The 24 micron source counts in deep Spitzer Space Telescope surveys

Galaxy source counts in the infrared provide strong constraints on the evolution of the bolometric energy output from distant galaxy populations. We present the results from deep 24 μm imaging from Spitzer surveys, which include ≈5 × 10⁴ sources to an 80% completeness of ≈60 μJy. The 24 μm counts rapidly rise at near-Euclidean rates down to 5 mJy, increase with a super-Euclidean rate between 0.4 and 4 mJy, and converge below ∼0.3 mJy. The 24 μm counts exceed expectations from nonevolving models by a factor of ≳10 at S_v ∼0.1 mJy. The peak in the differential number counts corresponds to a population of faint sources that is not expected from predictions based on 15 μm counts from the Infrared Space Observatory. We argue that this implies the existence of a previously undetected population of infrared-luminous galaxies at z ∼ 1-3. Integrating the counts to 60 μJy, we derive a lower limit on the 24 μm background intensity of 1.9 ± 0.6 nW m^-2 sr^-1 of which the majority (∼60%) stems from sources fainter than 0.4 mJy. Extrapolating to fainter flux densities, sources below 60 μJy contribute 0.8_-0.4 ^+0.9 n W m^-2 sr^-1 to the background, which provides an estimate of the total 24 μm background of 2.7_-0.7 ^+1.1 m^-2 sr^-1.