Normal Subgroup Growth of Linear Groups: the (G2; F4;E8)-Theorem
  Let G be a finitely generated group and M_n(G) the number of its normal subgroup subgroups of index at most n. For linear groups G we show that M_n(G) can grow polynomially in n only if the semisimple part of the Zariski closure of G has simple components only of type G2, F4 or E8 (and in this case indeed this can happened!) 
Michael Larsen and Alexander Lubotzky
math.GR