  We have reviewed the characteristics of the extended red emission (ERE) as
observed in many dusty astronomical environments, in particular, the diffuse
interstellar medium of the Galaxy. The spectral nature and the photon
conversion efficiency of the ERE identify the underlying process as highly
efficient photoluminescence by an abundant component of interstellar dust. We
have compared the photoluminescence properties of a variety of carbon- and
silicon-based materials proposed as sources for the ERE with the
observationally established constraints. We found that silicon nanoparticles
provide the best match to the spectrum and the efficiency requirement of the
ERE. If present in interstellar space with an abundance sufficient to explain
the intensity of the ERE, silicon nanoparticles will also contribute to the
interstellar 9.7 micron Si-O stretch feature in absorption, to the near- and
mid-IR nonequilibrium thermal background radiation, and to the continuum
extinction in the near- and far-UV. About 36% of the interstellar silicon
depleted into the dust phase would be needed in the form of silicon
nanoparticles, amounting to less than 5% of the interstellar dust mass. We
propose that silicon nanoparticles form through the nucleation of SiO in
oxygen-rich stellar mass outflows and that they represent an important
small-grain component of the interstellar dust spectrum.
