How is the "nodeMass" property defined?

Created by Andrew Benson on on 2010-11-01
mass definition
Last updated by:
Andrew Benson on on 2010-11-01

The nodeMass property is defined to be the total mass of each node in a merger tree. Therefore, it includes both dark and baryonic mass. Additionally, the mass of a node includes the mass of any satellite nodes that it may contain. The mean density of the node depends on the "virialDensityContrastMethod" parameter. By default this uses the 'spherical top hat' method which computes the density of the halos using the spherical top hat collapse model. The other option currently available is 'Bryan + Norman' which uses the fitting functions from to determine the halo density.

In either case, you can compute the mean density of the resulting halos at their virial radii. If you set "outputVirialData" to "true" then the quantity "nodeVirialRadius" will be output. You can find the mean density of each halo then using:

density = 3 nodeMass / 4 Pi nodeVirialRadius^3

and can convert it to a density contrast by dividing by the mean density of the Universe in whatever cosmology you chose.

If you need to compare with specific data, e.g. M200, then you can compute M200 from the output properties of the halos (e.g. using nodeMass, nodeVirialRadius and, if you're using NFW halos, darkMatterScaleRadius) and integrating the density profile appropriately.