Galois level and congruence ideal for p-adic families of finite slope Siegel modular forms

We consider families of Siegel eigenforms of genus 2 and finite slope, defined as local pieces of an eigenvariety and equipped with a suitable integral structure. Under some assumptions on the residual image, we show that the image of the Galois representation associated with a family is big, in the sense that a Lie algebra attached to it contains a congruence subalgebra of non-zero level. We call the Galois level of the family the largest such level. We show that it is trivial when the residual representation has full image. When the residual representation is a symmetric cube, the zero locus defined by the Galois level of the family admits an automorphic description: it is the locus of points that arise from overconvergent eigenforms for GL(2), via a p-adic Langlands lift attached to the symmetric cube representation. Our proof goes via the comparison of the Galois level with a 'fortuitous' congruence ideal. Some of the p-adic lifts are interpolated by a morphism of rigid analytic spaces from an eigencurve for GL(2) to an eigenvariety for GSp(4), while the remainder appear as isolated points on the eigenvariety.