| Abstract |
We present a clear detection of a broad Lyα absorber (BLA) with a
matching O VI line in the nearby universe. The BLA is detected at
z(\Ly\hspace{1.00006pt\alpha }) = 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({H\,\mathsc{i}}) = 55 \pm 1 km s-1 and b({H\,\mathsc{i}}) =
33 \pm 1 km s-1, separated in velocity by v ~ 115 km
s-1. The O VI, detected by the Far-Ultraviolet Spectroscopic
Explorer at z({O\,\mathsc{vi}}) = 0.01027, has a b({O\,\mathsc{vi}}) =
29 \pm 3 km s-1 and is kinematically well aligned with the
broader H I component. The non-detection of other ions such as C II, Si
II, Fe II, C III, Si III, C IV, Si IV, and N V at the same velocity as
the BLA and the O VI implies that the absorber is tracing highly ionized
gas. The different line widths of the BLA and O VI 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.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under NASA
contract NAS 05-26555, and the NASA-CNES/ESA Far-Ultraviolet
Spectroscopic Explorer mission, operated by the Johns Hopkins
University, supported by NASA contract NAS 05-32985. |