Dusting off the diffuse interstellar bands: DIBs and dust in
  extragalactic SDSS spectra
  Using over a million and a half extragalactic spectra we study the properties
of the mysterious Diffuse Interstellar Bands (DIBs) in the Milky Way. These
data provide us with an unprecedented sampling of the skies at high
Galactic-latitude and low dust-column-density. We present our method, study the
correlation of the equivalent width of 8 DIBs with dust extinction and with a
few atomic species, and the distribution of four DIBs - 5780.6A, 5797.1A,
6204.3A, and 6613.6A - over nearly 15000 squared degrees. As previously found,
DIBs strengths correlate with extinction and therefore inevitably with each
other. However, we show that DIBs can exist even in dust free areas.
Furthermore, we find that the DIBs correlation with dust varies significantly
over the sky. DIB under- or over-densities, relative to the expectation from
dust, are often spread over hundreds of square degrees. These patches are
different for the four DIBs, showing that they are unlikely to originate from
the same carrier, as previously suggested.
