Dissipational galaxy formation. I - Effects of gasdynamics

We present numerical simulations of hierarchical galaxy formation including gasdynamics. These simulations are conducted using a new, general-purpose program for evolving self-gravitating systems in three dimensions. The gravitational forces are calculated with a hierarchical tree algorithm, while the gasdynamic properties are determined by an approach known as smoothed particle hydrodynamics. In this method the complete thermodynamic state of the gas is known everywhere, so dissipational effects can be included by allowing the gas to cool radiatively. We use standard cooling curves that include Compton, as well as radiative, cooling. These simulations model the collapse of isolated constant density perturbations, initially in solid-body rotation and in Hubble-flow. The perturbations consist of dark and baryonic matter in a 10 to 1 ratio. Small-scale power is added using the Zel'dovich approximation assuming a power-law slope of either -2. 5 or 0. We are successful in making two component systems that resemble spiral galaxies: a thin disk made of gas and a dark matter halo. The disk transfers more than 50% of its original angular momentum to the dark halo and forms at an angle of ~30ᵈeg^ to the rotation axis of the dark halo. The disks form with a warp that is also a consequence of the angular momentum transport that acts during the collapse. Beyond ~5 kpc the gaseous disk has a flat rotation curve and an exponential surface density profile. Due to the efficient cooling that acts during the collapse, the gas never heats to the virial temperature but remains at less than 30, 000 K.