clusters: Update radio halo radius estimation

The simulated radio halo is assumed to have a radius of the falling
sub-cluster; while previously it is assumed to be 1/4 of the virial radius of
the merged cluster.

The new estimation can agree better with the currently observed radio halos,
which generally have a angular diameter size of ~2-7 [arcmin].

2 files changed, 25 insertions, 23 deletions

diff --git a/fg21sim/extragalactic/clusters/halo.py b/fg21sim/extragalactic/clusters/halo.py
index 832371e..6bfa4ca 100644
--- a/fg21sim/extragalactic/clusters/halo.py
+++ b/fg21sim/extragalactic/clusters/halo.py
@@ -166,13 +166,17 @@ class RadioHalo:
     @property
     def radius(self):
         """
-        The halo radius derived from the virial radius by a scaling
-        relation.
+        The estimated radius for the simulated radio halo.
+
+        NOTE
+        ----
+        The halo radius is assumed to be the virial radius of the falling
+        sub-cluster.  See ``helper.radius_halo()`` for more details.
 
         Unit: [kpc]
         """
-        mass = self.M_main + self.M_sub  # [Msun]
-        r_halo = helper.radius_halo(mass, self.z_merger)  # [kpc]
+        r_halo = helper.radius_halo(self.M_main, self.M_sub,
+                                    self.z_merger)
         return r_halo
 
     @property
diff --git a/fg21sim/extragalactic/clusters/helper.py b/fg21sim/extragalactic/clusters/helper.py
index b54340c..e553da9 100644
--- a/fg21sim/extragalactic/clusters/helper.py
+++ b/fg21sim/extragalactic/clusters/helper.py
@@ -54,16 +54,16 @@ def radius_virial(mass, z=0.0):
 
     Parameters
     ----------
-    mass : float
+    mass : float, `~numpy.ndarray`
         Total (virial) mass of the cluster
         Unit: [Msun]
-    z : float, optional
+    z : float, `~numpy.ndarray`, optional
         Redshift
         Default: 0.0 (i.e., present day)
 
     Returns
     -------
-    R_vir : float
+    R_vir : float, `~numpy.ndarray`
         Virial radius of the cluster
         Unit: [kpc]
     """
@@ -74,35 +74,33 @@ def radius_virial(mass, z=0.0):
     return R_vir
 
 
-def radius_halo(mass, z=0.0):
+def radius_halo(M_main, M_sub, z=0.0):
     """
-    Calculate the radius of (giant) radio halo for a cluster.
+    Calculate the (predicted) radius of (giant) radio halo for a cluster.
 
-    The halo radius is assumed to linearly scale with the virial radius,
-    and is estimated by:
-        R_halo = R_vir / 4
-    * halo radius is ~3-6 times smaller than the virial radius;
-      Ref.[cassano2007],Sec.(1)
-    * halo half radius is ~R500/4, therefore, R_halo ~ R_vir/4;
-      Ref.[zandanel2014],Sec.(6.2)
+    NOTE
+    ----
+    It can be intuitively assumed that a merger will generate turbulences
+    within a region of size of the falling sub-cluster.  And this
+    estimation can agree with the currently observed radio halos, which
+    generally have a angular diameter size ~2-7 [arcmin].
 
     Parameters
     ----------
-    mass : float
-        Total (virial) mass of the cluster
+    M_main, M_sub : float, `~numpy.ndarray`
+        Total (virial) masses of the main and sub clusters
         Unit: [Msun]
-    z : float, optional
+    z : float, `~numpy.ndarray`, optional
         Redshift
         Default: 0.0 (i.e., present day)
 
     Returns
     -------
-    R_halo : float
-        Radius of the (expected) giant radio halo
+    R_halo : float, `~numpy.ndarray`
+        Radius of the (simulated/predicted) giant radio halo
         Unit: [kpc]
     """
-    R_vir = radius_virial(mass=mass, z=z)  # [kpc]
-    R_halo = R_vir / 4.0  # [kpc]
+    R_halo = radius_virial(mass=M_sub, z=z)  # [kpc]
     return R_halo