2026ApJ..1000...47A

Mapping the physical conditions of the shocked plasma of young supernova remnants (SNRs) is crucial for understanding their explosion mechanisms, ejecta structure, and large-scale asymmetries. Using >350 ks of XRISM/Resolve high spectral resolution observations of Cassiopeia A (Cas A), the youngest known Galactic core-collapse SNR, we present the first microcalorimeter-based plasma parameter maps of any SNR. We tessellate Cas A into $1^{\prime} \rm{}\,\rm{}\,\times 1^{\prime} \rm{}\,\rm{}\,$regions and fit the broadband spectra as thermal emission from two pure-metal ejecta components-corresponding to intermediate-mass elements (IMEs) and iron-group elements (IGEs)-plus nonthermal synchrotron radiation. For robust inference, we introduce UltraSPEX, a Bayesian framework that couples the SPEX plasma code with the UltraNest nested-sampling algorithm, yielding full posterior distributions and exploration of parameter degeneracies. Key findings include enhanced Ar/Si and Ca/Si abundance ratios near the base of the Si-rich jets and a high Ni/Fe mass ratio (0.08 ± 0.015) in the base of the NE jet. IGE ejecta exhibit systematically higher Doppler velocities and broadenings than IME ejecta in most regions, with maximum differences of ∼800 and ∼1200 km s^–1, respectively; Ca shows distinct (faster) kinematics from other IMEs in several SE regions. The ionization timescale and electron temperature show a robust anticorrelation, particularly for IGEs. This relation and measured parameter values could be explained by semianalytical models with significant ejecta clumping (overdensities of ∼10 for IGEs and up to ∼100 for IMEs) and reduced historical reverse-shock velocities. Nonthermal emission accounts for a substantial fraction, with at least 47% of the 4-6 keV continuum, and dominates in the western regions, where the spectrum hardens.