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C. W. Danforth, J. M. Shull (U. Colorado)
The intergalactic medium (IGM) is predominantly studied through Ly\alpha absorption lines in the spectra of quasars and AGN (the "Ly\alpha Forest"). Most IGM research to date has been done at z>2 where the Ly\alpha forest is shifted within range of optical telescopes. It is only with the advent of space-based UV observatories such as HST and FUSE that the same methods can be used to survey the IGM in the local universe (z<1). However, the Ly\alpha forest only samples the cool, photoionized gas in the IGM. Cosmological simulations show that the fraction of baryons in cool gas has fallen from nearly 100% at high redshift to about 30% in the present universe. Meanwhile, the warm-hot ionized medium (WHIM, T=105-107 K) baryon fraction has risen to 30% at z=0. Relatively little is known about the WHIM since the best diagnostic lines are either X-ray transitions of OVII and OVIII, or far-UV transitions of Li-like OVI, NV, and CIV.
We summarize an ongoing survey of >200 low-redshift IGM absorbers (z<0.3) in FUSE and HST/STIS observations along 31 AGN sight lines. Multiple Lyman transitions give accurate measurements of HI column density and line width. Absorption in CIII and SiIII trace moderately ionized gas while OVI, NV, CIV, and SiIV absorbers are used to trace collisionally ionized WHIM gas. We see that the column densities of WHIM tracers are poorly correlated with that of cooler species. OVI shows a much steeper power-law distribution of column densities than HI indicating that some physical regulatory mechanism is in place for WHIM gas not present in cooler material. Shock heating through gravitational infall of clouds onto large-scale structure and/or feedback from galaxies are the leading sources of WHIM. We estimate the typical scale of metal-enriched IGM absorbers to be roughly 400 kpc and show that the canonical metallicity of 10% is largely valid. By measuring different metal lines, we can see patterns of enrichment in the IGM, determine the metallicity of the IGM, and place limits on the typical size of IGM absorbers. Finally, we summarize some of the outstanding problems in the low-z IGM and point out some of the biases inherent in the baryon census to date. This will pave the way for future investigations and future observations of the baryon distribution in the local universe.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.