The EDIBLES survey: III. C2-DIBs and their profiles

Context. An unambiguous identification of the carriers of the diffuse interstellar bands (DIBs) would provide important clues to the life cycle of interstellar matter. The so-called C-2-DIBs are a class of very weak bands that fall in the blue part of the optical spectrum and are associated with high column densities of the C-2 molecule. DIB profile structures constrain potential molecular carriers, but their measurement requires high signal-to-noise, high-resolution spectra and the use of sightlines without Doppler splitting, as typical for a single-cloud situation. Aims. Spectra from the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) conducted at the Very Large Telescope (ESO/Paranal) were explored to identify single-cloud and high C-2 column sightlines, extract the corresponding C-2-DIBs and study their strengths and profiles, and to investigate in detail any sub-structures. Methods. The target selection was made based on profile-fitting of the 3303 and 5895 angstrom Na I doublets and the detection of C-2 lines. The C-2 (2-0) (8750-8849 angstrom) Phillips system was fitted using a physical model of the host cloud. C-2 column densities, temperatures as well as gas densities were derived for each sightline. Results. Eighteen known C-2-DIBs and eight strong non-C-2 DIBs were extracted towards eight targets, comprising seven single-cloud and one multi-cloud line-of-sights. Correlational studies revealed a tight association of the former group with the C-2 columns, whereas the non-C-2 DIBs are primarily correlated with reddening. We report three new weak diffuse band candidates at 4737.5, 5547.4, and 5769.8 angstrom. We show for the first time that at least 14 C-2-DIBs exhibit spectral sub-structures which are consistent with unresolved rotational branches of molecular carriers. The variability of their peak separations among the bands for a given sightline implies that their carriers are different molecules with quite different sizes. We also illustrate how profiles of the same DIB vary among targets and as a function of physical parameters, and provide tables defining the sub-structures to be compared with future models and experimental results.