Imaging the Low Red-shift Cosmic Web
  The first image of a Cosmic Web, Lyman Limit System has just been made in HI emission within the Local Group filament connecting M31 and M33. The corresponding HI distribution function is in very good agreement with that of the QSO absorption lines, confirming the 30-fold increase in surface area expected between 10^19 cm^-2 and 10^17 cm^-2. The critical observational challenge is crossing the ``HI desert'', the range of log(N_HI) from about 19.5 down to 18, over which photo-ionization by the intergalactic radiation field produces an exponential decline in the neutral fraction from essentially unity down to a few percent. Nature is kinder again to the HI observer below log(N_HI) = 18, where the neutral fraction decreases only very slowly with log(N_HI). Average spectra of the M31/M33 filament suggest a kinetic temperature of 2x10^5 K for the parent ion population, consistent with significant shock-heating. The brightest knot within the filament, with only a 2x10^4 K apparent temperature, is suggestive of localized cooling and condensation. We have initiated two complimentary surveys that should lay the groundwork for a comprehensive study of the Cosmic Web phenomenon in HI emission. When combined with targeted optical and UV absorption line observations, the total baryonic masses and enrichment histories of the Cosmic Web could be determined over the complete range of environmental over-densities. 
Robert Braun (1) and David Thilker (2) ((1) ASTRON, (2) JHU)