def _surfdens(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _surfdens
        PURPOSE:
           evaluate the surface density for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the surface density
        HISTORY:
           2018-08-19 - Written - Bovy (UofT)
        """
        return (3.-self.alpha)/2./nu.pi*z*R**-self.alpha\
            *special.hyp2f1(0.5,self.alpha/2.,1.5,-(z/R)**2)