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Hello,
I have already calculated the stellar density projection for each slice, and now I am trying to calculate the mass of each galaxy in this slice. This is the code I used to calculate the stellar density projection. After reviewing your response in match-snapshot-particles-with-their-halosubhalo, I am still unable to compute the mass for each galaxy in this slice. Do you have any suggestions?

front = 'tngdm'
os.makedirs(f"{front}/star", exist_ok=True)

hdf5_file = 'sims.TNG/TNG300-1/simulation.hdf5'

part_len = 15625000000
loops = 500.0
bin_size = part_len/loops
x_min, x_max = 0,205000
y_min, y_max = 0,10250
z_min, z_max = 0,205000

total_particles_found = 0

for i in range(0, int(loops)):
    try:
        start = int(i*bin_size)
        stop = int((i+1)*bin_size - 1)

        print(f"Processing batch {i+1}/{int(loops)}: particles {start} to {stop}")

        with h5py.File(hdf5_file, 'r') as f:
            dm_positions = f['/Snapshots/99/PartType4/Coordinates'][start:stop, :]
            dm_mass = f['/Snapshots/99/PartType4/Masses'][start:stop]            

            w = np.where((dm_positions[:, 0] >= x_min) & (dm_positions[:, 0] <= x_max) &
                        (dm_positions[:, 1] >= y_min) & (dm_positions[:, 1] < y_max) &
                        (dm_positions[:, 2] >= z_min) & (dm_positions[:, 2] <= z_max))[0]

            if len(w) > 0:
                total_particles_found += len(w)
                print(f"Found {len(w)} particles in range for batch {i}")

                try:
                    coordinates_file = f"{front}/star/y9/coordinates_{i}.fits"
                    pos_table = Table([dm_positions[w, 0]/1000.0, 
                                     dm_positions[w, 1]/1000.0, 
                                     dm_positions[w, 2]/1000.0],
                                    names=('px', 'py', 'pz'))
                    pos_table.write(coordinates_file, format='fits', overwrite=True)


                    if os.path.exists(coordinates_file):
                        print(f"Successfully saved coordinates to {coordinates_file}")
                    else:
                        print(f"Warning: Failed to save coordinates file {coordinates_file}")


                    mass_file = f"{front}/star/y9/mass_{i}.fits"
                    mass_table = Table([dm_mass[w]],
                                        names=('masses',))
                    mass_table.write(mass_file, format='fits', overwrite=True)


                    if os.path.exists(mass_file):
                        print(f"Successfully saved density to {mass_file}")
                    else:
                        print(f"Warning: Failed to save density file {mass_file}")

                except Exception as e:
                    print(f"Error saving files for batch {i}: {str(e)}")
            else:
                print(f"No particles found in range for batch {i}")


        del dm_positions
        del dm_mass

    except Exception as e:
        print(f"Error processing batch {i}: {str(e)}")
        continue

print(f"Processing complete. Total particles found: {total_particles_found}")

What do you mean "the mass for each galaxy in this slice", what is your goal?

My main goal is to find the minimum galaxy mass of the stellar projection density field. And maybe the subhalo means the galaxy? I want to sum the stellar particles belonging to each subhalo to get the mass of each subhalo, but the calculation doesn't seem to provide subhalo-related information. However, if I follow the method you mentioned in the/match-snapshot-particles-with-their-halosubhalo , how can I obtain the stellar density field projection for each slice? I use the TNG300-1 data.

Every Subhalo (i.e. galaxy) has fields for stellar mass, and position (SubhaloMassType[4] and SubhaloPos). You can use these to find the minimum galaxy mass in any region of space, for example.