  We compute modifications to the jet spectrum in the presence of a dense
medium. We show that in the large-$N_c$ approximation and at leading
logarithmic accuracy the jet nuclear modification factor factorizes into a
quenching factor associated to the total jet color charge and a Sudakov
suppression factor which accounts for the energy loss of jet substructure
fluctuations. This factor, called the jet collimator, implements the fact that
subjets, that are not resolved by the medium, lose energy coherently as a
single color charge, whereas resolved large angle fluctuations suffer more
quenching. For comparison, we show that neglecting color coherence results in a
stronger suppression of the jet nuclear modification factor.
