symbols correspond to cross-correlation signal in different fields. One DR7 quasar in Boötes happens to lie on top of a very bright point source which biases that result, indicated by the empty red circles (therefore, it should be excluded from the sample). The filled red circles correspond to the cross-correlation between the new DR7 QSO sample in Boötes and the 100 $\micron$ map. The horizontal dotted line marks the zero signal expected if there is no correlation between quasars and cirrus. The vertical dashed line corresponds to the FWHM of the IRAS beam. Data points below the scale of the IRAS beam are strongly correlated. For the DR9 quasars, the cross-correlation with the 100 $\micron$ maps in the HeLMS S82 region exhibits a systematic negative signal. Data points from different fields are shifted slightly horizontally for visual clarity.[]{data-label="fig:qso_iris_all"}](dr7_allfields_100_scramble.eps "fig:"){height="2.7in" width="3.45in"} ![Cross-correlation between the SDSS DR7 (top) and SDSS-III DR9 (bottom) QSOs and the 100 $\micron$ maps. Different coloured symbols correspond to cross-correlation signal in different fields. One DR7 quasar in Boötes happens to lie on top of a very bright point source which biases that result, indicated by the empty red circles (therefore, it should be excluded from the sample). 