A Modern Census of the Broadest Diffuse Interstellar Bands

Abstract

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 Å) 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 Å DIB are compared with the corresponding strengths of five narrower DIBs known to trace different phases of the diffuse ISM: the 4963 Å C 2 -DIB (which can reside in diffuse molecular gas) and the 5780, 5797, 6284, and 6613 Å DIBs (which primarily trace atomic gas). The 6177 Å DIB correlates quite well with the 5780 and 6284 Å DIBs but shows no correlation with the 4963 Å C 2 -DIB—suggesting that its carrier is found primarily in the atomic gas.