Dusting off the diffuse interstellar bands: DIBs and dust in extragalactic sloan digital sky survey 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 eight DIBs with dust extinction and with a few atomic species, and the distribution of four DIBs - 5780.6, 5797.1, 6204.3, and 6613.6 Å - over nearly 15 000 deg². 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 overdensities, 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.