The 24 Micron Source Counts in Deep Spitzer Space Telescope Surveys**This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407.
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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 m imaging from Spitzer surveys, which include 5 104 sources to an 80% completeness of 60 Jy. The 24 m 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 m counts exceed expectations from nonevolving models by a factor of 10 at Sv 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 m 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 Jy, we derive a lower limit on the 24 m 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 Jy contribute 0.8-0.4+0.9 n W m-2 sr-1 to the background, which provides an estimate of the total 24 m background of 2.7-0.7+1.1 m-2 sr-1.
