The Velocity Field of Baryonic Gas in the Universe

The dynamic evolution of the baryonic intergalactic medium (IGM) caused bythe underlying dark matter gravity is governed by the Navier-Stokes equationsin which many cooling and heating processes are involved. However, it has longbeen recognized that the growth mode dynamics of cosmic matter clustering canbe sketched by a random force driven Burgers' equation if cooling and heatingare ignored. Just how well the dynamics of the IGM can be described as aBurgers fluid has not been fully investigated probably because cooling andheating are essential for a detailed understanding of the IGM. Using IGMsamples produced by a cosmological hydrodynamic simulation in which heating andcooling processes are properly accounted for, we show that the IGM velocityfield in the nonlinear regime shows the features of a Burgers fluid, that is,when the Reynolds number is high, the velocity field consists of an ensemble ofshocks. Consequently, (1) the IGM velocity $v$ is generally smaller than thatof dark matter; (2) for the smoothed field, the IGM velocity shows tightcorrelation with dark matter given by $v \simeq s v_{dm}$, with $s<1$, suchthat the lower the redshift, the smaller $s$; (3) the velocity PDFs areasymmetric between acceleration and deceleration events; (4) the PDF ofvelocity difference $\Delta v=v(x+r)-v(x)$ satisfies the scaling relation for aBurgers fluid, i.e., $P(\Delta v)=(1 r^y)F(\Delta v/r^y)$. We find the scalingfunction and parameters for the IGM which are applicable to the entire scalerange of the samples (0.26 - 8 h$^{-1}$ Mpc). These properties show that thesimilarity mapping between the IGM and dark matter is violated on scales muchlarger than the Jeans length of the IGM.Comment: 14 pages, 10 jpg-figures, accepted for publication in the Astrophysical Journal. References adde