A Massive Merging Galaxy Cluster
The collision of two clusters of galaxies as seen at multiple wavelengths (low energy X-ray in blue, high energy X-ray in red, and optical light in yellow). Astronomers were able to use X-ray spectroscopy to re-measure the redshift distance to a colliding cluster and found it to be much farther away than previously thought, resolving several puzzles.
Mergers of galaxy clusters are the most extreme events knwon in the universe, releasing the energy equivalent of up to a million galaxies shining for a million years. The shocks and turbulence generated during these mergers heat the gas between clusters (the intracluster medium) and accelerate charged particles to speeds close to that of light. The radiation emitted by these particles as they gyrate around intergalactic magnetic field lines produces radio wavelength structures seen as giant halos or diffuse arcs (called "radio relics"). Merging galaxy clusters and their radio structures are thus unique places to study particle acceleration under extreme conditions.
Among the set of known, large, merging galaxy clusters, those that host double relics belong to a rare subclass that usually has a simple merging geometry. ClG 0217+70 is one such multi-relic cluster merger with at least four relics that are not associated with any optical galaxy and are indicative of powerful shocks. The problem has been that this cluster is not very luminous, and so is not expected to host such powerful emissions. CfA astronomer Ralph Kraft is a member of an international team that used a highly ionized iron emission line in the X-rays to re-measure the distance to the cluster through its redshift. They report that the distance to the galaxy is actually about three times farther (about three billion light-years) than had been previously obtained from observations of optical spectral features. This implies that the sizes of the structures and their emitted energies are all considerably larger than previously thought; one of the radio arcs, for example, is actually over two thousand light-years long. Indeed, the new distance measurement means that this cluster is one of the largest ever found, with a mass of over a million billion solar-masses and extreme, shock-induced density discontinuities. The authors speculate that the merger might be the result of a slightly off-axis collision in which the matter (including the dark matter component) has begun to fall back together, producing a new set of shocks. The paper observes that the ability to use X-ray spectroscopy to measure galaxy redshifts is a powerful new technique that will become increasingly important in upcoming X-ray missions.
Reference(s):
"ClG 0217+70: A Massive Merging Galaxy Cluster with a Large Radio Halo and Relics," X. Zhang, A. Simionescu, J. S. Kaastra, H. Akamatsu, D. N. Hoang, C. Stuardi, R. J. van Weeren, L. Rudnick, R. P. Kraft, and S. Brown, Astronomy & Astrophysics 642, L3, 2020.