Cosmic Origins Spectrograph and FUSE Observations of T ~ 105 K Gas in a Nearby Galaxy Filament

Type Journal Article
Names Anand Narayanan, Bart P. Wakker, Blair D. Savage, Brian A. Keeney, J. Michael Shull, John T. Stocke, Kenneth R. Sembach
Publication The Astrophysical Journal
Volume 721
Issue 2
Pages 960-974
Date October 1, 2010
URL http://adsabs.harvard.edu/abs/2010ApJ...721..960N
Library Catalog NASA ADS
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.
Tags Galaxies: Halos, galaxies: clusters: individual: GH 158, galaxies: individual: NGC 5987 NGC 5982 NGC 5985 NGC 5987 NGC 5989, intergalactic medium, quasars: absorption lines, ultraviolet: general
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