def _surfdens(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _surfdens
        PURPOSE:
           evaluate the surface density
        INPUT:
           R - Cylindrical Galactocentric radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           Sigma (R,z)
        HISTORY:
           2018-08-19 - Written - Bovy (UofT)
        """
        return nu.exp(-self._alpha*R)
NAME:
           _surfdens
        PURPOSE:
           evaluate the surface density
        INPUT:
           R - Cylindrical Galactocentric radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           Sigma (R,z)
        HISTORY:
           2018-08-19 - Written - Bovy (UofT)