Initializing the Satellite Galaxy
=================================

The satellite galaxy comprises two key components: the **galaxy potential** as well as the **particle distribution**
which is actually what ``GalaRP`` uses to trace orbits. 




The Satellite Galaxy Potential
------------------------------

The satellite galaxy potential can be any combination of ``Gala`` potentials. For example:

.. code-block:: python

    import gala.potential as gp
    
    dm = gp.BurkertPotential(rho=5.93e-25 * u.g / u.cm**3, r0=11.87 * u.kpc, units=galactic)

    stars = gp.MiyamotoNagaiPotential(m=10**9.7 * u.Msun, a=2.5 * u.kpc, b=0.5 * u.kpc, units=galactic)
    gas = gp.MiyamotoNagaiPotential(m=10**9.7 * u.Msun, a=3.75 * u.kpc, b=0.75 * u.kpc, units=galactic)

    pot = gp.CompositePotential(dm=dm, stars=stars, gas=gas)

In this case we return an explicit composite potential, where you can access the components easily using ``pot["dm"]``,
for example. 

``GalaRP`` also has built-in potentials under the ``builtins.satellites`` module.

Note that a ``GalaRP``-defined mass profile should be generated using the following:

.. code-block:: python

    mass_profile = grp.gen_mass_profile(pot)


Particle Initialization
-----------------------

To run a ``GalaRP`` sim, the user needs to define some set of particles through which the RP orbits are calculated. 
The main particle classes included are a uniform grid of equally spaced particles, as well as a particle distribution
matching the density distribtion of a double-exponential model:

.. math::
   n(R,z) = n(0, 0)\exp\left(-\frac{R}{h_R}\right)\exp\left(-\frac{|z|}{h_z}\right)

For a uniform grid with a maximum radius of 10 kpc:

.. code-block:: python

    particles = grp.UniformGrid(n_particles=150, Rmax=10)
    particles.generate(mass_profile=mass_profile)

This will create a uniform grid with 50 particles from ``-Rmax`` to ``Rmax``. It will also automatically initialize the 
particles with a velocity dispersion of 10 km/s along each axis.


.. raw:: html

   <img src="_static/plots/particles_and_potential.png" width="80%%"  style="margin-bottom: 32px;"/>