The 24 Micron Source Counts in Deep Spitzer Space Telescope Surveys

Galaxy source counts in the infrared provide strong constraints on theevolution of the bolometric energy output from distant galaxy populations. Wepresent the results from deep 24 micron imaging from Spitzer surveys, whichinclude approximately 50,000 sources to an 80% completeness of 60 uJy. The 24micron counts rapidly rise at near-Euclidean rates down to 5 mJy, increase witha super-Euclidean rate between 0.4 - 4 mJy, and converge below 0.3 mJy. The 24micron counts exceed expectations from non-evolving models by a factor >10 at0.1 mJy. The peak in the differential number counts corresponds to a populationof faint sources that is not expected from predictions based on 15 microncounts from ISO. We argue that this implies the existence of a previouslyundetected population of infrared-luminous galaxies at z ~ 1-3. Integrating thecounts to 60 uJy, we derive a lower limit on the 24 micron background intensityof 1.9 +/- 0.6 nW m-2 sr-1 of which the majority (~ 60%) stems from sourcesfainter than 0.4 mJy. Extrapolating to fainter flux densities, sources below 60uJy contribute 0.8 {+0.9/-0.4} nW m-2 sr-1 to the background, which provides anestimate of the total 24 micron background of 2.7 {+1.1/-0.7} nW m-2 sr-1.Comment: Accepted to the ApJS (Spitzer special issue); 5 pages, 3 color figures, uses emulateapj clas