8. Since the flux from a source scales as S=L/(4.pi.D_L²), whilst the solid angle subtended by the object goes like O=(l/D_A)², the surface brightness S/O scales like (D_A/D_L)². Since D_A=D_L/(1+z)², it follows that surface brightness scales as (1+z)^-4. So it drops rapidly at high redshift.