Store BH birth density and metallicity

src/black_holes/EAGLE/black_holes.h

@@ -821,8 +821,9 @@ INLINE static void black_holes_create_from_gas(
    * in the FOF code. We update them here.
    * (i.e. position, velocity, mass, time-step have been set) */
 
-  /* Birth time */
+  /* Birth time and density */
   bp->formation_scale_factor = cosmo->a;
+  bp->formation_gas_density = hydro_get_physical_density(p, cosmo);
 
   /* Initial seed mass */
   bp->subgrid_mass = props->subgrid_seed_mass;

src/black_holes/EAGLE/black_holes_io.h

@@ -139,7 +139,7 @@ INLINE static void black_holes_write_particles(const struct bpart* bparts,
                                                int with_cosmology) {
 
   /* Say how much we want to write */
-  *num_fields = 29;
+  *num_fields = 30;
 
   /* List what we want to write */
   list[0] = io_make_output_field_convert_bpart(
@@ -328,6 +328,13 @@ INLINE static void black_holes_write_particles(const struct bpart* bparts,
       sound_speed_subgrid_gas,
       "Physical subgrid sound-speeds used in the subgrid-Bondi model.");
 
+  list[29] = io_make_output_field(
+      "BirthGasDensities", FLOAT, 1, UNIT_CONV_DENSITY, 0.f, bparts,
+      formation_gas_density,
+      "Physical densities of the converted part at the time of birth. "
+      "We store the physical density at the birth redshift, no conversion is "
+      "needed.");
+
 #ifdef DEBUG_INTERACTIONS_BLACK_HOLES
 
   list += *num_fields;

src/black_holes/EAGLE/black_holes_part.h

@@ -74,6 +74,9 @@ struct bpart {
     float formation_scale_factor;
   };
 
+  /*! Physical density of the converted part (internal units) */
+  float formation_gas_density;
+
   /*! Subgrid mass of the black hole */
   float subgrid_mass;
 

src/chemistry/EAGLE/chemistry.h

@@ -348,6 +348,9 @@ __attribute__((always_inline)) INLINE static void chemistry_bpart_from_part(
       p_data->metal_mass_fraction_from_AGB * gas_mass;
   bp_data->iron_mass_from_SNIa =
       p_data->iron_mass_fraction_from_SNIa * gas_mass;
+  bp_data->formation_metallicity = p_data->metal_mass_fraction_total;
+  bp_data->smoothed_formation_metallicity =
+      p_data->smoothed_metal_mass_fraction_total;
 }
 
 /**

src/chemistry/EAGLE/chemistry_io.h

@@ -271,7 +271,19 @@ INLINE static int chemistry_write_bparticles(const struct bpart* bparts,
                            "Masses of the BH particles in iron that have been "
                            "produced by SNIa stars");
 
-  return 9;
+  list[9] =
+      io_make_output_field("BirthMetallicities", FLOAT, 1, UNIT_CONV_NO_UNITS,
+                           0.f, bparts, chemistry_data.formation_metallicity,
+                           "Metallicities (metal mass fractions) of the gas "
+                           "particles the black holes formed from");
+
+  list[10] = io_make_output_field(
+      "SmoothedBirthMetallicities", FLOAT, 1, UNIT_CONV_NO_UNITS, 0.f, bparts,
+      chemistry_data.smoothed_formation_metallicity,
+      "Smoothed metallicities (metal mass fractions) of the gas particles the "
+      "black holes formed from");
+
+  return 11;
 }
 
 #ifdef HAVE_HDF5

src/chemistry/EAGLE/chemistry_struct.h

@@ -120,6 +120,12 @@ struct chemistry_bpart_data {
 
   /*! Iron mass coming from SNIa */
   float iron_mass_from_SNIa;
+
+  /*! Metallicity of converted part. */
+  float formation_metallicity;
+
+  /*! Smoothed metallicity of converted part. */
+  float smoothed_formation_metallicity;
 };
 
 #endif /* SWIFT_CHEMISTRY_STRUCT_EAGLE_H */