Ultrafine Structure in the λ5797 Diffuse Interstellar Absorption Band

High-resolution and high-S/N observations of the strong and relatively narrow λ5797 diffuse interstellar absorption band, recorded with the Ultra-High-Resolution Facility at the 3.9 m Anglo-Australian Telescope and the 0.9 m coudé feed of the Kitt Peak National Observatory, are reported. For selected lines of sight and using resolving powers of up to 600,000, a remarkable degree of intrinsic ultrafine structure is found within the band. There exists an almost exact correspondence between the structure in the spectra recorded toward μ Sgr, ζ Per, and ζ Oph, including one particularly narrow component with a width that is comparable to the widths of atomic and molecular absorption lines along these lines of sight. The results provide a new and extremely exacting test against which theoretical or laboratory candidates can be assessed, and they point to a new approach to studies of diffuse band carrier distributions through their velocity signatures.