Astron. Astrophys. 324, 534-548 (1997)

1. Introduction

Recently, there has been a dramatic increase in extra-galactic observational data through, e.g., the Hubble and COBE satellites, and very large ground based telescopes. These detailed observations have put new demands on theoretical models of galaxy formation.

Gas dynamics with proper energy dissipation is of fundamental importance in galaxy formation. Analytic models of gas dynamics are typically restricted to systems possessing a high degree of symmetry. Hierarchical galaxy formation, on the other hand, is a highly inhomogeneous process. In order to follow the three-dimensional evolution of the baryonic component in a proto-galaxy, numerical techniques are required.

Smooth particle hydrodynamics (SPH) is a fully Lagrangian numerical method for gas dynamics. SPH was introduced by Lucy (1977 ) and Gingold & Monaghan (1977 ) to avoid the limitations of grid-based methods. When coupled with a fast algorithm to calculate gravitational forces, this method has been used successfully to study hierarchical galaxy formation (Evrard 1988 , Hernquist & Katz 1989 , Navarro & White 1993 ).

In this paper we present an SPH implementation, coupled with a gravitational tree method, that has been specifically developed to study the formation of galaxies. The code is applied to several test cases to clarify the advantages and limitations of the implementation. The code used for the simulations described in this paper is fully Lagrangian, does not put restrictions on the geometry of the problem, and has a locally varying resolution both in space and time.

Simulating the formation of galaxies in a hierarchical model is a demanding task for any numerical method. Small objects form first, and the characteristic mass of objects then grows rapidly with time, as smaller objects merge into larger ones. At any given time there is a wide range of object masses, and the code must be able to handle many different scales simultaneously. When galaxies merge, part of the gas falls to the center of the new galaxy. This gas concentration effect is especially pronounced in simulations without star formation, where the baryonic mass remains gaseous throughout the simulation. The gas cores that collect at the center of dark matter halos are typically to small to be gravitationally resolved, but can still be the cause of much of the calculational cost. We avoid this problem by merging particles in unresolved gas cores, thereby limiting the hydrodynamical resolution to the gravitational resolution, and furthermore speeding up the calculations significantly.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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