2011ApJ...730...15N

We report on the detection of Ne VIII in the Hubble Space Telescope/Cosmic Origins Spectrograph (COS) spectrum of the intervening absorption system at z = 0.495096 toward PKS 0405-123 (z_em= 0.5726). The high signal-to-noise COS spectrum also covers absorption from H I, C III, O III, O IV, and O VI associated with this multiphase system. The Ne VIII is detected with high significance in both lines of the doublet, with integrated column densities of log N_a(Ne VIII 770) = 13.96±0.06 and log N_a(Ne VIII 780) = 14.08 ±0.07. We find the origin of Ne VIII consistent with collisionally ionized gas at T ∼ 5x10⁵ K with a large baryonic column density of N(H) ∼ 10¹⁹-10²⁰/cm2. The metallicity in the Ne VIII gas phase is estimated to be [Ne/H] ~-0.6 ±0.3 dex. The intermediate ions such as C III, O III, O IV, and H I are consistent with photoionization in lower ionization gas at T ∼ 10⁴ K. The O V and O VI in this absorber can have contributions from both the photoionized and collisionally ionized gas phases. The absorber is at|Δv| = 180 km/s systematic velocity and ρ = 110h^–1₇₀ kpc projected separation from an M_R= -19.6 galaxy of extended morphology. The collisionally ionized gas at T ∼ 5x10⁵ K detected in Ne VIII and O VI points to an origin in multiphase gas embedded in the hot halo of the galaxy, or in a nearby WHIM structure. The high-sensitivity UV spectroscopy afforded by COS has opened up new opportunities for discovering large reservoirs of missing baryons in the low-z universe through the detection of Ne VIII systems.