A Study of the Dark Core in A520 with Hubble Space Telescope: The Mystery Deepens

TL;DR

This study uses Hubble Space Telescope data to analyze the complex mass structure of galaxy cluster A520, confirming a significant dark core coincident with X-ray gas but lacking stellar luminosity, raising questions about cluster merger models.

Contribution

The paper provides a detailed weak-lensing analysis of A520 with higher source galaxy density, confirming the dark core with over 10 sigma significance and revealing complex filamentary structures.

Findings

Detection of the dark core with >10 sigma significance.

Identification of a 1.5 Mpc filamentary structure.

Discrepancy with typical merging cluster models.

Abstract

We present a Hubble Space Telescope/Wide Field Planetary Camera 2 weak-lensing study of A520, where a previous analysis of ground-based data suggested the presence of a dark mass concentration. We map the complex mass structure in much greater detail leveraging more than a factor of three increase in the number density of source galaxies available for lensing analysis. The "dark core" that is coincident with the X-ray gas peak, but not with any stellar luminosity peak is now detected with more than 10 sigma significance. The ~1.5 Mpc filamentary structure elongated in the NE-SW direction is also clearly visible. Taken at face value, the comparison among the centroids of dark matter, intracluster medium, and galaxy luminosity is at odds with what has been observed in other merging clusters with a similar geometric configuration. To date, the most remarkable counter-example might be the Bullet Cluster, which shows a distinct bow-shock feature as in A520, but no significant weak-lensing mass concentration around the X-ray gas. With the most up-to-date data, we consider several possible explanations that might lead to the detection of this peculiar feature in A520. However, we conclude that none of these scenarios can be singled out yet as the definite explanation for this puzzle.