Using CAESAR

Data Structure

Within the main.CAESAR object you will find a number of lists containing any number of group.Group objects. The primary lists are the halos and galaxies list; within each of these you will find every object identified by CAESAR. Below is a quick relationship diagram describing how objects are linked together:

CAESAR Data Structure

From this you can see that each Halo object has a list of galaxies, a link to its central galaxy, and a sub-list of satellite galaxies (those who are not a central). Each Galaxy object has a link to its parent Halo, a boolean describing if it is a central, and a sub-list linking to all of the satellite galaxies for its parent halo.

Usage

Usage of CAESAR all comes down to what you want to do. The real power of the code comes with the simple relationships that link each object. As we saw in the previous section, each group.Group has a set of relationships. We can exploit these to intuitively query our data. For example, say you wanted to get an array of all galaxy masses, how would you most efficiently do that? The easiest way (in my opinion) would be to use python’s list comprehension. Here is a quick example (assuming you have the main.CAESAR object loaded into the obj variable):

galaxy_masses = [i.masses['total'] for i in obj.galaxies]

This is basically a compact way of writing:

galaxy_masses = []
for gal in obj.galaxies:
    galaxy_masses.append(gal.masses['total'])

Now that in itself is not all that impressive. Things get a bit more interesting when we start exploiting the object relationships. As another example, say we wanted the parent halo mass of each galaxy? Lets see how that is done:

parent_halo_masses = [i.halo.masses['total'] for i in obj.galaxies]

Since each group.Galaxy has a link to its parent group.Halo, we have access to all of the parent halo’s attributes. We can also begin to add conditional statements to our list comprehension statements to further refine our results; let’s only look at the halo masses of massive galaxies:

central_galaxy_halo_masses = [i.halo.masses['total'] for i in obj.galaxies if i.masses['total'] > 1.0e12]

Obviously we can make these list comprehensions infinitely complicated, but I think you get the gist. The bottom line is: CAESAR provides a convenient and intuitive way to relate objects to one another.