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 gas with T ≳ 10⁵ K, predominantly collisionally ionized, with moderate densities of n_{H} ≲ 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 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 = 0.570 52 system has H I 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, 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) × 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.