Exploring the diffuse interstellar bands with the Sloan Digital Sky
  Survey
  We use star, galaxy and quasar spectra taken by the Sloan Digital Sky Survey
to map out the distribution of diffuse interstellar bands (DIBs) induced by the
Milky Way. We show that, after carefully removing the intrinsic spectral energy
distribution of each source, it is possible to measure statistical flux
fluctuations at the 1e-3 level, detect more than 20 DIBs and measure their
strength as a function of position on the sky. We create a map of DIB
absorption covering about 5000 square degrees and measure correlations with
various tracers of the interstellar medium: atomic and molecular hydrogen, dust
and polycyclic aromatic hydrocarbons (PAHs). After recovering known
correlations, we show that each DIB has a different dependence on atomic and
molecular hydrogen: while they are all positively correlated with N(HI), they
exhibit a range of behaviours with N(H2) showing positive, negative or no
correlation. We show that a simple parametrization involving only N(HI) and
N(H2) applied to all the DIBs is sufficient to reproduce a large collection of
observational results reported in the literature: it allows us to naturally
describe the relations between DIB strength and dust reddening (including the
so-called skin effect), the related scatter, DIB pair-wise correlations &
families, the affinity for $\sigma/\zeta$-type environments and other
correlations related to molecules. Our approach allows us to characterize DIB
dependencies in a simple manner and provides us with a metric to characterize
the similarity between different DIBs.
