Commit 971c1ed3 authored by Matthieu Schaller's avatar Matthieu Schaller
Browse files

Clarifications in the documentation of the EAGLE SF model

parent ae708c04
...@@ -424,9 +424,11 @@ pc^2} \right)^{-n} \times \left(\frac{\gamma}{G_{\rm N}}f_gP\right)^{(n-1)/2}`, ...@@ -424,9 +424,11 @@ pc^2} \right)^{-n} \times \left(\frac{\gamma}{G_{\rm N}}f_gP\right)^{(n-1)/2}`,
where :math:`n` is the exponent of the Kennicutt-Schmidt relation (typically where :math:`n` is the exponent of the Kennicutt-Schmidt relation (typically
:math:`n=1.4`) and :math:`A` is the normalisation of the law (typically :math:`n=1.4`) and :math:`A` is the normalisation of the law (typically
:math:`A=1.515\times10^{-4} {\rm M_\odot}~{\rm yr^{-1}}~{\rm kpc^{-2}}` for a :math:`A=1.515\times10^{-4} {\rm M_\odot}~{\rm yr^{-1}}~{\rm kpc^{-2}}` for a
Cabrier IMF). :math:`m_g` is the gas particle mass, :math:`\gamma` is the Chabrier IMF). :math:`m_g` is the gas particle mass, :math:`\gamma` is the
adiabatic index, :math:`f_g` the gas fraction of the disk and :math:`P` the adiabatic index, :math:`f_g` the gas fraction of the disk and :math:`P` the
total pressure of the gas including any subgrid turbulent terms. total pressure of the gas including any subgrid turbulent terms. The star
formation rate of the gas particles is stored in the particles and written to
the snapshots.
Once a gas particle has computed its star formation rate, we compute the Once a gas particle has computed its star formation rate, we compute the
probability that this particle turns into a star using :math:`Prob= probability that this particle turns into a star using :math:`Prob=
...@@ -434,7 +436,7 @@ probability that this particle turns into a star using :math:`Prob= ...@@ -434,7 +436,7 @@ probability that this particle turns into a star using :math:`Prob=
and convert the gas particle into a star or not depending on our luck. and convert the gas particle into a star or not depending on our luck.
The density threshold itself has a metallicity dependence. We use the *smoothed* The density threshold itself has a metallicity dependence. We use the *smoothed*
metallicty (metal mass fraction) of the gas (See :ref:`EAGLE_chemical_tracers`) metallicity (metal mass fraction) of the gas (See :ref:`EAGLE_chemical_tracers`)
and apply the relation :math:`n^*_{\rm H} = n_{\rm H,norm}\left(\frac{Z_{\rm and apply the relation :math:`n^*_{\rm H} = n_{\rm H,norm}\left(\frac{Z_{\rm
smooth}}{Z_0}\right)^{n_{\rm Z}}`, alongside a maximal value. The model is smooth}}{Z_0}\right)^{n_{\rm Z}}`, alongside a maximal value. The model is
designed such that star formation threshold decreases with increasing designed such that star formation threshold decreases with increasing
...@@ -445,10 +447,10 @@ the figure below. ...@@ -445,10 +447,10 @@ the figure below.
:width: 400px :width: 400px
:align: center :align: center
:figclass: align-center :figclass: align-center
:alt: Metal-dependance of the threshold for star formation in the :alt: Metal-dependence of the threshold for star formation in the
EAGLE model. EAGLE model.
The dependency of the SF threshold density on the metallicty of the gas The dependency of the SF threshold density on the metallicity of the gas
in the EAGLE model (black line). The function is described by the four in the EAGLE model (black line). The function is described by the four
parameters indicated on the figure. These are the slope of the parameters indicated on the figure. These are the slope of the
dependency, its position on the metallicity-axis and normalisation dependency, its position on the metallicity-axis and normalisation
...@@ -485,7 +487,7 @@ the relevant YAML parameters defining it is shown on the figure below. ...@@ -485,7 +487,7 @@ the relevant YAML parameters defining it is shown on the figure below.
ones assumed in the reference EAGLE model. ones assumed in the reference EAGLE model.
