Extragalactic Imprints in Galactic Dust Maps
  Extragalactic astronomy relies on the accurate estimation of source
photometry corrected for Milky Way dust extinction. This has motivated the
creation of a number of "Galactic" dust maps. We investigate whether these maps
are contaminated by extragalactic signals using the clustering-redshift
technique, i.e., by measuring a set of angular cross-correlations with
spectroscopic objects as a function of redshift. Our tomographic analysis
reveals imprints of extragalactic large-scale structure patterns in nine out of
10 Galactic dust maps, including all infrared-based maps as well as "stellar"
reddening maps. When such maps are used for extinction corrections, this
extragalactic contamination introduces redshift- and scale-dependent biases in
photometric estimates at the millimagnitude level. It can affect both
object-based analyses, such as the estimation of the Hubble diagram with
supernovae, as well as spatial statistics. The bias can be appreciable when
measuring angular correlation functions with low amplitudes, such as
lensing-induced correlations or angular correlations for sources distributed
over a broad redshift range. As expected, we do not detect any extragalactic
contamination for the dust map inferred from 21cm HI observations. Such a map
provides an alternative to widely used infrared-based maps but relies on the
assumption of a constant dust-to-gas ratio. We note that, using the WISE 12
micron map sensitive to polycyclic aromatic hydrocarbons (PAH), an indirect
dust tracer, we detect the diffuse extragalactic PAH background up to $z\sim2$.
Finally, we provide a procedure to minimize the level of biased magnitude
corrections in maps with extragalactic imprints.
