[BUG] Make Gizmo's creation_time field robust against empty chunks

yt/frontends/gizmo/fields.py

@@ -147,13 +147,16 @@ def _metal_density_field(data):
             setup_magnetic_field_aliases(self, ptype, magnetic_field)
 
     def setup_star_particle_fields(self, ptype):
-        def _creation_time(data):
+        def _creation_time(field, data):
+            t_form = data[ptype, "StellarFormationTime"]
+
+            if t_form.size == 0:
+                return data.ds.arr([], data.ds.unit_system["time"])
+
             if data.ds.cosmological_simulation:
-                a_form = data[ptype, "StellarFormationTime"]
-                z_form = 1 / a_form - 1
+                z_form = 1 / t_form - 1
                 creation_time = data.ds.cosmology.t_from_z(z_form)
             else:
-                t_form = data[ptype, "StellarFormationTime"]
                 creation_time = data.ds.arr(t_form, "code_time")
             return creation_time
 

yt/frontends/gizmo/tests/test_outputs.py

@@ -131,3 +131,31 @@ def test_star_particle_fields():
     derived_fields = ["creation_time", "age"]
     for field in derived_fields:
         assert (ptype, field) in ds.derived_field_list
+
+
+@requires_module("h5py")
+@requires_file("FIRE_M12i_ref11/snapshot_600.hdf5")
+def test_gizmo_creation_time_handles_empty_particle_chunks():
+    ds = yt.load("FIRE_M12i_ref11/snapshot_600.hdf5")
+
+    def young_stars(pfilter, data):
+        age = data[pfilter.filtered_type, "age"]
+        return age.in_units("Myr") <= 10
+
+    yt.add_particle_filter(
+        "young_stars",
+        function=young_stars,
+        filtered_type="PartType4",
+        requires=["age"],
+    )
+
+    ds.add_particle_filter("young_stars")
+
+    ad = ds.all_data()
+    com = ad.quantities.center_of_mass(
+        use_gas=False,
+        use_particles=True,
+        particle_type="young_stars",
+    )
+
+    assert com.shape == (3,)