import numpy as np

M = 5
a = 5

def model_density_distribution(r_bins: np.ndarray):
    """
    Generate a density distribution for a spherical galaxy model, as per the Hernquist model.
    See https://doi.org/10.1086%2F168845 for more information.
    """
    rho = M / (2 * np.pi) * a / (r_bins * (r_bins + a)**3)
    return rho