Weak-Lensing Analysis of the Galaxy Cluster Abell 85: Constraints on the Merger Scenarios of Its Southern Subcluster

TL;DR

This study uses deep Subaru/HSC weak-lensing data to map the dark matter distribution in Abell 85 and its surroundings, resolving substructures associated with the main halo, the southern subcluster, and the southwestern subcluster. By fitting multi-halo NFW profiles to the reduced tangential shear, the authors find a major merger in A85 with a mass ratio of roughly 2:1 between the main cluster and the southern subcluster ($M_{200c, ext{A85}} \approx 2.91\times10^{14}\,M_\odot$, $M_{200c, ext{S}} \approx 1.23\times10^{14}\,M_\odot$), while constraining the SW subcluster ($M_{200c, ext{SW}} \approx 0.64\times10^{14}\,M_\odot$). The convergence map, validated against galaxy distributions and X-ray features, supports a complex assembly history with ongoing interaction, complemented by the detection of nearby background clusters (A87, A89) in the same field. The work demonstrates the power of high-resolution WL in dissecting cluster mergers, and provides quantitative constraints on merger phase and mass assembly in A85, with implications for galaxy evolution and intracluster medium dynamics along the filamentary environment.

Abstract

Abell 85 is a nearby (z=0.055) galaxy cluster that hosts a sloshing cool core, a feature commonly reported in relaxed clusters. However, the presence of multiple past and ongoing mergers indicates that it is an active node within the Abell 85/87/89 complex. We present a weak gravitational lensing (WL) analysis using Subaru Hyper Suprime-Cam imaging data to understand its assembly history by investigating the dark matter components of the substructures. Our mass reconstruction resolves three substructures associated with the brightest cluster galaxy (main), the southern (S) subcluster, and the southwestern (SW) subcluster, with WL peak significances of $> 6σ$, $> 5σ$, and $> 4σ$, respectively. The locations of these mass peaks are consistent with those of the member galaxies. We estimate the masses of the main cluster ($M_{200c,main} = 2.91 \pm 0.72 \times 10^{14}\ M_\odot$) and the S subcluster ($M_{200c,S} = 1.23 \pm 0.52 \times 10^{14}\ M_\odot$) by fitting a multi-halo Navarro-Frenk-White profile. This $\sim$2:1 mass ratio indicates that the system is undergoing a major merger that is actively shaping the current dynamical state of Abell 85. Incorporating X-ray observations, we discuss the merger phase of the S subcluster and further examine the star-forming activity along the putative filament extending southeast of Abell 85.

Figures (7)

• Figure 1: Pseudo-color composite image of A85, overlaid with the XMM- Newton X-ray relative deviation surface brightness map in magenta and the MGCLS 1.28 GHz radio emission map in green. The background pseudo-color composite image is constructed using the $g$-, $g$+$i$-, and $i$-bands mapped to the blue, green, and red color channels, respectively. The X-ray relative deviation map is produced by dividing the X-ray surface brightness map with a 2D elliptical $\beta$-model centered on the BCG. The scale bar represents approximately 15 arcmin which corresponds to 1 Mpc at the cluster redshift. The BCG, as well as the southwestern (SW) and southern (S) subclusters identified by the X-ray observations, are annotated.
• Figure 2: PSF model correction for $i$-band. The distributions of stellar ellipticities ($e_1$–$e_2$) before and after the PSF correction are shown in black and red dots, respectively. We denote the mean and standard deviation of the ellipticity components. In addition, the distribution of the residual size $R_{res}$ is shown along with its corresponding mean and standard deviation. After applying the PSF correction, both the scatter in the stellar ellipticity and the size are significantly reduced and centered at zero.
• Figure 3: Remaining systematic diagnosis in the PSF modeling using the $\rho$ statistics. The $\rho_1(r)$ (black) and $\rho_2(r)$ (blue) correlation functions are plotted as a function of the angular separation in arcmin for the $i$-band. The amplitudes of the correlation functions remain at the level of $10^{-6}$, indicating that the residual systematics in the PSF modeling are negligible.
• Figure 4: (Top) The $i$-band magnitude distributions of all detected galaxies in the COSMOS field (gray) and in the A85 field (green). The $i$-band magnitude cut at 23 mag for source selection is determined by comparing the two distributions, indicated by the leftmost orange dashed line. (Bottom) The color–magnitude diagram of the objects in the A85 field, using $i$-band magnitude and $g-i$ color. The background density map represents the number density of all detected sources. The BCG and spectroscopically confirmed cluster members are marked in cyan and red, respectively. The red sequence, derived via iterative linear regression of the cluster members, is shown as a black dashed line. The $i$-band magnitude cuts are shown as the orange dashed lines. The selected source galaxies are shown as magenta dots.
• Figure 5: Comparison between the WL convergence signal-to-noise (S/N) map and the relative deviation X-ray surface brightness map. The background shows the pseudo-color composite image of A85. The yellow contours represent the WL S/N map, ranging from 3$\sigma$ to 6$\sigma$ in 1$\sigma$ intervals. The red contours indicate the relative deviation X-ray emission map, obtained by dividing a 2D elliptical $\beta$-model centered on the BCG. The red dashed line shows the XMM-Newton observation footprint. The scale bar in the top right denotes about 15 arcmin, corresponds to 1 Mpc scale at the cluster redshift. The X-ray emission peaks and the WL peaks are well matched with the positions of BCG, S, and SW subclusters.