Cosmological Simulations with Scale‐Free Initial Conditions. I. Adiabatic Hydrodynamics

We analyze hierarchical structure formation based on scale-free initialconditions in an Einstein-de Sitter universe, including a baryonic component.We present three independent, smoothed particle hydrodynamics (SPH)simulations, performed with two different SPH codes (TreeSPH and P3MSPH) at tworesolutions. Each simulation is based upon identical initial conditions, whichconsist of Gaussian distributed initial density fluctuations that have an n=-1power spectrum. The baryonic material is modeled as an ideal gas subject onlyto shock heating and adiabatic heating and cooling. The evolution is expectedto be self-similar in time, and under certain restrictions we identify theexpected scalings for many properties of the distribution of collapsed objectsin all three realizations. The distributions of dark matter masses, baryonmasses, and mass and emission weighted temperatures scale quite reliably.However, the density estimates in the central regions of these structures aredetermined by the degree of numerical resolution. As a result, mean gasdensities and luminosities obey the expected scalings only when calculatedwithin a limited dynamic range in density contrast. The temperatures andluminosities of the groups show tight correlations with the baryon masses,which can be well-represented by power-laws. The Press-Schechter (PS)approximation predicts the distribution of group dark matter and baryon massesfairly well, though it tends to overestimate the baryon masses. Combining thePS mass distribution with the measured relations for T(M) and L(M) predicts thetemperature and luminosity distributions reasonably, though there are somediscrepancies at high temperatures/luminosities. The three simulations agreewell for the properties of groups that are resolved by 32 or more particles.Comment: 40 pages, 16 embedded postscript figures, uses AASTEX 4.0 style. Minor wording changes, to appear in ApJ. Abridged abstrac