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 decreasing flux threshold to which resolved sources are removed from images. When all pixels brighter than a Vega magnitude of 24.6 are removed, the amplitude of the power spectrum at arcminute angular scales can be described with an extra component of z > 8 sources with a IRB contribution around 0.4 nW m−2 sr−1. The shape of the power spectrum, however, is more consistent with that expected for unresolved, faint galaxies at lower redshifts with Vega magnitudes fainter than 23 with a total 3.6 μm intensity between 0.1 to 0.8 nW m−2 sr−1. We confirm this assumption by showing that large-scale power decreases rapidly when the unresolved clustering spectrum is measured from a processed HDF-N IRAC image where locations of faint ACS sources with no IR counterparts were also masked. 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. Subject headings: large scale structure of universe — diffuse radiation — infrared: galaxies