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

Abstract

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, ergs s cm sr , is roughly one-third of the scattered light 5 1 2 1 I ≈ 1.2 # 10 ERE intensity, consistent with recent color measurements of diffuse Galactic light. The peak of the cirrus ERE (l 0 ∼ 6000 A ˚) is shifted toward short (bluer) wavelengths compared with the ERE in sources excited by intense ultraviolet radiation, such as H ii regions (A ˚); such a trend is seen in laboratory experiments on l ∼ 8000 0 hydrogenated amorphous carbon films.