Working with Units

In general, the raw output of cosmological simulations is scale free: coordinates and velocities use comoving coordinates and values are given in terms of the reduced Hubble constant h.

OpenCosmo allows you to convert between these conventions as needed based on what is most convinient for your analysis. The default convention is “comoving.” This leaves coordinates and velocities in comoving units, but absorbs the h factor into the values of the data. The other available conventions are “physical”, “scalefree” and “unitless.” Note that “scalefree” and “unitless” will have the same numerical values.

You can readily convert between these conventions with the opencosmo.Dataset.with_units() method:

import opencosmo as oc

ds = oc.read("galaxyproperties.hdf5")
ds = ds.with_units("physical")

You can also have work with a datasets in multiple unit conventions at the same time.

import opencosmo as oc

# comoving is the default
ds_comoving = oc.read("galaxyproperties.hdf5")
ds_physical = ds_comoving.with_units("physical")
# ds_comoving is not changed

When you filter a dataset with opencosmo.Dataset.filter(), the filtering will be performed in the same unit convention as the dataset. If you provide a unitless value to a filter, it will be interpreted as a value with the same units as the column in question.

import opencosmo as oc

ds = oc.read("haloproperties.hdf5").with_units("scalefree")
ds_filtered = ds.filter(ds.col("fof_halo_mass") > 1e10)
# This will filter out halos with mass greater than 1e10 * h^-1 Msun
ds_comoving = ds.with_units("comoving")
ds_filtered_comoving = ds_comoving.filter(ds_comoving.col("fof_halo_mass") > 1e10)
# This will filter out halos with mass greater than 1e10 Msun (no factor of h!)

If you change unit conventions after performing a filter, the filter will still be applied in the original unit convention. For example:

import opencosmo as oc

ds = oc.read("haloproperties.hdf5").with_units("scalefree")
ds_filtered = ds.filter(ds.col("fof_halo_mass") > 1e10).with_units("comoving")
print(ds_filtered.data["fof_halo_mass"].min())

should output a value of ~1.5 x 10^10 Msun, which is about 1e10/0.67.