The Excitation of Extended Red Emission: New Constraints on Its Carrier fromHubble Space TelescopeObservations of NGC 7023

The carrier of the dust-associated photoluminescence process causing theextended red emission (ERE) in many dusty interstellar environments remainsunidentified. Several competing models are more or less able to match theobserved broad, unstructured ERE band. We now constrain the character of theERE carrier further by determining the wavelengths of the radiation thatinitiates the ERE. Using the imaging capabilities of the Hubble SpaceTelescope, we have resolved the width of narrow ERE filaments appearing on thesurfaces of externally illuminated molecular clouds in the bright reflectionnebula NGC 7023 and compared them with the depth of penetration of radiation ofknown wavelengths into the same cloud surfaces. We identify photons withwavelengths shortward of 118 nm as the source of ERE initiation, not to beconfused with ERE excitation, however. There are strong indications from thewell-studied ERE in the Red Rectangle nebula and in the high-|b| Galacticcirrus that the photon flux with wavelengths shortward of 118 nm is too smallto actually excite the observed ERE, even with 100% quantum efficiency. Weconclude, therefore, that ERE excitation results from a two-step process. Whilenone of the previously proposed ERE models can match these new constraints, wenote that under interstellar conditions most polycyclic aromatic hydrocarbon(PAH) molecules are ionized to the di-cation stage by photons with E > 10.5 eVand that the electronic energy level structure of PAH di-cations is consistentwith fluorescence in the wavelength band of the ERE. Therefore, PAH di-cationsdeserve further study as potential carriers of the ERE. (abridged)Comment: Accepted for Publication in the Ap