A Modern Census of the Broadest Diffuse Interstellar Bands

We present the results of a ground-based observing campaign designed to produce the first homogeneous census of the population of the broadest (FWHM ≥ 6 ) diffuse interstellar bands (DIBs) in the Galaxy. New low-resolution optical spectra for 21 sightlines spanning a reddening range of three magnitudes, using stars of spectral types O7 through A3, were examined together with corresponding higher-resolution echelle spectra. A new pipeline developed for the reduction and analysis of the low-resolution spectra is presented. Of the 34 broad features reported in the literature, 22 are covered in this survey. Of those 22, 9 are confirmed as broad DIBs, 2 are rejected, and the remaining 11 are uncertain, due to severe blending with stellar and/or telluric lines. No new broad DIBs are identified. New homogeneous measurements of the strength of the broad 6177 DIB are compared with the corresponding strengths of five narrower DIBs known to trace different phases of the diffuse ISM: the 4963 C2-DIB (which can reside in diffuse molecular gas) and the 5780, 5797, 6284, and 6613 DIBs (which primarily trace atomic gas). The 6177 DIB correlates quite well with the 5780 and 6284 DIBs but shows no correlation with the 4963 C2-DIB-suggesting that its carrier is found primarily in the atomic gas.