Simulation of Gas and Dark Matter in Galaxy Formation

In the present work the gas and dark matter were simulated in lambda cold dark matter model using gadget-2 code. In the simulation one million gas and one million dark matter particles were simulated from the early universe (redshift z=30) to (redshift z=0) in a box of size (80Mpc/h) 3 . The gravity caused the collection of gas and dark matter particles to construct many clumps, and then these clumps developed with time. The clumps are clusters of galaxies formed in the simulation. The clusters surrounded by voids, while the filaments formed between the clusters. The clusters, voids, and filaments were clearly appeared in redshift z=0. In the simulation, the densitytemperature plane shows different concentrations of the non cooling and cooling of gas particles in different epochs. In the simulation sixteen processors of high performance supercomputer of Nottingham University-England were used. Introduction Dark matter is the unseen material that does not produce enough radiation to be observed directly. Several independent arguments suggest that the Universe dominated by dark matter.Although it was provided in 1933, but was detected in 2006 by its gravitational lensing. There are three different ways of measuring the amount of dark matter in clusters of galaxies which are galaxy orbit, measuring the temperature of the hot gas in clusters and the gravitational lensing. All of these methods agree on the indication that the total mass of a cluster is about fifty times the mass of its stars, implying huge amounts of dark matter. (Coles, 1998; Ramachers, 2001). Some of the dark matter could be ordinary or baryonic matter but there does not appear to be enough ordinary matter to account for all the dark matter. Most of it is probably extraordinary or nonbaryonic matter consisting of undiscovered particles that are called weakly interacting massive particles WIMPs (Bennett et al., 2007). Journal of Kirkuk University – Scientific Studies, vol.6, No.1, 2011 41 Dark matter is grouped into two general types, the cold dark matter and the hot dark matter. The first is non baryonic and non relativistic particles (Vitorio & Silk, 1984), the second is also non baryonic but it is ultrarelativistic particles (Masayuki & Ikeuchi, 1984). The evolution of the structures refers cold dark matter (Holtzman & Primack, 1993). According to the current standard paradigm, galaxies form and reside inside extended dark matter halos and the gravitation force of dark matters must be the primary force holding these structures together. Thus, it is strongly suspected that the gravitational attraction of dark matter is what pulled galaxies and clusters together in the first place (Bennett et al., 2007; White& Ress, 1978). The second component of the universe is the gas. It was predicted that the ratio of protons to neutrons during the era of nucleosynthesis of the Universe should be about 75% hydrogen and 25% helium mass (Bennett et al., 2007). Simulation All simulations of large-scale structures and galaxy formations depend on the existence of dark matter and gas particles which are the basic components of the universe. In the present work one million dark matter particles were simulated with one million gas particles in the conditions of cosmological density Ω=0.25, dark energy density ΩΛ=0.75, baryon density Ωb=0.04, fluctuation amplitude σ8=0.9 and Hubble constant H0=100h kms -1 Mpc -1 . Simulation of one million particles of dark matter was run in abox (80Mpc/h) 3 . The run time took four hours to complete on two processors. The same dark matter particles with the addition of a noncooling gas component consisting of another one million particles were simulated in the same box. This run took eight hours to complete using four processors. One million dark matters and one million cooled gases were simulated also. This run took more than forty eight hours to complete on sixteen processors. From the simulations, inside the box clumps of dark matter and gas together constructed which represents clusters of galaxies. The filaments between the clusters and the voids around the clusters are also constructed with time, so the concentration of particles differs at different redshifts. In the present work, as mentioned above, the Hubble constant is:  0 H 100 h km s -1 Mpc -1 . (1) But the uncertainty (h) has the value: h=0.73, so:  0 H 73 km s -1 Mpc -1 . (2) Journal of Kirkuk University – Scientific Studies, vol.6, No.1, 2011