Properties of the cosmological filament between two clusters: A possible detection of a large-scale accretion shock by Suzaku

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
e-mail: h.akamatsu@sron.nl
² Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, 565-0871 Osaka, Japan
³ Graduate School of Science and Engineering, Kagoshima University, 1-21-35, Korimoto, Kagoshima, 890-0065 Kagoshima, Japan
⁴ Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, 192-0397 Tokyo, Japan
⁵ Research Center for Measurement in Advanced Science, Faculty of Science, Rikkyo University 3-34-1, Nishi-Ikebukuro, Toshima-ku, 171-8501 Tokyo, Japan
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Center for Computational Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8571 Ibaraki, Japan
⁸ Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjyuku-ku, Tokyo 162-8601, Japan

Received: 25 January 2017
Accepted: 18 April 2017

Abstract

We report on the results of a Suzaku observation of the plasma in the filament located between the two massive clusters of galaxies, Abell 399 and Abell 401. Abell 399 (z = 0.0724) and Abell 401 (z = 0.0737) are expected to be in the initial phase of a cluster merger. In the region between the two clusters, we find a clear enhancement in the temperature of the filament plasma from 4 keV (expected value of a typical cluster temperature profile) to kT ~ 6.5 keV. Our analysis also shows that filament plasma is present out to a radial distance of 15′ (1.3 Mpc) from a line connecting the two clusters. The temperature profile is characterized by an almost flat radial shape with kT ~ 6–7 keV within 10′ or ~0.8 Mpc. Across r = 8′ from the axis, the temperature of the filament plasma shows a drop from 6.3 keV to 5.1 keV, indicating the presence of a shock front. The Mach number based on the temperature drop is estimated to be ℳ ~ 1.3. We also successfully determined the abundance profile up to 15′ (1.3 Mpc), showing an almost constant value (Z = 0.3 solar) at the cluster outskirts. We estimated the Compton y parameter to be ~ 14.5 ± 1.3 × 10^-6, which is in agreement with the Planck results (14–17 × 10^-6 on the filament). The line-of-sight depth of the filament is l ~ 1.1 Mpc, which indicates that the geometry of filament is likely a pancake shape rather than cylindrical. The total mass of the filamentary structure is ~7.7 × 10¹³M_⊙. We discuss a possible interpretation of the drop of X-ray emission at the rim of the filament, which was pushed out by the merging activity and formed by the accretion flow induced by the gravitational force of the filament.