Discovery of a Very Hot Phase of the Milky Way Circumgalactic Medium with Non-solar Abundance Ratios

We present the discovery of a very hot gas phase of the Milky Way circumgalactic medium (CGM) at T ≈ 10⁷ K, using deep XMM-Newton Reflection Grating Spectrometer observations of the blazar 1ES 1553+113. The hot gas, coexisting with a warm-hot phase at T ≈ 10⁶ K is α-enhanced, with [{{O}}/{Fe}]={0.9}_-0.3^+0.7, indicating core-collapse supernovae enrichment. Additionally, we find [Ne/O] and [{{N}}/{{O}}]={0.7}_-0.2^+1.6, such that N/Ne is consistent with solar. Along with the enrichment by asymptotic giant branch stars and core-collapse supernovae, this indicates that some oxygen has depleted onto dust and/or transited to cooler gas phase(s). These results may affect previous baryonic and metallic mass estimations of the warm-hot and hot CGM from the observations of oxygen emission and absorption. Our results provide insights on the heating, mixing, and chemical enrichment of the Milky Way CGM, and provide inputs to theoretical models of galaxy evolution.