Probing the Warm-Hot Circumgalactic Medium with broad OVI and X-rays

Abstract: Most of the baryonic mass in the circumgalactic medium (CGM) of a spiral galaxy is believed to be warm-hot, with temperature around $10^6$K. The narrow OVI absorption lines probe a somewhat cooler component at $\log \rm T(K)= 5.5$, but broad OVI 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 OVI together with the non-detection of Lya was indicative of warm-hot gas. The strongest line expected to be present at $\approx 10^6$K is OVII $\lambda 21.602$. There is a hint of an absorption line at the redshifted wavelength, but the line is not detected with better than $2\sigma$ significance. A physical model, taking into account strengths of several other lines, provides better constraints. Our best-fit absorber model has $\log \rm T(K) =6.3\pm 0.2$ and $\log \rm N_{H} (cm^{-2})=20.7^{+0.3}_{-0.5}$. These parameters are consistent with the warm-hot plasma model based on UV observations; other OVI models of cooler gas phases are ruled out at better than $99$% confidence. Thus we have suggestive, but not conclusive evidence for the broad OVI absorber probing the warm-hot gas from the shallow observations of this pilot program. About 800ks of XMM-Newton observations will detect the expected absorption lines of OVII and OVIII unequivocally. Future missions like XRISM, Arcus and Athena will revolutionize the CGM science.