Cosmic Origins Spectrograph and fuse observations of T ∼ 105 K gas in a nearby galaxy filament

Abstract

We present a clear detection of a broad Lyα absorber (BLA) with a matching OVI line in the nearby universe. The BLA is detected at z(Lyα) = 0.01028 in the high signal-to-noise ratio spectrum of Mrk 290 obtained using the Cosmic Origins Spectrograph. The Lyα absorption has two components, with b(HI) = 55 ± 1 km s-1 and b(HI) = 33 ± 1 km s-1, separated in velocity by v ∼ 115 km s-1. The OVI, detected by the Far-Ultraviolet Spectroscopic Explorer at z(OVI) = 0.01027, has a b(OVI) = 29±3 km s-1 and is kinematically well aligned with the broader Hi component. The non-detection of other ions such as CII, SiII, FeII, CIII, SiIII, C IV, SiIV, and NV at the same velocity as the BLA and the OVI implies that the absorber is tracing highly ionized gas. The different line widths of the BLA and OVI suggest a temperature of T = 1.4 × 105 K in the absorber. Photoionization, collisional ionization equilibrium as well as non-equilibrium collisional ionization models do not explain the ion ratios at this temperature. The observed line strength ratios and line widths favor an ionization scenario in which both ion-electron collisions and UV photons contribute to the ionization in the gas. Such a model requires a low metallicity of ∼ - 1.7 dex, ionization parameter of log U ∼ -1.4, a large total hydrogen column density of N(H) ∼ 4×1019 cm-2, and a path length of ∼400 kpc. The line of sight to Mrk 290 intercepts at the redshift of the absorber, a megaparsec scale filamentary structure extending over ∼20° in the sky, with several luminous galaxies distributed within ∼1.5 h -1 Mpc projected distance from the absorber. The collisionally ionized gas phase of this absorber is most likely tracing a shock-heated gaseous structure, consistent with a few different scenarios for the origin including an overdense region of the warm-hot intergalactic medium in the galaxy filament or highly ionized gas in the extended halo of one of the galaxies in the filament. In general, BLAs with metals provide an efficient means to study T ∼ 105-106 K gas in galaxy halos and in the intergalactic medium. A substantial fraction of the baryons missing from the present universe is predicted to be in such environments in the form of highly ionized plasma. © 2010. The American Astronomical Society. All rights reserved.