IR Background Anisotropies in Spitzer GOODS Images and Constraints on First Galaxies

We describe the angular power spectrum of unresolved 3.6 μm IR light in Spitzer GOODS fields. The amplitude of the anisotropy spectrum decreases with the decreasing flux threshold to which resolved sources are removed from images. When all pixels above a flux threshold of 24.6 Vega magnitudes are removed, the power spectrum of the remaining pixels has a shape on arcminute angular scales that is consistent with unresolved, faint galaxies at 1<z<4 with flux below 23 Vegamag and a total 3.6 μm intensity between 0.1 and 0.8 nW m^-2 sr^-1. The amplitude of unresolved IR background clustering is further reduced when regions containing optical ACS sources with no IR counterparts are also masked out. We take this as evidence that the most likely identification of the sources responsible for the excess power are low-redshift galaxies. Our anisotropy measurements, however, allow for the possibility that unresolved sources at z>8 contribute to the IR background with a 3.6 μm intensity below about 0.6 nW m^-2 sr^-1. Based on resolved counts and unresolved fluctuations, we find that, at most, about 7.0 nW m^-2 sr^-1 can be ascribed to galaxies.