COSMIC ORIGINS SPECTROGRAPH DETECTION OF Ne VIII TRACING WARM-HOT GAS TOWARD PKS 0405-123

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 similar to 5 x 10(5) K with a large baryonic column density of N(H) similar to 10(19)-10(20) cm(-2). The metallicity in the Ne VIII gas phase is estimated to be [Ne/H] similar to -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 similar to 10(4) 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 vertical bar Delta v vertical bar = 180 km s(-1) systematic velocity and rho = 110h(70)(-1) kpc projected separation from an M-R = -19.6 galaxy of extended morphology. The collisionally ionized gas at T similar to 5 x 10(5) 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.