On some l-adic representations of Gal(Qbar/Q) attached to noncongruence subgroups

The l-adic parabolic cohomology groups attached to noncongruence subgroups of SL_2(Z) are finite-dimensional representations of Gal(Qbar/F) for some number field F. We exhibit examples (with F=Q) giving rise to Galois representations whose images are open subgroups of the full group of symplectic similitudes (of arbitrary dimension). The determination of the image of the Galois group relies on Katz's classification theorem for reductive subalgebras of gl(N) containing a principal nilpotent element; the paper also contains a short conceptual proof of this theorem, suggested by I. Gronowski.