| 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. |