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 50 out of 50 Main -21.0 1000 1120 3 4.938 × 10 10 25
Esmeralda 50 out of 50 Main -20.0 640 1250 3 0.931 × 10 10 15
Consuelo 50 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.

Respective sizes of the 4 LasDamas simulation boxes

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.

The LasDamas simulations were run thanks to a Teragrid allocation and the use of RPI and NYU computing resources.

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.