DETECTION OF EXTENDED RED EMISSION IN THE DIFFUSE INTERSTELLAR-MEDIUM

Extended red emission (ERE) has been detected in many dusty astrophysi cal objects, raising the question of whether ERE is present only in di screte objects or if it is an observational feature of all dust, i.e., present in the diffuse interstellar medium. In order to answer this q uestion, we determined the blue and red intensities of the radiation f rom the diffuse interstellar medium (ISM) and examined the red intensi ty for the presence of an excess above that expected for scattered lig ht. The diffuse ISM blue and red intensities were obtained by subtract ing the integrated star and galaxy intensities from the blue and red m easurements made by the Imaging Photopolarimeters (IPPs) aboard the Pi oneer 10 and 11 spacecraft. The unique characteristic of the Pioneer m easurements is that they were taken outside the zodiacal dust cloud an d, therefore, are free from zodiacal light. The color of the diffuse I SM was found to be redder than the Pioneer intensities. If the diffuse ISM intensities were entirely caused by scattering from dust (i.e., d iffuse Galactic light or DGL), the color of the diffuse ISM would be b luer than the Pioneer intensities. Finding a redder color implies the presence of an excess red intensity. Using a model for the DGL, we fou nd the blue diffuse ISM intensity to be entirely attributable to the D GL. The red DGL was calculated using the blue diffuse ISM intensities and the approximately invariant color of the DGL calculated with the D GL 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 similar to 10 S-10(V)(G2V). This represents the li kely detection of ERE in the diffuse ISM since Ha emission cannot acco unt 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 N-HI and ERE intensity is (1.43 +/- 0.31) x 10(-29) ergs s(-1) Angstrom(-1) sr(-1) H atom(-1), from which the ERE photon conversion efficiency was estim ated at 10% +/- 3%.