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

We present initial results from the first optical spectroscopic survey ofhigh latitude Galactic cirrus clouds. The observed shape of the cirrus spectrumdoes not agree with that of scattered ambient Galactic starlight. This mismatchcan be explained by the presence of Extended Red Emission (ERE) in the diffuseinterstellar medium, as found in many other astronomical objects, probablycaused by photoluminescence of hydrocarbons. The integrated ERE intensity,I_ERE \approx 1.2 x 10^{-5} erg s^{-1} cm^{-2} sr^{-1}, is roughly a third ofthe scattered light intensity, consistent with recent color measurements ofdiffuse Galactic light. The peak of the cirrus ERE (lambda_{0} \sim 6000 AA) isshifted towards short (bluer) wavelengths compared to the ERE in sourcesexcited by intense ultraviolet radiation, such as HII regions (lambda_{0} sim8000 AA); such a trend is seen in laboratory experiments on hydrogenatedamorphous carbon films.Comment: 7 pages, 2 figures. Accepted for publication in ApJ Letter