Cosmological Model
The current phase of LasDamas uses a single cosmological model for all simulations:
| Omega_m | Omega_L | Omega_b | Hubble / 100 (h) | sigma_8 | n s |
|---|---|---|---|---|---|
| 0.25 | 0.75 | 0.04 | 0.7 | 0.8 | 1.0 |
The Simulations
Mocks are generated from many realizations of four different boxes. We are in the processes of running 50 realizations per box, where each has the same initial power spectrum, but a different random seed.
| Name | Number Completed | Galaxy Samples | Box Size | Number of Particles | Particle Mass | softening |
|---|---|---|---|---|---|---|
| (r-band limit) | (Mpc / h) | (Msun / h) | (kpc/h) | |||
| Oriana | 40 out of 50 | LRG & Main -22.0 | 2400 | 1280 3 | 45.730 × 10 10 | 53 |
| Carmen | 41 out of 50 | Main -21.0 | 1000 | 1120 3 | 4.938 × 10 10 | 25 |
| Esmeralda | 30 out of 50 | Main -20.0 | 640 | 1250 3 | 0.931 × 10 10 | 15 |
| Consuelo | 4 out of 50 | Main -19.0/-18.0 | 420 | 1400 3 | 0.187 × 10 10 | 8 |
Below Image: Smoothed distribution of halos (mass-weighted) of a 40 Mpc/h thick slice of the four LasDamas simulation boxes. One can see their relative sizes.
Initial Conditions
CMBfast (Seljak & Zaldarriaga 1996) is used to compute the power spectrum of density fluctuations. An initial density field is generated and initial positions and velocities computed for the particles using the 2LPT code, developed and tested by one of the members of LasDamas (Scoccimarro). 2LPT computes initial conditions using second-order Lagrangian perturbation theory. This method is more accurate than the traditional Zeldovich approximation (ZA) because it accounts for very early nonlinear gravitational evolution, which can have a significant impact on the properties of the highest density peaks. 2LPT initial conditions have been tested extensively by Crocce, Pueblas, & Scoccimarro (2006).
Oriana and Carmen are started at an initial redshift of z = 49; Esmeralda and Consuelo at z = 99.
Gravitational Evolution
Once the positions and velocities are generated, the gravitational evolution is performed using the publicly available Gadget-2 code (Springel et al. 2005). We only use collisionless dark matter particles and utilize its TreePM functionality — a hybrid of tree and particle mesh — to speed computation and increase the long range force accuracy.
Halo Identification
Bound groups of dark matter particles (halos) are identified using a parallel friends-of-friends (FOF) code. The implementation used, ntropy-fofsv, is built with the Ntropy framework (Gardner, Connolly, & McBride 2007). The FOF algorithm (Davis et al. 1985) links together particles separated by less than a specified linking length. Two different linking lengths are currently being investigated: 0.2 and 0.156 times the mean interparticle separation.