The robustness of cosmological hydrodynamic simulation predictions to changes in numerics and cooling physics

Abstract

We test and improve the numerical schemes in our smoothed particle hydrodynamics (SPH) code for cosmological simulations, including the pressure-entropy formulation (PESPH), a time-dependent artificial viscosity, a refined timestep criterion, and metalline cooling that accounts for photoionisation in the presence of a recently refined Haardt & Madau (2012) model of the ionising background. The PESPH algorithm effectively removes the artificial surface tension present in the traditional SPH formulation, and in our test simulations it produces better qualitative agreement with mesh-code results for Kelvin-Helmholtz instability and cold cloud disruption. Using a set of cosmological simulations, we examine many of the quantities we have studied in previous work. Results for galaxy stellar and HI mass functions, star formation histories, galaxy scaling relations, and statistics of the Lyα forest are robust to the changes in numerics and microphysics. As in our previous simulations, cold gas accretion dominates the growth of high-redshift galaxies and of low mass galaxies at low redshift, and recycling of winds dominates the growth of massive galaxies at low redshift. However, the PESPH simulation removes spurious cold clumps seen in our earlier simulations, and the accretion rate of hot gas increases by up to an order of magnitude at some redshifts. The new numerical model also influences the distribution of metals among gas phases, leading to considerable differences in the statistics of some metal absorption lines, most notably NeVIII.

The impact of wind scalings on stellar growth and the baryon cycle in cosmological simulations

“…We also include the Hubble flow while calculating the velocity divergence. Our fiducial code leads to considerable improvements in resolving the instabilities at fluid interfaces in subsonic flows and produces consistent results with other state-of-art hydrodynamic codes in various numerical tests (Sembolini et al 2016a,b;Huang et al 2019).…”

“…We implemented the new wind algorithm into our SPH code based on gadget-3 (see Springel (2005) for reference). The code includes several recent numerical improvements in the SPH technique (Huang et al 2019). To summarise, we use the pressure-entropy formulation (Hopkins 2013) of SPH to integrate the fluid equations and a quintic spline kernel to measure fluid quantities over 128 neighbouring particles.…”

“…We identify galaxies using skid and so as in Huang et al (2019) and measure the stellar mass and halo mass for every galaxy that we identify. First, we will focus on comparing the galactic stellar mass functions at four different redshifts and discuss the effects of changing the mass loading factor and the wind speed separately.…”

“…It is also implemented with the numerical improvements to the SPH hydrodynamics introduced by Huang et al (2019). We will focus on those predictions that have changed significantly from Huang et al (2019) and from our previously published work with our new wind algorithm. Figure 14 shows that the GSMFs from our fiducial simulation, shown as black lines in each panel, are mostly consistent with observations at all redshifts from z = 0 to z = 4.…”

“…We use a uniform ultraviolet background (Haardt & Madau 2012) to calculate the ionization level of each ion. We normalise the strength of the background to match the Lyman α decrement measurements (Huang et al 2019). From these mock spectra, we further obtain observational quantities such as column densities and equivalent widths for each ion by fitting their line profiles using the Voigt profile fitting software AUTOVP (Davé et al 1997).…”