We have performed stacking image analyses of galaxies over the Galactic extinction map constructed by \citet SFD98 .
We select \sim 10 ^ { 7 } galaxies in total from the Sloan Digital Sky Survey ( SDSS ) DR7 photometric catalog .
We detect clear signatures of the enhancement of the extinction in r -band , \Delta A _ { r } , around galaxies , indicating that the extinction map is contaminated by their FIR ( far infrared ) emission .
The average amplitude of the contamination per galaxy is well fitted to \Delta A _ { r } ( m _ { r } ) = 0.64 \times 10 ^ { 0.17 ( 18 - m _ { r } ) } ~ { } { [ mmag ] } .
While this value is very small , it is directly associated with galaxies and may have a systematic effect on galaxy statistics .
Indeed this correlated contamination leads to a relatively large anomaly of galaxy surface number densities against the SFD extinction A _ { SFD } discovered by \citet Yahata2007 .
We model the radial profiles of stacked galaxy images , and find that the FIR signal around each galaxy does not originate from the central galaxy alone , but is dominated by the contributions of nearby galaxies via galaxy angular clustering .
The separation of the single galaxy and the clustering terms enables us to infer the statistical relation of the FIR and r -band fluxes of galaxies and also to probe the flux-weighted cross-correlation of galaxies , down to the magnitudes that are difficult to probe directly for individual objects .
We repeat the same stacking analysis for SDSS DR6 photometric quasars and discovered the similar signatures but with weaker amplitudes .
The implications of the present results for galaxy and quasar statistics and for correction to the Galactic extinction map are briefly discussed .