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

Abstract

We report on the detection of Ne VIII in the HST/Cosmic Origins Spectrograph\nspectrum of the intervening absorption system at z = 0.495096 towards PKS\n0405-123. The high S/N COS spectrum also covers absorption from H I, C III, O\nIII, O IV and O VI associated with this multiphase system. The Ne VIII is\ndetected with high significance in both lines of the doublet, with integrated\ncolumn densities of log Na(Ne VIII 770) = 13.96 +/- 0.06 dex and log Na(Ne VIII\n780) = 14.08 +/- 0.07 dex. We find the origin of Ne VIII consistent with\ncollisionally ionized gas at T ~ 5 x 10^5 K with a large baryonic column\ndensity of N(H) ~ 10^{19} - 10^{20} cm^-2. The metallicity in the Ne VIII gas\nphase is estimated to be [Ne/H] ~ -0.6 +/- 0.3 dex. The intermediate ions such\nas C III, O III, O IV and H I are consistent with photoionization in lower\nionization gas at T ~ 10,000 K. The O V and O VI in this absorber can have\ncontributions from both the photoionized and collisionally ionized gas phases.\nThe absorber is at 180 km/s systematic velocity and 110 kpc projected\nseparation from a M_R = -19.6 galaxy of extended morphology. The collisionally\nionized gas at T ~ 5 x 10^5 K points to an origin in multiphase gas embedded in\nthe hot halo of the galaxy, or in a nearby WHIM structure. The high sensitivity\nUV spectroscopy afforded by COS has opened up new opportunities for discovering\nlarge reservoirs of "missing baryons" in the low-z universe through the\ndetection of Ne VIII systems.\n