Detection of Extended Red Emission in the Diffuse Interstellar Medium

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Abstract
Extended red emission (ERE) has been detected in many dusty astrophysical objects, raising the question of whether ERE is present only in discrete objects or if it is an observational feature of all dust, i.e., present in the diffuse interstellar medium. In order to answer this question, we determined the blue and red intensities of the radiation from the diffuse interstellar medium (ISM) and examined the red intensity for the presence of an excess above that expected for scattered light. The diffuse ISM blue and red intensities were obtained by subtracting the integrated star and galaxy intensities from the blue and red measurements made by the Imaging Photopolarimeters (IPPs) aboard the Pioneer 10 and 11 spacecraft. The unique characteristic of the Pioneer measurements is that they were taken outside the zodiacal dust cloud and, therefore, are free from zodiacal light. The color of the diffuse ISM was found to be redder than the Pioneer intensities. If the diffuse ISM intensities were entirely caused by scattering from dust (i.e., diffuse Galactic light or DGL), the color of the diffuse ISM would be bluer than the Pioneer intensities. Finding a redder color implies the presence of an excess red intensity. Using a model for the DGL, we found the blue diffuse ISM intensity to be entirely attributable to the DGL. The red DGL was calculated using the blue diffuse ISM intensities and the approximately invariant color of the DGL calculated with the DGL model. Subtracting the calculated red DGL from the red diffuse ISM intensities resulted in the detection of an excess red intensity with an average value of ~10 S10(V)G2 V. This represents the likely detection of ERE in the diffuse ISM since Hα emission cannot account for the strength of this excess and the only other known emission process applicable to the diffuse ISM is ERE. Thus, ERE appears to be a general characteristic of dust. The correlation between NH I and ERE intensity is (1.43 ± 0.31) × 10-29 ergs s-1 Å-1 sr-1 H atom-1, from which the ERE photon conversion efficiency was estimated at 10% ± 3%.