  Using a cosmological simulation at redshift 5, we find that the baryon-rich
cores of intergalactic filaments extending outward from galaxies commonly form
isothermal gas cylinders in regions favorable to their formation. The central
gas density is typically about 500 times the cosmic mean total density, and the
temperature is typically 1-2 times 10^4 K, just above the Lyman alpha cooling
floor. These findings argue that the hydrodynamic properties of the gas are
more important than the dark matter in determining this structure. It is
noteworthy that the temperature and ionization state of the gas completely
determine a finite total mass per unit length of an isothermal cylinder. Our
findings may have implications for understanding the "cold mode" mechanism of
gas transport into galaxies.
