XRISM-Subaru views of Abell 754: Energetic ICM motions revealed by XRISM/Resolve

We present high-resolution X-ray spectroscopy of the merging cluster Abell 754 using XRISM/Resolve. In GO1 phase, XRISM/Resolve observed Abell 754 in two deep pointings, targeting the eastern primary core (114 ks) and the middle of the X-ray filamentary structure (190 ks). Spectral fits to full field-of-view data reveal a line-of-sight velocity difference of 656 ± 35 km s⁻¹ between the two pointing, corresponding to a bulk Mach number of 0.45±0.03. Velocity dispersions are measured to be 220⁺²⁶₋₂₉ km s⁻¹ and 279⁺²⁴₋₂₃ km s⁻¹ in the eastern and middle pointing, respectively. Within the eastern core, the velocity dispersion shows spatial variation, reaching 497⁺¹⁴⁴₋₁₁₇ km s⁻¹ in the southern core with high temperature—among the largest values yet reported in galaxy clusters to date. Narrow-band analysis of the Fe–K complex in this region reveals systematically higher temperatures derived from He-like and H-like Fe line ratio compared to those obtained via broadband fits, indicating multi-phase structures. Two-temperature modeling further separates a cooler core phase from a hotter, shock or turbulence-heated phase whose velocity is blueshifted, similar to that of the middle pointing. These results point to a mixing interface where post-shock gas from the south overlaps, in projection, with cooler core gas, inflating the observed line widths in this region. Weak-lensing analysis with Subaru/HSC and Suprime-cam confirms that the eastern component is about twice as massive as the western one, consistent with disruption and gas stripping of the latter. The curved morphology of the eastern X-ray core, together with the measured kinematics, is naturally explained by an off-axis, post–core-passage merger that imparts angular momentum and drives large-scale rotational and fallback flows.