We give a complete description of the arboreal Galois representation of a certain postcritically finite cubic polynomial over a large class of number fields and for a large class of basepoints. This is the first such example that is not conjugate to a power map, Chebyshev polynomial, or Lattès map. The associated Galois action on an infinite ternary rooted tree has Hausdorff dimension bounded strictly between that of the infinite wreath product of cyclic groups and that of the infinite wreath product of symmetric groups. We deduce a zero-density result for prime divisors in an orbit under this polynomial. We also obtain a zero-density result for the set of places of convergence of Newton's method for a certain cubic polynomial, thus resolving the first nontrivial case of a conjecture of Faber and Voloch.

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Let K be a number field and let S be a finite set of places of K which contains all the Archimedean places. For any φ(z) ∈ K(z) of degree d ≥ 2 which is not a d-th power in K(z), Siegel's theorem implies that the image set φ(K) contains only finitely many S-units. We conjecture that the number of such S-units is bounded by a function of |S| and d (independently of K and φ). We prove this conjecture for several classes of rational functions, and show that the full conjecture follows from the Bombieri-Lang conjecture.

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