BDM

BDM tables store the halo catalogues determined using the Bound Density Maximum (BDM) algorithm. Two different versions exist, using either the standard overdensity criterion with 360*ρback (background density; BDMV) to define halos, or halos are cut-off at 200*ρcrit (critical density; BDMW).

Halos are divided into two groups: distinct halos and subhalos (see explanation and illustration at Halo finders).
Distinct halos are those which are not lying inside the virial radius of a larger halo (i.e. halos with
larger virial radius) and the value of their hostFlag is “-1”, subhalos get the bdmId of the parent halo.

NOTE: Subhalos which lie at the opposite side of the (periodic) cosmological box from their host are not marked as subhalos in the current version of BDM halos!

Column Type UCD Unit Description
bdmId bigint meta.id; meta.main unique id for bdm-halo, e.g.

bdmId = ( snapnum*108 +(number in BDM catalogue) )

The power of 10 can be different for each simulation database.

snapnum smallint time.epoch number of snapshot
NInCat int meta.id number of halo in BDM catalogue file (= “Nhalo”)
hostFlag bigint meta.id index (flag) indicating if substructure or host; -1 for distinct halo, bdmId of parent halo for subhalos
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 in halo
Mvir real phys.mass 1/h Msun halo mass – mass of bound particles within Rvir
Mtot real phys.mass 1/h Msun halo mass – mass of all particles within Rvir
Rvir real phys.size.radius 1/h Mpc;
for MDPL: 1/h kpc
virial radius
NOTE: For a small fraction of distinct halos the ‘virial’ radius
is smaller than the formal virial radius. This is due to overlapping of distinct halos.
If radii of two halos overlap, then the smaller halo has its radius reduced. This affects
about 2% of halos.
Vrms real phys.veloc.dispMeasure km/s root mean square proper velocities of bound dark matter particles
Vcir real phys.veloc km/s maximum circular velocity in the halo
conc real phys.size; arith.ratio concentration of the halo as found from Vcirc / sqrt(G Mvir / Rvir) assuming a NFW profile
Xoff real phys.size; arith.ratio offset parameter = ratio of (distance from the halo center to the center of mass) to (the virial radius)
virialRatio real arith.ratio virial ratio = 2 K/Epot - 1, K = kinetic energy, Epot = potential energy
spin real spin parameter with K used instead of Etot. Use virialRatio for the definition of Etot.
Rrms real phys.size.radius 1/h Mpc;
for MDPL: 1/h kpc
root mean square distance of halo particles Rrms = sqrt(sum(r_i**2))
axisratio_2_1 real ratio of intermediate to major axis (as estimated in a spherical region via the modified inertia tensor)
axisratio_3_1 real ratio of minor to major axis
axis1_x real x-component of major axis (eigenvector with largest eigenvalue, from inertia tensor)
axis1_y real y-component of major axis
axis1_z real z-component of major axis
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, i.e. index along the Peano-Hilbert curve

Example

Get the 10 most massive BDM halos at z=0:

SELECT * FROM MDR1.BDMV WHERE snapnum = 85
ORDER BY Mvir DESC
LIMIT 10

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

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