.. _mockstreams:
*******************************
Generating mock stellar streams
*******************************
Introduction
============
This module contains functions for generating mock stellar streams using a
variety of methods that approximate the formation of streams in N-body
simulations.
Some imports needed for the code below::
>>> import astropy.units as u
>>> import numpy as np
>>> import gala.potential as gp
>>> import gala.dynamics as gd
>>> from gala.dynamics.mockstream import mock_stream
>>> from gala.units import galactic
.. warning::
The API for this subpackage is experimental and may change in a future
release. This code has not been tested extensively -- feedback is welcome!
Model parametrization
=====================
The core implementation follows the parametrization used by [fardal15]_ (except
where noted below), which outlines a general way to specify the position and
velocity distributions of stars stripped from the progenitor system at the
time of stripping. In cylindrical coordinates :math:`(R,\phi,z)` in the orbital
plane of the progenitor system (at position :math:`(R_p,\phi_p,z_p)`) the
position of the
:math:`i` th star at the time it is stripped from the satellite is given by:
.. math::
R_i &= R_p \pm k_R\,r_{\rm tide}\\
\phi_i &= \phi_p + k_\phi\,\frac{r_{\rm tide}}{R_p}\\
z_i &= k_z\,r_{\rm tide}
where :math:`r_{\rm tide}` is the tidal radius and the parameters
:math:`(k_R,k_\phi,k_z)` allow completely general specification of the
distribution of stripped stars (e.g., they may be constants, drawn from
probability distributions, functions of time). Similarly, in velocity:
.. math::
v_{R,i} &= v_{R,p} + k_{vR}\,v_{R,p}\\
v_{\phi,i} &= v_{\phi,p} + k_{v\phi}\,\Omega_p\,k_R\,r_{\rm tide}\\
v_{z,i} &= v_{z,p} + k_{vz}\,\Omega_p\,r_{\rm tide}
where :math:`\Omega_p` is the orbital frequency of the progenitor.
Because this framework is very general, we can use it to reproduce many of
the methods that have been used in the literature. Two examples are included
below: the *Streakline* method from [kuepper12]_ and the method from
[fardal15]_.
.. note::
The method outlined above allows for arbitrary mass-loss histories or
distributions of stripping times, however, currently only uniform mass-loss
is supported.
Streakline streams
------------------
In the *Streakline* method, star particles are released with the same angular
velocity as the satellite at the tidal radius with no dispersion. This is
equivalent to fixing the :math:`k` parameters to:
.. math::
k_R &= 1.\\
k_{v\phi} &= 1.\\
k_\phi &= k_z = k_{vR} = k_{vz} = 0
We can set these parameters when generating a stream model with
`~gala.dynamics.mockstream.mock_stream`, but first we need to specify a
potential and integrate the orbit of the progenitor system. We'll use a
spherical NFW potential, a progenitor mass :math:`m=10^4~{\rm M}_\odot` and
initial conditions that place the progenitor on a mildly eccentric orbit:
>>> pot = gp.NFWPotential.from_circular_velocity(v_c=250*u.km/u.s,
... r_s=10*u.kpc,
... units=galactic)
>>> prog_mass = 1E4*u.Msun
>>> prog_w0 = gd.PhaseSpacePosition(pos=[15, 0, 0.]*u.kpc,
... vel=[75, 150, 30.]*u.km/u.s)
>>> prog_orbit = gp.Hamiltonian(pot).integrate_orbit(prog_w0, dt=0.5, n_steps=4000)
>>> fig = prog_orbit.plot(['x', 'y'])
.. plot::
:align: center
:context: close-figs
import astropy.units as u
import numpy as np
import gala.potential as gp
import gala.dynamics as gd
from gala.units import galactic
pot = gp.NFWPotential.from_circular_velocity(v_c=250*u.km/u.s, r_s=10*u.kpc,
units=galactic)
prog_mass = 1E4*u.Msun
prog_w0 = gd.PhaseSpacePosition(pos=[15, 0, 0.]*u.kpc,
vel=[75, 150, 0.]*u.km/u.s)
prog_orbit = gp.Hamiltonian(pot).integrate_orbit(prog_w0, dt=0.5, n_steps=4000)
fig = prog_orbit.plot(['x', 'y'])
We now have to define the ``k`` parameters. This is done by defining an iterable
with 6 elements corresponding to
:math:`(k_R, k_\phi, k_z, k_{vR}, k_{v\phi}, k_{vz})`. We also have to set the
dispersions in these parameters to 0.
>>> k_mean = [1., 0, 0, 0, 1., 0]
>>> k_disp = np.zeros(6)
Now we generate the mock stream. We will release a star particle every time-step
from both Lagrange points by setting ``release_every=1``:
>>> stream = mock_stream(pot, prog_orbit, prog_mass,
... k_mean=k_mean, k_disp=k_disp, release_every=1)
>>> fig = stream.plot(['x', 'y'], marker='.', alpha=0.25)
.. plot::
:align: center
:context: close-figs
from gala.dynamics.mockstream import mock_stream
k_mean = [1., 0, 0, 0, 1., 0]
k_disp = np.zeros(6)
stream = mock_stream(pot, prog_orbit, prog_mass,
k_mean=k_mean, k_disp=k_disp, release_every=1)
stream.plot(['x', 'y'], marker='.', alpha=0.25)
Or, zooming in around the progenitor:
.. plot::
:align: center
:context: close-figs
fig = stream.plot(['x', 'y'], marker='.', alpha=0.25)
prog_end = prog_orbit[-1]
fig.axes[0].set_xlim(prog_end.x.value-1., prog_end.x.value+1.)
fig.axes[0].set_ylim(prog_end.y.value-1., prog_end.y.value+1.)
Fardal streams
--------------
[fardal15]_ found values for the ``k`` parameters and their dispersions by
matching to *N*-body simulations. For this method, these are set to:
>>> k_mean = [2., 0, 0, 0, 0.3, 0]
>>> k_disp = [0.5, 0, 0.5, 0, 0.5, 0.5]
With the same potential and progenitor orbit as above, we now generate a mock
stream using this method:
>>> stream2 = mock_stream(pot, prog_orbit, prog_mass,
... k_mean=k_mean, k_disp=k_disp,
... release_every=1)
>>> fig = stream2.plot(['x', 'y'], marker='.', alpha=0.25)
.. plot::
:align: center
:context: close-figs
k_mean = [2., 0, 0, 0, 0.3, 0]
k_disp = [0.5, 0, 0.5, 0, 0.5, 0.5]
stream2 = mock_stream(pot, prog_orbit, prog_mass,
k_mean=k_mean, k_disp=k_disp, release_every=1)
stream2.plot(['x', 'y'], marker='.', alpha=0.25)
Or, again, zooming in around the progenitor:
.. plot::
:align: center
:context: close-figs
fig = stream2.plot(['x', 'y'], marker='.', alpha=0.25)
prog_end = prog_orbit[-1]
fig.axes[0].set_xlim(prog_end.x.value-1., prog_end.x.value+1.)
fig.axes[0].set_ylim(prog_end.y.value-1., prog_end.y.value+1.)
References
==========
.. [fardal15] `Fardal, Huang, Weinberg (2015) `_
.. [kuepper12] `Küpper, Lane, Heggie (2012) `_
API
---
.. automodapi:: gala.dynamics.mockstream
:no-heading:
:headings: ^^