Abstract
In order to understand the origin of clustered anisotropies detected in Spitzer images between 3.6 and 8 μm, 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 μm of 130 ± 5 nJy above the background. The wealth of multiwavelength 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 less than or similar to 0.03L_*, _(UV) galaxy with a 90% confidence interval for the redshift of 1.9-2.7. These sources produce a 3.6 μm absolute background intensity between 0.1 and 0.35 nW m^(−2) sr^(−1), and the clustered IR light could account for ~30%-50% of fluctuation power in the IR background at 4' angular scales. Although the exact redshift distribution of these sources is unknown, these galaxies appear to contain 5%-20% of the comoving stellar mass density at z ~ 2.5.
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