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

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 $σ_{los} = 1455 \pm 83$ 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).

Figures (10)

• Figure 1: Color–magnitude diagram for galaxies in the Peanut cluster field. Brown circles denote galaxies from Peanut, while blue circles indicate galaxies without measured spectroscopic redshifts ($z_{\rm spec}$) at the start of this work. The red dashed line shows the best-fitting relation between the $r-z$ color and the $r$-band magnitude for the galaxies from Peanut. The red shaded area illustrates a region for target selection for subsequent spectroscopic observations with BTA.
• Figure 2: Galaxies in the Peanut cluster field on a $z$-band image from the DESI LIS survey. Red circles mark the five galaxies with spectroscopic redshifts reported in Peanut. Blue circles indicate 115 galaxies brighter than $r < 22$ within 2$^{\prime}$ of the X-ray source center that are possible cluster members.
• Figure 3: Finding charts graphically showing the outlines of the spectrograph slits with their labels and position angles indicated. The images are taken from the DESI LIS survey in $z$-band.
• Figure 4: Examples of galaxy spectra. Left: observed cluster galaxy spectra (blue) with selected spectral features indicated, together with template spectra shifted to the corresponding galaxy redshifts (red). Right: $\chi^2$ distributions obtained from comparison of the observed spectra with the templates.
• Figure 5: Measured redshifts of galaxies in the Peanut cluster. The left panel shows a DESI LIS $z$-band image. Galaxies for which we measured spectroscopic redshifts are marked in blue, with the corresponding redshift values indicated. The galaxy shown in red has a spectroscopic redshift measured by the DESI survey. The right panel shows the projected positions of the cluster galaxies; circle colors and sizes correspond to the measured redshifts and $r$-band magnitudes, respectively.