def get_galaxies_in_orbit(m_a=10.e11|units.MSun,
m_b=10.e11|units.MSun,
ecc=0.5,
r_min=25.|units.kpc,
t_start=None):
"""
binary galaxy with orbit of given parameters
-- if ecc=>1, start at t_start (p625, t_start=-10=-10*100Myr)
-- if ecc<1, start at apocenter (p644)
"""
converter=nbody_system.nbody_to_si(m_a+m_b,1|units.kpc)
semi = r_min/(1.-ecc)
# relative position and velocity vectors at the pericenter using kepler
kepler = Kepler_twobody(converter)
kepler.initialize_code()
kepler.initialize_from_elements(mass=(m_a+m_b), semi=semi, ecc=ecc, periastron=r_min) # at periastron
# evolve back till initial position
if ( ecc<1. ):
kepler.return_to_apastron()
else:
kepler.transform_to_time(t_start)
# get time of the orbit
t_orbit = kepler.get_time()
rl = kepler.get_separation_vector()
r = [rl[0].value_in(units.AU), rl[1].value_in(units.AU), rl[2].value_in(units.AU)] | units.AU
vl = kepler.get_velocity_vector()
v = [vl[0].value_in(units.kms), vl[1].value_in(units.kms), vl[2].value_in(units.kms)] | units.kms
kepler.stop()
# assign particle atributes
galaxies = Particles(2)
galaxies[0].mass = m_a
galaxies[0].position = (0,0,0) | units.AU
galaxies[0].velocity = (0,0,0) | units.kms
galaxies[1].mass = m_b
galaxies[1].position = r
galaxies[1].velocity = v
# identification
galaxies[0].id = 'a0'
galaxies[1].id = 'b0'
galaxies.move_to_center()
return galaxies, t_orbit
