al. 2010; Bonfield et al. 2011; Dai et al. 2012). But additionally DSFGs are expected to occur as satellite galaxies in the dark matter halos where quasars are found[^7]. A last effect is that dark matter halos correlated with the quasar-hosting halo are also expected to host DSFGs. The SPIRE instrumental noise should not correlate with quasars and therefore only contributes to the noise in the measured cross-correlation signal between quasars and the CIB. Similarly, Galactic dust emission should not have any intrinsic correlation with the quasars. However, due to the selection of the quasars, there might be (anti-)correlation between the observed quasar number density field and the intensity of the cirrus emission. There are two effects of Galactic dust on the selection of QSOs. The first effect is reddening which means QSOs with extreme reddening will not be selected as targets for spectroscopy. The second effect is extinction, which can cause the QSOs to drop out of the target list even before they reach the colour selection stage. QSO populations with steeper number counts will be more sensitive to extinction.

To explicitly check the potential correlation between the quasar samples and Galactic cirrus emission, we make use of the 