Probing the Hot Circumgalactic Medium with Broad O VI and X-Rays

Most of the baryonic mass in the circumgalactic medium (CGM) of a spiral galaxy is believed to be warm-hot, with temperature around 10⁶ K. The narrow O VI absorption lines probe a somewhat cooler component at $\mathrm{log}\,T({\rm{K}})=5.5$ , but broad O VI absorbers have the potential to probe the hotter CGM. Here we present 376 ks Chandra LETG observations of a carefully selected galaxy in which the presence of broad O VI together with the non-detection of Lyα was indicative of hot gas. The strongest line expected to be present at ≈10⁶ K is O VII λ21.602. There is a hint of an absorption line at the redshifted wavelength, but the line is not detected with better than 2σ significance. A physical model, taking into account strengths of several other lines, provides better constraints. Our best-fit absorber model has $\mathrm{log}\,T({\rm{K}})=6.3\pm 0.2$ and $\mathrm{log}{N}_{{\rm{H}}}({\mathrm{cm}}^{-2})={20.7}_{-0.5}^{+0.3}$ . These parameters are consistent with the hot plasma model based on UV observations; other O VI models of cooler gas phases are ruled out at better than 99% confidence. Thus we have suggestive, but not conclusive evidence for the broad O VI absorber probing the hot gas from the shallow observations of this pilot program. About 800 ks of XMM-Newton observations will detect the expected absorption lines of O VII and O VIII unequivocally. Future missions like XRISM, Arcus, and Athena will revolutionize the CGM science.