Detection of two intervening Ne viii absorbers probing warm gas at z ∼ 0.6

Abstract

We report on the detection of two Ne VIII absorbers, at z = 0.61907 and z = 0.57052 in the HST/COS spectrum of background quasars SDSS J 080908.13+461925.6 and SBS 1122+594 respectively. The Ne VIII 770 line is at $\sim 3\sigma$ significance. In both instances, the NeVIII is found to be tracing gas with $T \gtrsim 10^5$ K, predominantly collisionally ionized, with moderate densities of $n_{H} \lesssim 10^{-4}$ $cm^{-3}$, sub-solar metallicities and total hydrogen column densities of $N(H) \gtrsim 10^{19}$ $cm^{2}$. In the z = 0.61907 absorber, the low, intermediate ions and O VI are consistent with origin in photoionized gas, with the O VI potentially having some contribution from the warm collisional phase traced by Ne VIII. The z = 0.57052 system has H I absorption in at least three kinematically distinct components, with one of them having $b (H I) = 49 {\pm} 11$ $km s^{-1}$. The intermediate ionization lines, O VI and Ne VIII are coincident in velocity with this component. Their different line widths suggest warm temperatures of $T = (0.5 - 1.5) \times 10^5$ K. Both absorbers are residing in regions where there are several luminous ($ \gtrsim L^*$) galaxies. The absorber at z = 0.57052 is within the virial radius of a $2.6L^*$ galaxy, possibly associated with shock heated circumgalactic material.