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Register a new userExtended Red Emission: Photoluminescence by Interstellar Nanoparticles
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Interstellar dust in nebulae and in the diffuse interstellar medium of galaxies contains a component which responds to illumination by ultraviolet photons with efficient luminescence in the 500 nm to 1000 nm spectral range, known as Extended Red Emission (ERE). We review the techniques of detection and the observational characteristics of the ERE in a wide range of astrophysical environments. We then discuss results of the analysis of ERE observations, leading to a set of specific constraints that any proposal for the ERE carrier must confront. Finally, we review specific models that have been advanced over the past two decades to explain the ERE phenomenon. Despite promising progress on several fronts, no completely satisfactory model for the ERE carrier/process exists at this time.
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Nearby interstellar clouds at high Galactic latitudes are ideal objects in which the interaction of interstellar dust with photons from the well-characterized interstellar radiation field can be studied. Scattering and UV-excited photoluminescence at optical wavelengths as well as thermal emission at mid- and far-infrared wavelengths are observable manifestations of such interactions. Here we report initial results from an optical imaging survey of optically thin high-Galactic-latitude clouds, which is designed to study the surface brightness, structure, and spectral energy distribution of these objects. The primary aim of this paper is to study the extended red emission (ERE) that has been reported at high Galactic latitudes in earlier investigations and which is attributed to ultraviolet-excited photoluminescence of an as yet unidentified component of interstellar dust. We find strong evidence for dust emission in the form of a broad (>1000 A FWHM) ERE band with peak emission near 600 nm wavelength and peak intensity of ~ 5x10^-9 (erg cm^-2 s^-1 A^-1 sr^-1) in optically-thin clouds. This amounts to about 30% of the total optical surface brightness of these clouds, the remainder being consistent with expectations for dust-scattered light. This supports claims for the ubiquitous presence of the ERE carrier throughout the diffuse interstellar medium of the Milky Way Galaxy. We suggest that the ERE carrier is involved in the radiative processing of about 20% to 30% of the dust-absorbed UV/optical luminosity of the Milky Way galaxy, with the bulk of this energy being emitted in the near- to mid-infrared spectral regions.
We present a study showing cooperative behavior of light emitting quantum dots at room temperature, with large increases in radiative decay rates and efficiencies, in the presence of small gold nanoparticles (1.5 - 4 nm radii) in low fractions. This is a size-regime of metal particles where the expected effect on emission from independent emitters is vain non-radiative loss. But the addition of such metal particles in low fractions induces a strong evolution of the super-radiant modes of emission among quantum dots and aids their survival of thermal fluctuations; exhibiting a phase transition. While an increase of size of metal particles results in an increase in local thermal fluctuations to revert to the behavior of apparently independent emitters. Our theoretical evaluations of their possible collective modes of emission in the presence of metal nanoparticles predict such experimental observations. Two different types of self-assembled nanoscale structures containing quantum dots were experimentally studied. This included the effect of the fractions and size of metal particles on the collective modes of emission in each type of structure; each type of structure had samples of different nominal sizes (and emission energies) of dots to establish generality. First, quantum dots collected in cylindrical cavities surrounded by randomly located gold particles were experimentally studied in large ensembles using polymer templates. The other type of nanostructure was a colloidal monolayer of quantum dots closely packed along with small gold nanoparticles. A cross-over between collective and independent regimes is observed based on the size of metal particles, and also at larger number fractions in the closely packed structure. Time-resolved photoluminescence measurements were also used to confirm this increase in the quantum efficiency and radiative decay rates of the dots.
Extended Red Emission (ERE) was recently attributed to the photo-luminescence of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH$^{++}$), or charged PAH dimers. We analysed the visible and mid-infrared (mid-IR) dust emission in the North-West and South photo-dissociation regions of the reflection nebula NGC 7023.Using a blind signal separation method, we extracted the map of ERE from images obtained with the Hubble Space Telescope, and at the Canada France Hawaii Telescope. We compared the extracted ERE image to the distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral and ionized PAHs (PAH$^0$ and PAH$^+$) obtained with the Spitzer Space Telescope and the Infrared Space Observatory. ERE is dominant in transition regions where VSGs are being photo-evaporated to form free PAH molecules, and is not observed in regions dominated by PAH$^+$. Its carrier makes a minor contribution to the mid-IR emission spectrum. These results suggest that the ERE carrier is a transition species formed during the destruction of VSGs. Singly ionized PAH dimers appear as good candidates but PAH$^{++}$ molecules seem to be excluded.
Interstellar iron in the form of metallic iron nanoparticles may constitute a component of the interstellar dust. We compute the stability of iron nanoparticles to sublimation in the interstellar radiation field, finding that iron clusters can persist down to a radius of $simeq 4.5,$AA, and perhaps smaller. We employ laboratory data on small iron clusters to compute the photoelectric yields as a function of grain size and the resulting grain charge distribution in various interstellar environments, finding that iron nanoparticles can acquire negative charges particularly in regions with high gas temperatures and ionization fractions. If $gtrsim 10%$ of the interstellar iron is in the form of ultrasmall iron clusters, the photoelectric heating rate from dust may be increased by up to tens of percent relative to dust models with only carbonaceous and silicate grains.
We present a survey of diffuse O VI emission in the interstellar medium obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). Spanning 5.5 years of FUSE observations, from launch through 2004 December, our data set consists of 2925 exposures along 183 sight lines, including all of those with previously-published O VI detections. The data were processed using an implementation of CalFUSE v3.1 modified to optimize the signal-to-noise ratio and velocity scale of spectra from an aperture-filling source. Of our 183 sight lines, 73 show O VI 1032 emission, 29 at > 3-sigma significance. Six of the 3-sigma features have velocities |v_LSR| > 120 km/s, while the others have |v_LSR| < 50 km/s. Measured intensities range from 1800 to 9100 LU, with a median of 3300 LU. Combining our results with published O VI absorption data, we find that an O VI-bearing interface in the local ISM yields an electron density n_e = 0.2--0.3 cm^-3^ and a path length of 0.1 pc, while O VI-emitting regions associated with high-velocity clouds in the Galactic halo have densities an order of magnitude lower and path lengths two orders of magnitude longer. Though the O VI intensities along these sight lines are similar, the emission is produced by gas with very different properties.
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