XRISM reveals complex multi-temperature structures in the Abell 2029 galaxy cluster

Abstract The Resolve micro-calorimeter onboard XRISM is set to significantly advance our understanding of the complex intracluster medium (ICM) in galaxy clusters. We present ${\sim}500$ ks XRISM observations covering the central and two northern regions of the Abell 2029 galaxy cluster. Resolve enables us to distinguish multiple emission lines from hydrogen-like and helium-like iron (Fe) ions. This study focuses on the multi-temperature structure of Abell 2029 using line-ratio diagnostics. Using a single-temperature collisionally ionized equilibrium (CIE) model, we measure average plasma temperatures of 6.73 keV, 7.61 keV, and 8.14 keV in the central, inner northern, and outer northern regions, respectively, spanning a radial range up to $700$ kpc. To further investigate thermal structure, we derive excitation and ionization temperatures by comparing observed emission-line flux ratios with atomic database predictions. Significant deviations from the single-temperature CIE model in the central and inner northern regions indicate the presence of multi-phase gas. The excitation and ionization temperatures range from 2.85 to 8.5 keV in the central region, 4.3 to 9.8 keV in the inner northern region, and 8.3 to 10.4 keV in the outer northern region. These temperature distributions are largely consistent with the previously observed temperature gradient of A2029. However, Resolve detects two notably cooler components—3.42 keV in the central region and ${\sim}4.3$ keV in the inner northern region—likely associated with displaced cool gas due to gas sloshing. Additionally, we thermally resolve a 2.85 keV gas component at the core of A2029—potentially a significant development in our understanding of gas cooling. We propose that this cooler gas is a direct product of ongoing cooling processes in A2029, having already cooled to its present temperature. If this temperature structure is stable and no heating mechanism is present, this reservoir is likely to cool to even lower temperatures and form stars.