A Very Rich Bimodal Galaxy Cluster Merger: RXC J0032.1+1808

TL;DR

RXC J0032.1+1808 is analyzed as a very rich bimodal galaxy cluster merger using archival optical, lensing, X-ray, and radio data plus a new Keck/DEIMOS redshift survey. The system exhibits two X-ray peaks separated by $1.27'\,(\sim397\text{ kpc})$ along the north-south axis, with a near-equal mass distribution ($M_{north}/M_{south}=1.06\pm0.07$) and global properties of $T_X=8.5^{+1.1}_{-0.9}$ keV and $L_X=(1.04\pm0.03)\times10^{45}$ erg s$^{-1}$ in the $0.5-10.0$ keV band; a radio relic on the northern outskirts supports a post-pericenter scenario and non-head-on geometry. The Keck/DEIMOS survey of 43 secure member redshifts yields a line-of-sight velocity offset between subclusters of $v_{los}=76\pm364$ km s$^{-1}$, implying a small LOS component. Hydrodynamic simulations are used to demonstrate that the merger is not head-on, that it occurred $\approx 395$–$560$ Myr after pericenter, and that the viewing angle foreshortens the projected subcluster separation by a factor of about $\sim2$, providing a coherent dynamical picture consistent with the observed morphology, temperature structure, and relic location.

Abstract

The galaxy cluster RXC J0032.1+1808 has been well-studied with optical imaging and gravitational lensing mass maps, both of which reveal an elongated morphology in the north-south direction. We find that its X-ray morphology is bimodal, suggesting that it is in the process of merging; combined with a previously reported detection of a radio relic, we suggest that the system is seen after first pericenter. We extract the global X-ray temperature and unabsorbed luminosity from archival XMM-Newton data, finding $T_X=8.5^{+1.1}_{-0.9}$ keV and $L_X=1.04 \pm 0.03 \times 10^{45}$ erg s$^{-1}$ at 90\% confidence in the $0.5$--$10.0$ keV energy range. We conduct a redshift survey of member galaxies and find that the line-of-sight relative velocity between the two subclusters is $76\pm364$ km/s. We use publicly available hydrodynamic simulations to show that it cannot be a head-on merger, that it is observed ${\approx}395$--560 Myr after pericenter, and that the viewing angle must be one that foreshortens the apparent subcluster separation by a factor ${\approx}2$.

Figures (2)

• Figure 1: True-color HST/ACS image using the F435W, F606W, and F814W filters, with linearly spaced, point source subtracted X-ray surface brightness contours (red) and RELICS/GLAFIC convergence contours (white). Numbers denote redMaPPer BCG candidates. The dashed line indicates the boundary between north and south subclusters used in §\ref{['sec-z']}.
• Figure 2: Linearly spaced LoTSS (green) and XMM-Newton contours (red) over a true-color $griz$ image from the DESI Legacy Survey. The radio relic likely traces a shock, suggesting that the two subclusters have already experienced a pericenter passage. This field is larger than that of Figure \ref{['fig-relicsACS']}, illustrating that the relic is rather far from the system center compared to the subclusters. The dashed line is the same as in Figure \ref{['fig-relicsACS']}. The artifact at lower right is from a 7th magnitude star outside the region shown.