The discovery of presolar nanodiamonds (see Section 5.4) and TiC nanocrystals (see Section 5.5) in primitive meteorites implies that there must exist such nano-sized species in interstellar space. In addition, as already mentioned in Section 1, the presence of nanoparticles in the ISM is clearly indicated by --
(1) The ubiquitous distinctive set of
"UIR" emission bands at 3.3, 6.2, 7.7, 8.6, and 11.3 µm.
This emission, accounting for ~ 20% of
the total power radiated by dust, is closely explained
by transiently heated PAHs with an abundance
C / H 
45
parts per million (ppm) and a log-normal size distribution
peaking at ~ 6 Å (with ~ 100 carbon atoms; see
Li & Draine 2001b
for details).
(2) The mid-IR emission at
60
µm.
This emission, first discovered by the IRAS broadband
photometry at 12 and 25 µm and later confirmed by
the COBE-DIRBE observations, cannot be explained by
large grains (a
250 Å)
heated by the interstellar radiation field to equilibrium temperatures
15-25 K since the
predicted emission intensities
from large grains are smaller than the IRAS 12 and 25 µm
emission intensities by several orders of magnitude.
In the diffuse ISM, the emission
at
60 µm accounts for ~ 35% of
the total power radiated by dust. It is well recognized that
this emission arises from nanoparticles
(a
250 Å) stochastically heated by
single UV/visible photons to temperatures
significantly higher than their time-averaged temperatures (see
Draine & Li 2001
and references therein). Even at 60 µm, the nanoparticle
component contributes
~ 70% of the total emission power of the diffuse ISM
detected by the COBE-DIRBE photometers (see Fig.8 of
Li & Draine 2001b).
(3) To a lesser degree, the far-UV extinction
rise.
The far-UV interstellar extinction continues to rise up to
= 0.1
µm, and there does not appear to
be any evidence of saturation even at this wavelength
(see Whittet 2003).
Since it is generally true that a grain
absorbs and scatters light most effectively at wavelengths
comparable to its size
2
a
(Krügel 2003),
we can therefore conclude that there must be
appreciable numbers of interstellar grains with a
0.1 µm / 2
16 nm
(e.g., in the size distribution derived by
Weingartner & Draine
[2001a],
grains smaller than 2 nm provides
80% of the total
surface area, although they contain only
6% of the total
dust mass).
However, as remarked earlier (see Section 1), the
far-UV extinction curve is not able to tell us the details of the size
distribution of the nanoparticle component.
(4) The "anomalous" Galactic foreground microwave emission in the 10-100 GHz region. This emission, discovered unexpectedly in recent experiments to study the angular structure in the cosmic background radiation, was found to be positively correlated with interstellar dust, as traced by the 100 µm IRAS map or the 140 µm COBE-DIRBE map (see Draine 1999 and references therein). However, the dust thermal emission at microwave frequencies extrapolated from the 100-3000 µm far-IR emission radiated by large grains in thermal equilibrium with the interstellar radiation field falls far below the observed microwave emission (Draine 1999). This "anomalous" emission also significantly exceeds the free-free emission from interstellar plasma (Draine & Lazarian 1998a). As described by Draine and his coworkers (Draine & Lazarian 1998b; Draine & Li 2003), a number of physical processes, including collisions with neutral atoms and ions, "plasma drag" (due to interaction of the electric dipole moment of the grain with the electric field produced by passing ions), and absorption and emission of photons, can drive nanoparticles to rapidly rotate, with rotation rates reaching tens of GHz. The rotational electric dipole emission from these spinning nanoparticles, the very same grain component required to account for the "UIR" emission and the IRAS 12 and 25 µm emission, was shown to be capable of accounting for the "anomalous" microwave emission (Draine & Lazarian 1998a, b; Draine 1999).
(5) The photoelectric heating of the diffuse ISM. Grains are long thought to be an important energy source for the interstellar gas through ejection of photoelectrons since (a) photons with energies below the ionization potential of H (~ 13.6 eV) do not couple directly to the gas; and (b) other heating sources such as cosmic rays, magnetic fields, and turbulence are unimportant as a global heating source for the diffuse ISM.
The photoelectric heating starts from the absorption of a far-UV photon by a dust grain, followed by ejection of an electron which then collisionally heats the interstellar gas by transferring (to the gas) the excess energy left over after overcoming the work function (the binding energy of the electron to the grain) and the electrostatic potential of the grain (if it is charged).
In the diffuse ISM, nanoparticles (and in particular,
angstrom-sized PAH molecules) are much more efficient
in heating the gas than large grains (see
Tielens & Peeters 2002
and references therein)
since (a) the mean free path of an electron in a solid
is just ~ 10 Å and therefore photoelectrons
created inside a large grain rarely reach the grain surface;
and (b) the total far-UV absorption is dominated by
the nanoparticle component.
