Bulk and turbulent gas motions in the interacting galaxy cluster Abell 3395 South observed with XRISM

Abstract

We investigate the gas motions in the core region of the Abell~3395 South subcluster (A3395S) using high-resolution X-ray spectroscopy with XRISM/Resolve. By analyzing the Fe~XXV He$α$ emission line, we directly measure the line-of-sight bulk and turbulent velocities of the intracluster medium. We find that the one-dimensional turbulent velocity is low, at the level of $124\pm21~{\rm km\,s^{-1}}$, while a significant line-of-sight bulk velocity of $263\pm23~{\rm km\,s^{-1}}$ is detected. The coexistence of low turbulence and finite bulk motion suggests that A3395S has not yet reached a dynamically relaxed state. These results are consistent with the non-detection of a radio halo in A3395S, implying that turbulent particle reacceleration is currently inefficient in the cluster core. This study demonstrates that high-resolution X-ray spectroscopy with XRISM provides a powerful means to directly constrain intracluster medium dynamics in merging galaxy clusters, and it provides a reference for future comparative studies of A3395N and A3391 within the same large-scale structure.

Figures (3)

• Figure 1: (a) eROSITA X-ray image of the Abell 3391/95 system in the 0.3--2.0 keV band, particle background-subtracted, corrected for Galactic absorption, exposure-corrected, and smoothed with a 32${}^{\prime\prime}$ Gaussian kernel. The XRISM/Resolve field of view is overlaid as a green box. A single ASKAP/EMU radio contour at $0.0001~{\rm Jy\,beam^{-1}}$ is shown in black Brueggen21. Some of the radio structure around A3391 is due to imaging artefacts. The regions used for the optical galaxy redshift analysis in Section \ref{['subsec:optical_data']} are indicated: a yellow circle for A3391, and white and cyan semicircles for A3395N and A3395S, respectively. (b) XRISM/Resolve X-ray image of A3395S, with the same field of view outlined in green and ASKAP/EMU radio contours overlaid. The position of the brightest cluster galaxy (BCG) is marked with a cross. The radio contour levels in panel (b) are $0.0001, 0.001, 0.01, 0.05,$ and $0.1~{\rm Jy\,beam^{-1}}$. Alt text: Two-panel X-ray images illustrating the large-scale environment and the central region of the Abell 3391/95 system. Panel (a) shows the three clusters and the intercluster regions. Panel (b) focuses on the southern subcluster A3395S, where radio emission associated with the central radio galaxy traces extended jet structures.
• Figure 2: Redshift distributions of galaxies associated with (a) A3391, (b) A3395N, and (c) A3395S, compiled from NED. The dot-dashed vertical lines indicate the redshifts of the brightest cluster galaxies (BCGs), while the dashed and dotted vertical lines show the median redshift of each cluster and its standard deviation, respectively. Alt text: Three panels showing galaxy redshift distributions for A3391, A3395N, and A3395S. Each panel displays a distinct redshift peak corresponding to the associated cluster, with differing widths that reflect variations in the galaxy velocity dispersion among the three systems.
• Figure 3: (a) Resolve spectrum of A3395S in the 2--10 keV band fitted with the bapec model. (b) Zoom-in of the Fe xxv He$\alpha$ and Fe xxvi Ly$\alpha$ line complex. The black crosses represent the data points. In panel (a), the spectrum is grouped to achieve a statistical significance of 5$\sigma$ per bin for display purposes. In panel (b), the spectrum is grouped to a fixed bin width, with one bin consisting of eight original channels (4 eV per channel). The red solid line shows the total best-fit model, while the blue solid, cyan dashed, and magenta dotted curves indicate the ICM emission, the non--X-ray background (NXB), and the cosmic X-ray background (CXB) components, respectively. Alt text: Two-panel X-ray spectra of A3395S. Panel (a) presents the overall spectral shape in the hard X-ray band, while panel (b) focuses on the iron emission line complex, illustrating the detailed line structure used to constrain the kinematic properties of the intracluster medium.