ar X iv : a st ro - p h / 05 02 17 5 v 2 4 O ct 2 00 5 The velocity field of baryonic gas in the universe

The dynamic evolution of the baryonic intergalactic medium (IGM) caused by the underlying dark matter gravity is governed by the Navier-Stokes equations in which many cooling and heating processes are involved. However, it has long been recognized that the growth mode dynamics of cosmic matter clustering can be sketched by a random force driven Burgers’ equation if cooling and heating are ignored. Just how well the dynamics of the IGM can be described as a Burgers fluid has not been fully investigated probably because cooling and heating are essential for a detailed understanding of the IGM. Using IGM samples produced by a cosmological hydrodynamic simulation in which heating and cooling processes are properly accounted for, we show that the IGM velocity field 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 of shocks. Consequently, (1) the IGM velocity v is generally smaller than that of dark matter; (2) for the smoothed field, the IGM velocity shows tight correlation with dark matter given by v ≃ svdm, with s < 1, such that the lower the redshift, the smaller s; (3) the velocity PDFs are asymmetric between acceleration and deceleration events; (4) the PDF of velocity difference ∆v = v(x + r) − v(x) satisfies the scaling relation for a Burgers fluid, i.e., P (∆v) = (1/r)F (∆v/r). We find the scaling function and parameters for the IGM which are applicable to the entire scale range of the samples (0.26 8 h Mpc). These properties show that the similarity mapping between the IGM and dark matter is violated on scales much larger than the Jeans length of the IGM. Subject headings: cosmology: theory large-scale structure of universe