Recent studies show that grains smaller than 10 nm
are responsible for
96% of the total
photoelectric heating of the gas, with half of this provided by
grains smaller than 15 Å
(Bakes & Tielens 1994;
Weingartner & Draine
2001b).
(6) The Extended Red Emission (ERE) ?
This emission, first detected in the Red Rectangle
(Schmidt, Cohen, & Margon
1980),
is characterized by a broad, featureless band between
~ 5400 Å and 9500 Å, with a width
600 Å
FWHM 1000
Å and a peak of maximum emission at
6100 Å
p
8200 Å,
depending on the physical conditions of the environment
where the ERE is produced.
The ERE has been seen in a wide variety of dusty environments:
the diffuse ISM of our Galaxy, reflection nebulae,
planetary nebulae, HII regions, and other galaxies
(see Witt & Vijh 2004
for a review). The ERE is generally attributed to photoluminescence (PL)
by some component of interstellar dust, powered by UV/visible photons.
The observational evidence shows that
(a) the ERE carriers must have a photon conversion efficiency
PL
(the number ratio of PL photons
to exciting photons) substantially larger than 10%
as estimated from the correlation of ERE intensity
with HI column density at high Galactic latitudes
(Gordon, Witt, & Friedmann
1998),
and (b) the carriers can be easily modified or destroyed
by intense UV radiation
(Witt 2000).
This suggests that the ERE carriers are very likely
in the nanometer size range because (a) in general,
nanoparticles are expected to luminesce efficiently
through the recombination of the electron-hole pair
created upon absorption of an energetic photon,
since in such small systems the excited electron
is spatially confined and the radiationless transitions
that are facilitated by Auger and defect related recombination
are reduced (see Section 4);
and (b) small nanoparticles may be photolytically
more unstable and/or more readily photoionized in
regions where the radiation intensity exceeds certain
levels of intensity and hardness, and thus resulting in
both a decrease in the ERE intensity and a redshift of
the ERE peak wavelength, since (i) photoionization would
quench the luminescence of nanoparticles,
and (ii) the smaller grains would be selectively
removed due to size-dependent photofragmentation
(Smith & Witt 2002).
Proposed ERE carriers include (a) "classic" submicron-sized carbonaceous materials: HAC (Duley 1985; Witt & Schild 1988), QCC (Sakata et al. 1992), and coal (Papoular et al. 1996); (b) nanometer-sized carbon-based materials: PAHs (d'Hendecourt et al. 1986), and carbon nanoparticles (Seahra & Duley 1999); (c) nanometer-sized silicon-based materials: crystalline silicon nanoparticles (Witt et al. 1998; Ledoux et al. 1998, 2001; Smith & Witt 2002); and (d) particle-bombarded silicate grains (Koike et al. 2002).
The carbon-based models appear to be ruled out: (a) submicron-sized carbon materials appear to be unable to simultaneously match the observed ERE spectra and the required PL efficiency (Witt et al. 1998); (b) although high photoluminescence efficiencies can be obtained by PAHs, the lack of spatial correlation between the ERE and the PAH IR emission bands in some regions (see Li & Draine 2002a and references therein), together with nondetection of ERE emission in reflection nebulae illuminated by stars with effective temperatures T* < 7000 K (Darbon, Perrin, & Sivan 1999), whereas PAHs emission bands have been seen in such regions (e.g., see Uchida, Sellgren, & Werner 1998) and are expected for the PAH emission model (Li & Draine 2002b), seems to argue against PAHs as ERE carriers; (c) the carbon nano-cluster hypothesis put forward by Seahra & Duley (1999) appears to be invalid as indicated by nondetection in NGC 7023 of the predicted 1 µm ERE peak (Gordon et al. 2000), although they argued that these carbon nanoparticles with mixed sp2 / sp3 bonding would be able to meet both the ERE profile and the PL efficiency requirements.
The silicon nanoparticle (SNP) model, originally proposed by Witt et al. (1998) and Ledoux et al. (1998), seems promising. Experimental data show that SNPs provide so far the best match to the observed ERE spectra and to the quantum efficiency requirement. However, this model also has difficulty: we calculated the thermal emission expected from such particles, both in a reflection nebula such as NGC 2023 and in the diffuse ISM; we found that SNPs would produce a strong emission feature at 20 µm which is not seen in the observational spectra; therefore we concluded that if the ERE is due to SNPs, they must be either in clusters or attached to larger grains (see Li & Draine 2002a for details).