  In order to understand the origin of clustered anisotropies detected in
Spitzer images between 3.6 and 8 microns, we stack the Spitzer IRAC/Great
Observatories Origins Deep Survey (GOODS) images at pixel locations
corresponding to faint, z_{AB}~27 mag, optical sources with no obvious IR
counterparts. We obtain a strong detection of the sources with a stacked median
flux at 3.6 microns of 130+/-5 nJy above the background. The wealth of
multi-wavelength imaging data in GOODS enables a similar stacking analysis to
be undertaken at various wavelengths between the ultraviolet and near-infrared
bands. We obtain strong stacked detections of these optically faint sources
over the entire wavelength range which places constraints on the average
properties of these sources. We find that the flux spectrum of the median,
stacked source is consistent with a L<0.03 L_{*,UV} galaxy with a 90%
confidence interval for the redshift of 1.9-2.7. These sources produce a 3.6
micron absolute background intensity between 0.1 and 0.35 nW/m^2/sr and the
clustered IR light could account for ~30-50% of fluctuation power in the IR
background at 4 arcminute angular scales. Although the exact redshift
distribution of these sources is unknown, these galaxies appear to contain
5-20% of the co-moving stellar mass density at z~2.5.

Astrophysics