Spectroscopic galaxy redshifts in the Peanut cluster - a massive nearly head-on cluster merger shortly after pericenter passage

TL;DR

This study presents spectroscopic redshift measurements of galaxies in the Peanut cluster, analyzing its structure and mass to determine if it is a single massive cluster or a merging system, revealing its extreme nature.

Contribution

First spectroscopic redshift survey of the Peanut cluster, providing insights into its dynamical state and mass, and assessing evidence for a merger or single massive cluster.

Findings

Velocity distribution suggests possible subclusters but remains ambiguous.

Estimated cluster mass is approximately 2 x 10^15 solar masses.

The cluster is comparable to extreme systems like the Bullet cluster.

Abstract

The Peanut cluster (SRGe J023820.8+200556, SRGe CL0238.3+2005, $z_{spec}$ = 0.42) has recently emerged as a candidate for a rare, massive merger, potentially analogous to the Bullet cluster. We present the results of optical identification and spectroscopic redshift measurements for 31 galaxies in the Peanut cluster, including 26 new redshifts obtained with the 6-m telescope BTA (Big Telescope Alt-azimuthal) at SAO RAS between October 2024 and January 2025. The derived distribution of line-of-sight velocities reveals the possible presence of two subclusters with a line-of-sight velocity difference of ~2000 km/s. However, statistical tests and the Dressler-Schectman test show that the hypothesis that the observed velocity distribution can be described by a normal distribution for a single cluster cannot be ruled out, and the evidence for the existence of two gravitationally bound substructures remains ambiguous. Assuming a single cluster with the normal velocity distribution, the estimated galaxy velocity dispersion is $\sigma_{los} = 1455 \pm 83$ km/s, corresponding to the total cluster mass of $M_{200} = 2 \times 10^{15} M_\odot$ based on the mass-velocity dispersion scaling relation. In either scenario -- a single extremely massive cluster or an ongoing merger -- the Peanut cluster appears to be a very rare and peculiar object, comparable to such extreme systems as the Bullet cluster (1E 0657-56) or El Gordo (ACT-CL J0102-4915).