3.3.19.1  :  FOF

The table stores all the friend-of-friends (FOF) groups extracted from the raw output from the Millennium-II Simulation. These FOF groups form the basis that the SUBFIND algorithm uses to detect the subhalos. For further description, see Boylan-Kolchin et al. 2009.

columntypeUCDunitdescription
fofId long meta.id; The id of this FOF group.
snapNum int time The snapshot number where this FOF group was identified.
This column corresponds to the snapnum column in the Snapshots table in the Millennium-II database.
redshift float time The redshift of the snapshot to which this FOF group belongs.
np int meta.number Number of simulation particles in this FOF group.
mass float phys.mass 1010 Msun/h The mass of the halo, calculated as np times the mass per particle.
x float pos.cartesian.x Mpc/h The X-coordinate of this FOF group's position in comoving coordinates (1).
y float pos.cartesian.y Mpc/h The Y-coordinate of this FOF group's position in comoving coordinates (1).
z float pos.cartesian.z Mpc/h The Z-coordinate of this FOF group's position in comoving coordinates (1).
ix integer pos.cartesian.x Zone index along x-direction, based on 20 bins (2)
See the text on spatial indexes.
iy integer pos.cartesian.x Zone index along y-direction, based on 20 bins (2)
See the text on spatial indexes.
iz integer pos.cartesian.x Zone index along z-direction, based on 20 bins (2)
See the text on spatial indexes.
phKey int The Peano-Hilbert index for the position of this halo.
cmx float pos.cartesian.x Mpc/h The X-coordinate of this FOF group's center of mass in comoving coordinates.
cmy float pos.cartesian.y Mpc/h The Y-coordinate of this FOF group's center of mass in comoving coordinates.
cmz float pos.cartesian.z Mpc/h The Z-coordinate of this FOF group's center of mass in comoving coordinates.
cvx float phys.veloc km/sec The X-component of the peculiar center-of-mass velocity of this FOF group.
cvy float phys.veloc km/sec The Y-component of the peculiar center-of-mass velocity of this FOF group.
cvz float phys.veloc km/sec The Z-component of the peculiar center-of-mass velocity of this FOF group.
m_crit200 float phys.mass 1010 Msun/h The mass within the radius where the FOF group has an overdensity 200 times the critical density of the simulation.
r_crit200 float phys.radius Mpc/h The radius within which the FOF group has an overdensity 200 times the critical density of the simulation.
m_mean200 float phys.mass 1010 Msun/h The mass within the radius where the FOF group has an overdensity 200 times the mean density of the simulation.
r_mean200 float phys.radius Mpc/h The radius within which the FOF group has an overdensity 200 times the mean density of the simulation.
m_tophat float phys.mass 1010 Msun/h The mass within the radius where the FOF group has an overdensity corresponding to the value at virialization in the top-hat collapse model for this cosmology.
r_tophat float phys.radius Mpc/h The radius within which the FOF group has an overdensity corresponding to the value at virialization in the top-hat collapse model for this cosmology.
numSubs integer Number of subhalos in this FOF halo. Note that the dominant subhalo counts as a subhalo.
firstSubhaloId subhaloId of the dominant subhalo within this FOF group (-1 if numSubs=0, i.e. if there are no bound substructures in the FOF group).

Notes:
(1) The (x, y, z) coordinates of FOF groups are determined as the minimum of the gravitational potential for the dominant subhalo. If there is no bound subhalo associated with a FOF group, then the coordinates (x, y, z) are set to the center-of-mass coordinates (cmx, cmy, cmz)
(2) The zone index for each FOF group is based on its (x, y, z) coordinates.