Abstract :
Space infrared observations from cooled telescopes have been providing significant information on our understanding of interstellar dust grains because they are sensitive to “diffuse emission” from the interstellar medium of the entire Galaxy, which is difficult to observe with warm telescopes. This paper reviews the latest observations of interstellar dust grains by the Infrared Telescope in Space (IRTS) and the Infrared Space Observatory (ISO) missions. Particular emphasis is placed on the observations of the mid-infrared excess, which was discovered by Infrared Astronomical Satellite (IRAS). IRTS and ISO observations have unambiguously revealed the presence of the unidentified infrared (UIR) emission bands in the diffuse infrared galactic light as well as in cirrus clouds. The correlation of the UIR band distribution with the far-infrared intensity indicates that the UIR-emitting material is well mixed with the submicron sized dust grains. Further studies of the UIR bands in different environments show that the band strengths are linearly proportional to the intensity of incident radiation field as long as the incident intensity is in the range from unity to 100 times the solar vicinity value. The relative band strengths do not vary appreciably in this range. These results are in agreement with the prediction of the temperature-fluctuating dust emission or infrared fluorescence for the excess emission. Under radiation fields stronger than 1000 times the solar vicinity value, a possible deviation from linearity is indicated, suggesting that the properties of the UIR band emitters may vary under harsh conditions. On the other hand, the excess emission in the range 25–60 μm varies more rapidly with the incident radiation field, indicating the different nature of this component. Prospects of the observations of dust grains by future missions are also discussed.
