover, observations from it support suggestions that dark matter lies in a ring around the visible parts of galaxies.
However, while the relative speeds observed in Abell 4067 are enormous, they are low compared to those seen in the Bullet Cluster. As a consequence, Chon has been able to detect a clear trail of debris, which might have been disturbed by a more violent encounter.
The outer layers of the impacting galaxies are being stripped away by the gravity of Abell 4067, and the way this is happening could tell us a lot about the distribution of dark matter in the two objects. It appears that Abell 4067 has been relatively inactive for quite a while, further simplifying the interactions between it and the incoming group of galaxies.
Chon believes that further study of the interaction will allow us to estimate the mass of the colliding set of galaxies, as well as how much gas has been left in a trail behind the smaller bunch. More generally, extra telescope time trained on Abell 4067 could tell us a lot about the way groups of galaxies interact and provide more detail on how dark matter is distributed within them.