Light rays are similar to light cones (Light Cone Generator) in how they stack multiple datasets together to span a redshift interval. Unlike light cones, which stack randomly oriented projections from each dataset to create synthetic images, light rays use thin pencil beams to simulate QSO sight lines. A sample script can be found in the cookbook under Light Ray.
A ray segment records the information of all grid cells intersected by the
ray as well as the path length, dl
, of the ray through the cell. Column
densities can be calculated by multiplying physical densities by the path
length.
Below follows the creation of a light ray from multiple datasets stacked together. However, a light ray can also be made from a single dataset. For an example of this, see cookbook-single-dataset-light-ray.
The arguments required to instantiate a
LightRay
object are the same as
those required for a
LightCone
object: the simulation parameter file, the
simulation type, the nearest redshift, and the furthest redshift.
from yt.analysis_modules.cosmological_observation.api import LightRay
lr = LightRay("enzo_tiny_cosmology/32Mpc_32.enzo",
simulation_type="Enzo",
near_redshift=0.0, far_redshift=0.1)
Additional keyword arguments are:
use_minimum_datasets
(bool): If True, the minimum number of datasets
is used to connect the initial and final redshift. If false, the light
ray solution will contain as many entries as possible within the redshift
interval. Default: True.deltaz_min
(float): Specifies the minimum Delta-z between
consecutive datasets in the returned list. Default: 0.0.minimum_coherent_box_fraction
(float): Used with
use_minimum_datasets
set to False, this parameter specifies the
fraction of the total box size to be traversed before rerandomizing the
projection axis and center. This was invented to allow light rays with
thin slices to sample coherent large scale structure, but in practice
does not work so well. Try setting this parameter to 1 and see what
happens. Default: 0.0.time_data
(bool): Whether or not to include time outputs when
gathering datasets for time series. Default: True.redshift_data
(bool): Whether or not to include redshift outputs
when gathering datasets for time series. Default: True.Once the LightRay object has been instantiated, the
LightRay,make_light_ray()
function will trace out the rays in each dataset and collect information for all the
fields requested. The output file will be an HDF5 file containing all the
cell field values for all the cells that were intersected by the ray. A
single LightRay object can be used over and over to make multiple
randomizations, simply by changing the value of the random seed with the
seed
keyword.
lr.make_light_ray(seed=8675309,
fields=['temperature', 'density'],
get_los_velocity=True)
The keyword arguments are:
seed
(int): Seed for the random number generator. Default: None.start_position
(list of floats): Used only if creating a light ray
from a single dataset. The coordinates of the starting position of the
ray. Default: None.end_position
(list of floats): Used only if creating a light ray
from a single dataset. The coordinates of the ending position of the ray.
Default: None.trajectory
(list of floats): Used only if creating a light ray
from a single dataset. The (r, theta, phi) direction of the light ray.
Use either end_position
or trajectory
, not both.
Default: None.fields
(list): A list of fields for which to get data.
Default: None.solution_filename
(string): Path to a text file where the
trajectories of each subray is written out. Default: None.data_filename
(string): Path to output file for ray data.
Default: None.get_los_velocity
(bool): If True, the line of sight velocity is
calculated for each point in the ray. Default: True.redshift
(float): Used with light rays made from single datasets to
specify a starting redshift for the ray. If not used, the starting
redshift will be 0 for a non-cosmological dataset and the dataset redshift
for a cosmological dataset. Default: None.njobs
(int): The number of parallel jobs over which the slices for
the halo mask will be split. Choose -1 for one processor per individual
slice and 1 to have all processors work together on each projection.
Default: 1Note
As of yt-3.0
, the functionality for recording properties of the nearest halo to each element of the ray no longer exists. This is still available in yt-2.x
. If you would like to use this feature in yt-3.x
, help is needed to port it over. Contact the yt-users mailing list if you are interested in doing this.
Once you have created a LightRay, you can use it to generate an
Absorption Spectrum. In addition, you can use the
RayCallback
to
Overplot the Path of a Ray on your plots.