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# GadgetUnits.jl

This package provides some basic unit conversion functionality to work with the SPH code Gadget2 by Volker Springel. It is based on the unit conversions listed by Klaus Dolag on his HowTo page (restricted). Further functionality provides tools for cosmological simulations.

If you use `GadgetUnits.jl`

in publications please cite it.

# Unit Conversion

GadgetUnits.jl uses Unitful.jl and UnitfulAstro.jl to store the unit conversion factors with actual units in place.
You can convert the internal units of Gadget into cgs units by defining the object `GadgetPhysicalUnits`

:

```
GU = GadgetPhysicalUnits(l_unit::T=3.085678e21, m_unit::T=1.989e43, v_unit::T=1.e5;
a_scale::T=1.0, hpar::T=1.0,
γ_th::T=5.0/3.0, γ_CR::T=4.0/3.0, xH::T=0.76) where T
```

where the keyword arguments are:

`a_scale = 1.0`

: Cosmological scale factor of the simulation. Can be passed with the header`h`

as`h.time`

.`hpar = 1.0`

: Hubble constant as 'little h'. Can be passed with header`h`

as`h.h0`

.`γ_th = 5.0/3.0`

: Adiabatic index of gas.`γ_CR = 4.0/3.0`

: Adiabatic index of cosmic ray component.`xH = 0.76`

: Hydrogen fraction of the simulation, if run without chemical model.

This returns an object of type `GadgetPhysicalUnits`

with the following properties:

Field Name | Meaning |
---|---|

`x_cgs::T` |
position in cm |

`x_physical::T` |
position in kpc |

`v_cgs::T` |
velocity in cm/s |

`v_physical::T` |
velocity in km/s |

`m_cgs::T` |
mass in g |

`m_msun::T` |
mass in Msun |

`m_physical::T` |
mass in 10^10 Msun |

`t_s::T` |
time in sec |

`t_Myr::T` |
time in Myr |

`E_cgs::T` |
energy in erg |

`E_eV::T` |
energy in eV |

`B_cgs::T` |
magnetic field in Gauss |

`rho_physical::T` |
density in 10^10 Msun/kpc^3 |

`rho_cgs::T` |
density in `g/cm^3` |

`rho_ncm3::T` |
density in `n_p/cm^3` |

`T_K::T` |
temperature in K |

`T_eV::T` |
temperature in eV |

`P_th_cgs::T` |
thermal pressure in Ba |

`P_CR_cgs::T` |
cosmic ray pressure in Ba |

To convert, say positions of gas particles from a cosmological simulation to physical units you can use:

```
h = read_header(filename)
pos = read_snap(filename, "POS", 0)
GU = GadgetPhysicalUnits(a_scale=h.time, hpar=h.h0)
pos .*= GU.x_cgs
```

If you have different units than the standard Gadget ones you can call the object cunstructor with different values

`GU = GadgetPhysicalUnits(your_l_unit, your_m_unit, your_v_unit; kwargs...)`

Converting the units can then be done with Unitful.jl and UnitfulAstro.jl.
So if you want to convert the position units from the default `cm`

to `Mpc`

you can do this as:

```
using Unitful
using UnitfulAstro
pos = read_snap(filename, "POS", 0)
pos = @. pos * GU.x_cgs |> u"Mpc"
```

If you want to get rid of the units, for example if you need basic datatypes again for a function
you can use the funtion `ustrip`

:

`pos = ustrip(pos)`

## Primitive unit type

If you want to have the same functionality, but without using `Unitful.jl`

you can construct a similar object:

```
GU = GadgetPhysical(l_unit::T=3.085678e21, m_unit::T=1.989e43, v_unit::T=1.e5;
a_scale::T=1.0, hpar::T=1.0,
γ_th::T=5.0/3.0, γ_CR::T=4.0/3.0, xH::T=0.76) where T
```

This uses the same conversions, but leaves out the actual unit strings.