Extended Red Emission in High Galactic Latitude Interstellar Clouds

Abstract

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 conduct this ongoing survey with remotely operated, fast, short focal length (0.5 m) telescopes equipped with absolutely calibrated CCD cameras yielding a field of view of 2° × 3°. The telescopes are located at New Mexico Skies at 7300 ft (2225 m) altitude near Mayhill, New Mexico. The optical surface brightness of our objects is typically a few percent of the brightness of the dark night sky, implying that the cloud SEDs must be deduced from differential surface brightness photometry in different filter bands. 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 ~5 × 10−9 ergs 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.