Optical Spectroscopy of Galactic Cirrus Clouds: Extended Red Emission in the Diffuse Interstellar Medium

We present initial results from the first optical spectroscopic survey of high latitude Galactic cirrus clouds. The observed shape of the cirrus spectrum does not agree with that of scattered ambient Galactic starlight. This mismatch can be explained by the presence of Extended Red Emission (ERE) in the diffuse interstellar medium, as found in many other astronomical objects, probably caused by photoluminescence of hydrocarbons. The integrated ERE intensity, IERE ≈ 1.2×10 −5 erg s cm sr, is roughly a third of the scattered light intensity, consistent with recent color measurements of diffuse Galactic light. The peak of the cirrus ERE (λ0 ∼ 6000 Å) is shifted towards short (bluer) wavelengths compared to the ERE in sources excited by intense ultraviolet radiation, such as Hii regions (λ0 ∼ 8000 Å); such a trend is seen in laboratory experiments on hydrogenated amorphous carbon films. Subject headings: Galaxy: solar neighborhood–ISM: clouds–ISM: dust, extinction–ISM: molecules–ISM: reflection nebulae Observations carried out at the University of California’s Lick Observatory, Mount Hamilton, California Alfred P. Sloan Research Fellow