An a priori detectability proxy.

def detectability(self,**kwargs):
        """
        An a priori detectability proxy.
        """
        distance_modulus = kwargs.get('distance_modulus')
        distance = mod2dist(distance_modulus)
        stellar_mass = kwargs.get('stellar_mass')
        extension = kwargs.get('extension')

        # Normalized to 10^3 Msolar at mod=18
        norm = 10**3/mod2dist(18)**2
        detect = stellar_mass / distance**2
        detect /= norm