Snapshots

The snapshots are stored in either Gadget-II format or HDF5. They can be read using the readgadget.py and readsnap.py scripts. If you have Pylians installed you already have them.

The snapshots only contain 4 blocks:

  • Header: This block contains general information about the snapshot such as redshift, number of particles, box size, particle masses…etc.

  • Positions: This block contains the positions of all particles. Stored as 32-floats

  • Velocities: This block contains the velocities of all particles. Stored as 32-floats

  • IDs: This block contains the IDs of all particles. Stored as 32-integers. (This block may be removed in the future to reduce the size of the snapshots)

An example on how to read a snapshot is this:

import numpy as np
import readgadget

# input files
snapshot = '/home/fvillaescusa/Quijote/Snapshots/h_p/snapdir_002/snap_002'
ptype    = [1] #[1](CDM), [2](neutrinos) or [1,2](CDM+neutrinos)

# read header
header   = readgadget.header(snapshot)
BoxSize  = header.boxsize/1e3  #Mpc/h
Nall     = header.nall         #Total number of particles
Masses   = header.massarr*1e10 #Masses of the particles in Msun/h
Omega_m  = header.omega_m      #value of Omega_m
Omega_l  = header.omega_l      #value of Omega_l
h        = header.hubble       #value of h
redshift = header.redshift     #redshift of the snapshot
Hubble   = 100.0*np.sqrt(Omega_m*(1.0+redshift)**3+Omega_l)#Value of H(z) in km/s/(Mpc/h)

# read positions, velocities and IDs of the particles
pos = readgadget.read_block(snapshot, "POS ", ptype)/1e3 #positions in Mpc/h
vel = readgadget.read_block(snapshot, "VEL ", ptype)     #peculiar velocities in km/s
ids = readgadget.read_block(snapshot, "ID  ", ptype)-1   #IDs starting from 0

In the simulations with massive neutrinos it is possible to read the positions, velocities and IDs of the neutrino particles. Notice that the field should contain exactly 4 characters, that can be blank: "POS ", "VEL ", "ID  ". The number in the name of the snapshot represents its redshift:

  • 000 ——> z=3

  • 001 ——> z=2

  • 002 ——> z=1

  • 003 ——> z=0.5

  • 004 ——> z=0