Modelling the Spoon IRS diagnostic diagram
  We explore whether our models for starbursts, quiescent star-forming galaxies
and for AGN dust tori are able to model the full range of IRS spectra measured
with Spitzer. The diagnostic plot of 9.7 mu silicate optical depth versus 6.2
mu PAH equivalent width, introduced by Spoon and coworkers in 2007, gives a
good indication of the age and optical depth of a starburst, and of the
contribution of an AGN dust torus. However there is aliasing between age and
optical depth at later times in the evolution of a starburst, and between age
and the presence of an AGN dust torus.
  Modeling the full IRS spectra and using broad-band 25-850 mu fluxes can help
to resolve these aliases. The observed spectral energy distributions require
starbursts of a range of ages with initial dust optical depth ranging from
50-200, optically thin dust emission ('cirrus') illuminated by a range of
surface brightnesses of the interstellar radiation field, and AGN dust tori
with a range of viewing angles.
