Frontier Fields Clusters: Chandra and JVLA View of the Pre-Merging Cluster MACS J0416.1-2403

TL;DR

This study provides a detailed multiwavelength analysis of the pre-merging galaxy cluster MACS J0416.1-2403, revealing its complex structure, shock features, and an unusual radio halo, offering insights into early merger stages.

Contribution

It presents the first comprehensive multiwavelength characterization of MACS J0416.1-2403, highlighting its pre-merging state and the presence of a potentially new type of radio halo.

Findings

The NE subcluster has a compact core with an X-ray cavity but is not a cool core.

A density discontinuity suggests interaction with a less massive structure.

The cluster hosts a radio halo with lower-than-expected power, possibly a nascent or ultra-steep spectrum halo.

Abstract

Merging galaxy clusters leave long-lasting signatures on the baryonic and non-baryonic cluster constituents, including shock fronts, cold fronts, X-ray substructure, radio halos, and offsets between the dark matter and the gas components. Using observations from Chandra, the Jansky Very Large Array, the Giant Metrewave Radio Telescope, and the Hubble Space Telescope, we present a multiwavelength analysis of the merging Frontier Fields cluster MACS J0416.1-2403 (z=0.396), which consists of a NE and a SW subclusters whose cores are separated on the sky by ~250 kpc. We find that the NE subcluster has a compact core and hosts an X-ray cavity, yet it is not a cool core. Approximately 450 kpc south-south west of the SW subcluster, we detect a density discontinuity that corresponds to a compression factor of ~1.5. The discontinuity was most likely caused by the interaction of the SW subcluster with a less massive structure detected in the lensing maps SW of the subcluster's center. For both the NE and the SW subclusters, the dark matter and the gas components are well-aligned, suggesting that MACS J0416.1-2403 is a pre-merging system. The cluster also hosts a radio halo, which is unusual for a pre-merging system. The halo has a 1.4 GHz power of (1.06 +/- 0.09) x 10^{24} W Hz^{-1}, which is somewhat lower than expected based on the X-ray luminosity of the cluster. We suggest that we are either witnessing the birth of a radio halo, or have discovered a rare ultra-steep spectrum halo.