  Giant impurity excitations with large binding energies are powerful probes
for exploring new regimes of far out of equilibrium dynamics in few- and
many-body quantum systems, as well as for in-situ observations of correlations.
Motivated by recent experimental progress in spectroscopic studies of Rydberg
excitations in ensembles of ultracold atoms, we develop a new theoretical
approach for describing multiscale dynamics of Rydberg excitations in quantum
Bose gases. We find that the crossover from few- to many-body dynamics
manifests in a dramatic change in spectral profile from resolved molecular
lines to broad Gaussian distributions representing a superpolaronic state in
which many atoms bind to the Rydberg impurity. We discuss signatures of this
crossover in the temperature and density dependence of the spectra.
