%%time
import yt

ds = yt.load('IsolatedGalaxy/galaxy0030/galaxy0030')

# 'deposit' means this is a deposited field - i.e. yt needs to construct it using a CIC operation.
# 'all' means that this field is based on all of the particles.  This is only important if your dataset has multiple particle types (i.e. dark matter, stars, etc.)
# 'cic' means we are asking for a cloud-in-cell deposition.  There's also 'density' (nearest-neighbot) 'mass' and 'count'.
field = ('deposit', 'all_cic')

ad = ds.all_data()

print ad[field]

slc = yt.SlicePlot(ds, 2, field)

slc.show()

[field for field in ds.derived_field_list if field[0] == 'deposit']

n_particles = len(ds.all_data()['all', 'particle_mass'])
print 'This dataset contains %s particles' % n_particles