The Widespread Presence of Nanometer-size Dust Grains in the
  Interstellar Medium of Galaxies
  Interstellar dust spans a wide range in size distribution, ranging from
ultrasmall grains of a few Angstroms to micrometer-size grains. While the
presence of nanometer-size dust grains in the Galactic interstellar medium was
speculated six decades ago and was previously suggested based on early infrared
observations, systematic and direct analysis of their properties over a wide
range of environments has been lacking. Here we report the detection of
nanometer-size dust grains that appear to be universally present in a wide
variety of astronomical environments, from Galactic high-latitude clouds to
nearby star-forming galaxies and galaxies with low levels of nuclear activity.
The prevalence of such a grain population is revealed conclusively as prominent
mid-infrared continuum emission at wavelength short than 10\mu m seen in the
Spitzer/IRS data, characterized by temperatures of ~300-400K that are
significantly higher than the equilibrium temperatures of common,
submicron-size grains in typical galactic environments. We propose that the
optimal carriers of this pervasive, featureless hot dust component are very
small carbonaceous (e.g., graphite) grains of nanometer size that are
transiently heated by single-photon absorption. This grain population accounts
for ~1.4% of the total infrared emission at ~5-3000\mu m and ~0.4% of the total
interstellar dust mass.
