TOPOLOGY OF LARGE-SCALE STRUCTURE BY GALAXY TYPE - HYDRODYNAMIC SIMULATIONS

The topology of large-scale structure is studied as a function of gala xy type using the genus statistic. In hydrodynamical cosmological cold dark matter simulations, galaxies form on caustic surfaces (Zeldovich pancakes) and then slowly drain onto filaments and clusters. The earl iest forming galaxies in the simulations (defined as ''ellipticals'') are thus seen at the present epoch preferentially in clusters (tending toward a meatball topology), while the latest forming galaxies (defin ed as ''spirals'') are seen currently in a spongelike topology. The to pology is measured by the genus (= number of ''doughnut'' holes minus number of isolated regions) of the smoothed density-contour surfaces. The measured genus curve for all galaxies as a function of density obe ys approximately the theoretical curve expected for random-phase initi al conditions, but the early-forming elliptical galaxies show a shift toward a meatball topology relative to the late-forming spirals. Simul ations using standard biasing schemes fail to show such an effect. Lar ge observational samples separated by galaxy type could be used to tes t for this effect.