COS OBSERVATIONS OF METAL LINE AND BROAD LYMAN-α ABSORPTION IN THE MULTI-PHASE O VI AND Ne VIII SYSTEM AT z = 0.20701 TOWARD HE 0226-4110

Observations of the QSO HE 0226-4110 (zem = 0.495) with the Cosmic Origins Spectrograph (COS) from 1134 to 1796 Å with a resolution of ∼17 km s–1 and signal-to-noise ratio (S/N) per resolution element of 20-40 are used to study the multi-phase absorption system at z = 0.20701 containing O VI and Ne VIII. The system was previously studied with lower S/N observations with Far-Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS). The COS observations provide more reliable measures of the H I and metal lines present in the system and reveal the clear presence of broad Lyα (BLA) absorption with b = 72(+13, –6) km s–1 and log N(H I) = 13.87 ± 0.08. Detecting BLAs associated with warm gas absorbers is crucial for determining the temperature, metallicity, and total baryonic content of the absorbers. The BLA is probably recording the trace amount of thermally broadened H I in the collisionally ionized plasma with log T ∼ 5.7 that also produces the O VI and Ne VIII absorption. The total hydrogen column in the collisionally ionized gas, log N(H) ∼ 20.1, exceeds that in the cooler photoionized gas in the system by a factor of ∼22. The oxygen abundance in the collisionally ionized gas is [O/H] = –0.89 ± 0.08 ± 0.07. The absorber probably occurs in the circumgalactic environment (halo) of a foreground L = 0.25L* disk galaxy with an impact parameter of 109 h70–1 kpc identified by Mulchaey and Chen.