2.10. Kinetic Energy in the IGM

Intergalactic baryons have peculiar velocities that are driven by the gravitational field of the dark matter distribution and by nongravitational interactions. The former is part of the primeval energy in category 4 in the inventory. The latter is expected to be most important near the virialized regions of galaxies, where the streaming motion has been largely transformed by shocks into thermal energy at the temperature T ~ 2 × 10⁶ K associated with the nominal velocity dispersion = 160 km s^-1 around L ~ L_* galaxies (eq. [10]). This shocked matter may be responsible for the O VI absorption lines in the Local Group (Sembach et al. 2003; Cen et al. 2001). The product of the internal energy per unit mass of plasma at this temperature with the mass fraction in entry 3.1a is

(128)

If an appreciable fraction of the kinetic energy produced by supernovae (eq. [126]) were deposited as kinetic energy in galactic haloes this energy would be dissipated by hydrodynamic processes rather than the cosmological redshift, but still may make a significant addition to equation (128).

Baryonic matter well outside the nominal virialized regions of galaxies and larger systems - at distances greater than about r₂₀₀ from clusters of galaxies and r_v ~ 200 kpc from L ~ L_* galaxies - is observed as Ly absorption systems (Penton, Stocke & Shull 2004). The primeval peculiar motions of this matter are perturbed by photoionisation that produces kinetic temperatures on the order of 10⁴ K. The product of this kinetic energy per unit mass with the intergalactic mass fraction (entry 3.1b) is about one percent of the kinetic energy in the warm intergalactic component (eq. [128]).

We use equation (128) for category 10. We caution, however, that a comparable amount of kinetic energy may be deposited by supernova winds.