Discovery of a very hot phase of the Milky Way CGM with non-solar abundance ratios

Abstract: We present the discovery of a very hot gas phase of the Milky Way circumgalactic medium (CGM) at T $\approx 10^7$ K, using deep XMM-Newton RGS observations of 1ES 1553+113. The hot gas, coexisting with a warm-hot phase at T $\approx 10^6$ K is $\alpha-$enhanced, with [O/Fe] = 0.9$^{+0.7}_{-0.3}$, indicating core-collapse supernovae enrichment. Additionally we find [Ne/O] and [N/O] = $0.7^{+1.6}_{-0.2}$, such that N/Ne is consistent with solar. Along with the enrichment by AGB 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.