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 = 0.619 07 and 0.570 52 in the Hubble Space Telescope/Cosmic Origins Spectrograph spectrum of background quasars SDSS J080908.13 + 461925.6 and SBS 1122 + 594, respectively. The Ne VIII 770 line is at ~3σ significance. In both instances, the Ne VIII is found to be tracing gaswith T≳105 K, predominantly collisionally ionized, with moderate densities of nH ≲ 10-4 cm-3, sub-solar metallicities and total hydrogen column densities of N(H) ≳ 1019 cm-2. In the z = 0.619 07 absorber, the low, intermediate ions and OVI are consistent with origin in photoionized gas, with the OVI potentially having some contribution from the warm collisional phase traced by Ne VIII. The z = 0.570 52 system has HI absorption in at least three kinematically distinct components, with one of them having b(H I) = 49±11 km s-1. The intermediate-ionization lines, OVI and Ne VIII, are coincident in velocity with this component. Their different line widths suggest warm temperatures of T = (0.5-1.5) × 105 K. Both absorbers are residing in regions where there are several luminous (≳L*) galaxies. The absorber at z = 0.570 52 is within the virial radius of a 2.6L* galaxy, possibly associated with shock-heated circumgalactic material.