Tracing differential reddening with Diffuse Interstellar Bands. The
  globular cluster M 4 as a testbed
  Diffuse interstellar bands (DIBs) are weak absorption features of
interstellar origin present in the optical and infrared spectra of stars. Their
use as a tool to trace the structure of the Galactic ISM is gaining relevance
in the recent years. Here we present an experiment to test our ability to trace
differential reddening on the plane of the sky by using the information
relative to the DIB at $\lambda$6614 extracted from the spectra of cool stars.
For that we made use of archive FLAMES data of the globular cluster M4, as well
as WISE and Planck images for reference. We found a global positive trend
between the distribution of the strength of the DIB, as traced by its
equivalent width, and the amount of Galactic reddening, as traced by Planck.
This result supports the use of DIBs to trace the small scale structure of the
Galactic ISM.
