Units

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CAESAR leverages yt’s symbolic units. Every meaningful quantity should have a unit attached to it. One of this advantages this provides is that you no longer have to keep track of little h or remembering if you are dealing with comoving or physical coordinates.

Working with units

Let’s take a look at some quick examples of what the units look like, and how we might take advantage of the easy conversion methods. Say we have a CAESAR object with obj.simulation.redshift=2 . Caesar generally defaults its length units to be comoving kpc (kpccm):

In [1]: obj.galaxies[0].radii['total']
Out[1]: 22.2676089684 kpccm

Note the cm tacked on, which stands for comoving.

Because this particular galaxy is at z=2 we may want to convert that radius to physical kiloparsecs:

In [2]: obj.galaxies[0].radii['total'].to('kpc')
Out[2]: 7.42253557308 kpc

or to physical kpc/h (using obj.simulation.hubble_constant=0.7):

In [3]: obj.galaxies[0].radii['total'].to('kpc/h')
Out[3]: 5.19577490116 kpc/h

When adding and subtracting quantities, they will be all be converted to the units of the first quantity. You don’t have to worry about homogenizing the units yourself!

Assigning units

Quantities that are added or subtracted must have convertible units. This means you cannot add a simple number to a quantity with symbolic units; you must first assign a unit to that number (or array).

To assign a unit, you can use the yt functions YTQuantity and YTArray:

In [4]: from yt import YTQuantity
In [5]: x = YTQuantity(10, 'Mpc')
In [6]: print(x.to('kpc'))
Out[6]: 10000.0 kpc

Similarly, use YTArray for arrays.

Removing units

If you need to get rid of the units and return a value for any reason, simply append .d to the quantity:

In [7]: print(x.d)
Out[7]: 10
In [8]: print(x.to('kpc').d)
Out[8]: 10000.0

For further information and tutorials regarding yt’s units please visit the symbolic unit page.

The various units and unit systems that are available in yt are described here .