Spherical varieties and non-ordinary families of cohomology classes

Abstract

We give a construction of non-ordinary $p$-adic families of classes in the cohomology of locally symmetric spaces associated to spherical pairs of reductive groups. In the \'etale case, we show how to map these classes into Galois cohomology. The methods developed in this paper can be used to give new constructions of $p$-adic families of Euler systems and $p$-adic $L$-functions. As an example, we show how the constructions of this paper can be used to construct norm-compatible classes associated to non-ordinary Siegel modular forms, generalising $p$-part of the Lemma--Flach Euler system constructed by Loeffler--Skinner--Zerbes.