Joint JWST-DECam Lensing Reveals That the Bullet Cluster Is a Minor Merger

Abstract

We present the first robust virial masses of the Bullet Cluster's three individual components from a joint weak+strong lensing analysis combining JWST/NIRCam and DECam observations. Despite its status as the benchmark system for dark matter and merger studies, inferred mass ratios for the Bullet Cluster have spanned a wide range from $\sim$2:1 to $\gtrsim$10:1 over more than two decades. We revisit this tension through three key advances: (1) JWST's exceptional data quality enables us to resolve three distinct halos, (2) DECam's wide-field coverage beyond its virial radius eliminates the need for extrapolation, and (3) high-fidelity strong-lensing priors mitigate weak-lensing model bias. We obtain $M_{200c}=15.11_{-2.10}^{+2.48}\times 10^{14}{M}_{\odot }$ for the main cluster and $1.49_{-0.25}^{+0.32}\times 10^{14}{M}_{\odot }$ for the subcluster, yielding a mass ratio of $10.14_{-2.47}^{+3.22}$, definitively classifying the Bullet Cluster as a minor merger. This result reconciles the long-standing tension in the mass ratio and provides updated initial parameters for future modeling of this iconic system.

21 pages, 12 figures, 3 tables, accepted for publication in ApJ