FOF

The FOF-tables contain the halo catalogues which were produced by Stefan Gottlöber using the Friends-of-Friends algorithm described in Halos gone MAD: The Halo Finder Comparison Project by Alexander Knebe et al. 2010.

FOF tables exist for different linking lengths, please look up the values in the corresponding table overview for each simulation in the Simulations chapter.
All FOF tables share the same structure as given below. All halos containing at least 20 particles are included.

Column Type UCD Unit Description
fofId bigint meta.id; meta.main unique id for FOF-group
usually calculated like this:

fofId = (snapnum*10 + level)*108 +(rank in FOF-file)

but powers of 10 vary with the simulation to which the FOF catalogue belongs

snapnum smallint time.epoch number of snapshot; look into redshifts-table to find the corresponding redshift
level smallint level of substructure, indicates the different linking lengths; also see linkLength-table
NInFile int meta.id line number of FOF-group in original catalogue file (= rank in file)
x real pos.cartesian.x 1/h Mpc comoving position, x-component
y real pos.cartesian.y 1/h Mpc comoving position, y-component
z real pos.cartesian.z 1/h Mpc comoving position, z-component
vx real phys.veloc; pos.cartesian.x km/s peculiar velocity, x-component
vy real phys.veloc; pos.cartesian.y km/s peculiar velocity, y-component
vz real phys.veloc; pos.cartesian.z km/s peculiar velocity, z-component
np int meta.number number of particles inside the FOF group (“halo”)
mass real phys.mass 1/h Msun total mass of the FOF group
size real phys.size.radius 1/h Mpc “radius” of the FOF group, if it were a sphere, computed based on the volume
disp real phys.dispMeasure 1/h Mpc dispersion
disp_v real phys.veloc.dispersion km/s velocity dispersion
delta real phys.density rho_b overdensity (mass/volume) in terms of mean background density
spin real spin parameter (Gottlöber & Yepes 2007, ApJ 664, 117)
angMom_x real phys.angMomentum; pos.cartesian.x 1/h2 Msun Mpc km/s angular momentum, x-component
angMom_y real phys.angMomentum; pos.cartesian.y 1/h2 Msun Mpc km/s angular momentum, y-component
angMom_z real phys.angMomentum; pos.cartesian.z 1/h2 Msun Mpc km/s angular momentum, z-component
angMom real phys.angMomentum 1/h2 Msun Mpc km/s absolute value of angular momentum
axis1 real major axis of the ellipsoidal shape, not normalized
axis2 real intermediate axis of the ellipsoidal shape, not normalized
axis3 real minor axis of the ellipsoidal shape, not normalized
axis1_x real direction of major axis, unit eigenvector of the inertia tensor, x-component
axis1_y real direction of major axis, y-component
axis1_z real direction of major axis, z-component
axis2_x real direction of intermediate axis, unit eigenvector of the inertia tensor, x-component
axis2_y real direction of intermediate axis, y-component
axis2_z real direction of intermediate axis, z-component
axis3_x real direction of minor axis, unit eigenvector of the inertia tensor, x-component
axis3_y real direction of minor axis, y-component
axis3_z real direction of minor axis, z-component
ix int pos.cartesian.x spatial index, x-direction
iy int pos.cartesian.y spatial index, y-direction
iz int pos.cartesian.z spatial index, z-direction
phkey int Peano-Hilbert key

Example

Get the 10 most massive FOF groups at z=0 from the MDR1-database:

SELECT * FROM MDR1.FOF WHERE snapnum = 85 
ORDER BY mass DESC
LIMIT 10

This query retrieves the first 10 records from the FOF-table for redshift 0 (snapshot number 85) after sorting them by decreasing mass and hence returns the most massive objects.

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