clusters/halo: Add function "speed_sound()"

Also add "gamma" the adiabatic index of ideal gas to utils/units.py

1 files changed, 28 insertions, 0 deletions

diff --git a/fg21sim/extragalactic/clusters/helper.py b/fg21sim/extragalactic/clusters/helper.py
index 1c6e48e..d77c63e 100644
--- a/fg21sim/extragalactic/clusters/helper.py
+++ b/fg21sim/extragalactic/clusters/helper.py
@@ -33,6 +33,10 @@ References
 .. [zandanel2014]
    Zandanel, Pfrommer & Prada 2014, MNRAS, 438, 124
    http://adsabs.harvard.edu/abs/2014MNRAS.438..124Z
+
+.. [zhuravleva2014]
+   Zhuravleva et al. 2014, Nature, 515, 85;
+   http://adsabs.harvard.edu/abs/2014Natur.515...85Z
 """
 
 import logging
@@ -256,6 +260,30 @@ def density_energy_electron(spectrum, gamma):
     return e_re
 
 
+def speed_sound(kT):
+    """
+    The adiabatic sound speed in cluster ICM.
+
+    Parameters
+    ----------
+    kT : float
+        The cluster ICM temperature
+        Unit: [keV]
+
+    Returns
+    -------
+    cs : float
+        The speed of sound in cluster ICM.
+        Unit: [km/s]
+
+    Reference: Ref.[zhuravleva2014],Appendix(Methods)
+    """
+    # The gas adiabatic index
+    gamma = AC.gamma
+    cs = np.sqrt(gamma * kT*AUC.keV2erg / (AC.mu * AC.u))  # [cm/s]
+    return cs * AUC.cm2km  # [km/s]
+
+
 def velocity_impact(M_main, M_sub, z=0.0):
     """
     Estimate the relative impact velocity between the two merging