Detection of Far Infrared Emission from Galaxies and Quasars in the
  Galactic Extinction Map by Stacking Analysis
  We have performed stacking image analyses of galaxies over the Galactic
extinction map constructed by Schlegel, Finkbeiner & Davis (1998). We select
~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 Yahata et al. (2007). 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.
