A geometric approach to the sup-norm problem for automorphic forms: the case of newforms on $G{L}_2({\mathbb{F}}_q(T))$ with squarefree level

The sup-norm problem in analytic number theory asks for the largest value taken by a given automorphic form. We observe that the function-field version of this problem can be reduced to the geometric problem of finding the largest dimension of the $i$th stalk cohomology group of a given Hecke eigensheaf at any point. This problem, in turn, can be reduced to the intersection-theoretic problem of bounding the “polar multiplicities” of the characteristic cycle of the Hecke eigensheaf, which in known cases is the nilpotent cone of the moduli space of Higgs bundles. We solve this problem for newforms on $GL_2 (\mathbb A_{\mathbb F_q(t)})$ of squarefree level, leading to bounds on the sup-norm that are stronger than what is known in the analogous problem for newforms on $G{L}_2({\mathbb{A}}_{\mathbb{Q}})$ (i.e. classical holomorphic and Maass modular forms.)