MACS J0025.4-1222

Lore & Background

MACS J0025.4-1222 was created when two galaxy clusters, each almost a quadrillion times the mass of the Sun, merged at speeds of millions of miles per hour. As they collided, the hot gas in each cluster interacted with the hot gas in the other and slowed down, while the dark matter, which interacts only weakly, did not. This resulted in a separation between the normal matter (shown in pink in the composite image) and the dark matter (shown in blue).

The composite image was made from separate exposures by the Hubble Space Telescope's ACS and WFPC2 detectors and the Chandra ACIS detector. Hubble's visible-light images showed gravitational lensing, which allowed astronomers to infer the distribution of total mass (both dark matter and normal matter), colored blue. Chandra data accurately mapped the distribution of normal matter, mostly hot gas glowing in X-rays, shown in pink. From these observations, it was determined that most of the mass in the two blue regions was dark matter.

The international team of astronomers was led by Marusa Bradac of the University of California, Santa Barbara, and Steve Allen of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and the Stanford Linear Accelerator Center (SLAC).

Reader's Guide

MACS J0025.4-1222 is significant because it provides independent, direct evidence for dark matter, similar to the earlier discovered Bullet Cluster. The clear separation between the intergalactic gas (normal matter) and the mass centroids of the colliding clusters supports the view that dark matter particles interact with each other only very weakly, almost entirely through gravity. By using gravitational lensing from Hubble to map total mass and X-ray data from Chandra to map hot gas, astronomers could directly observe that most of the mass in the blue regions was dark matter, which did not slow down during the collision like the normal gas did. This separation is a key observational signature that dark matter exists and behaves differently from ordinary matter, reinforcing the standard cosmological model where dark matter is collisionless. The study led by Bradac and Allen thus strengthens the case for dark matter as a fundamental component of the universe.

Did You Know?

• MACS J0025.4-1222 is part of the MAssive Cluster Survey (MACS).
• The two merging clusters each have a mass of almost a quadrillion times the mass of the Sun.
• The Hubble images used for the study were taken on November 5, 2006, and June 6, 2007.
• The separation between normal matter (pink) and dark matter (blue) provides direct evidence that dark matter particles interact with each other only very weakly.