In EAGLE, an entropy floor is already in use, so that the pressure of the gas is In EAGLE, an entropy floor is already in use, so that the pressure of the gas is
mentained high enough to prvent fragmentation of the gas. In such a scenario, maintained high enough to prevent fragmentation of the gas. In such a scenario,
there is no need for the internal EoS described above. And, of course, in such a there is no need for the internal EoS described above. And, of course, in such a
scenario, the gas can have a pressure above the floor. The code hence uses scenario, the gas can have a pressure above the floor. The code hence uses
:math:`P = \max(P_{\rm gas}, P_{\rm floor}, P_{\rm EoS})`. :math:`P = \max(P_{\rm gas}, P_{\rm floor}, P_{\rm EoS})`.
...@@ -515,6 +517,25 @@ The code applying this star formation law is located in the directory ...@@ -515,6 +517,25 @@ The code applying this star formation law is located in the directory
``src/star_formation/EAGLE/``. To simplify things, all constants are converted ``src/star_formation/EAGLE/``. To simplify things, all constants are converted
to the internal system of units upon reading the parameter file. to the internal system of units upon reading the parameter file.
Snapshots contain an extra field to store the star formation rates of the gas
particles. If a particle was star forming in the past but isn't any more, the
field will contain negative number either corresponding to the last
scale-factor where the particle was star forming (cosmological runs) or the last
time where it was star forming (non-cosmological runs).
+------------------------+--------------------------------------+-------------+-------------------------------------+
| Name | Description | Units | Comments |
+========================+======================================+=============+=====================================+
| ``StarFormationRates`` | | Star formation rates of the gas if | [U_M / U_t] | | The quantity is not drifted so |
| | | star forming. Negative numbers | | | corresponds to the rate the last |
| | | indicate the last time the gas was | | | time the particle was active. |
| | | star-forming. | | | |
+------------------------+--------------------------------------+-------------+-------------------------------------+
Note that the star formation rates are expressed in internal units and not in
solar masses per year as is the case in many other codes. This choice ensures
consistency between all the fields written to the snapshots.
For a normal EAGLE run, that section of the parameter file reads: For a normal EAGLE run, that section of the parameter file reads:
.. code:: YAML .. code:: YAML
...@@ -527,8 +548,8 @@ For a normal EAGLE run, that section of the parameter file reads: ...@@ -527,8 +548,8 @@ For a normal EAGLE run, that section of the parameter file reads:
KS_normalisation: 1.515e-4 # Normalization of the Kennicutt-Schmidt law in Msun / kpc^2 / yr. KS_normalisation: 1.515e-4 # Normalization of the Kennicutt-Schmidt law in Msun / kpc^2 / yr.
KS_exponent: 1.4 # Exponent of the Kennicutt-Schmidt law. KS_exponent: 1.4 # Exponent of the Kennicutt-Schmidt law.
min_over_density: 57.7 # Over-density above which star-formation is allowed. min_over_density: 57.7 # Over-density above which star-formation is allowed.
KS_high_density_threshold_H_p_cm3: 1e3 # Hydrogen number density above which the Kennicut-Schmidt law changes slope in Hydrogen atoms per cm^3. KS_high_density_threshold_H_p_cm3: 1e3 # Hydrogen number density above which the Kennicutt-Schmidt law changes slope in Hydrogen atoms per cm^3.
KS_high_density_exponent: 2.0 # Slope of the Kennicut-Schmidt law above the high-density threshold. KS_high_density_exponent: 2.0 # Slope of the Kennicutt-Schmidt law above the high-density threshold.
EOS_entropy_margin_dex: 0.5 # (Optional) Logarithm base 10 of the maximal entropy above the EOS at which stars can form. EOS_entropy_margin_dex: 0.5 # (Optional) Logarithm base 10 of the maximal entropy above the EOS at which stars can form.
KS_max_density_threshold_H_p_cm3: 1e5 # (Optional) Hydrogen number density above which a particle gets automatically turned into a star in Hydrogen atoms per cm^3. KS_max_density_threshold_H_p_cm3: 1e5 # (Optional) Hydrogen number density above which a particle gets automatically turned into a star in Hydrogen atoms per cm^3.
threshold_norm_H_p_cm3: 0.1 # Normalisation of the metal-dependant density threshold for star formation in Hydrogen atoms per cm^3. threshold_norm_H_p_cm3: 0.1 # Normalisation of the metal-dependant density threshold for star formation in Hydrogen atoms per cm^3.
......
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