Small-scale-structure of the interstellar medium probed through diffuse band observations

Abstract

The carriers of the diffuse interstellar band spectrum represent an important baryonic component of the interstellar medium (ISM) and it is expected that their identification will contribute significantly to the understanding of the chemistry and physics of interstellar clouds. It is widely held that the carriers are linked to the presence of dust grains on account of the good correlation of their strengths with interstellar reddening, so they offer an important potential route to improving our understanding of the composition and chemistry of grains and grain surfaces. In addition to the challenge of making the spectral assignments, an important current question concerns the spatial distribution and physical state of interstellar material, with recent observational atomic and molecular line absorption studies suggesting that diffuse clouds are more ‘clumpy’ than previously thought. We describe here high signal-to-noise optical observations made at the Anglo-Australian Telescope using UCLES that were undertaken to investigate the spatial distribution of diffuse band carriers. We describe the first detection of ‘small-scale-structure’ in the diffuse band carrier distribution in the ISM, and comment on the possibilities that these data hold for contributing to the solution of the diffuse band problem and our understanding of the nature of small-scale-structure in the diffuse ISM.