  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 micron imaging from Spitzer surveys, which
include approximately 50,000 sources to an 80% completeness of 60 uJy. The 24
micron counts rapidly rise at near-Euclidean rates down to 5 mJy, increase with
a super-Euclidean rate between 0.4 - 4 mJy, and converge below 0.3 mJy. The 24
micron counts exceed expectations from non-evolving models by a factor >10 at
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 micron
counts from ISO. 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 uJy, we derive a lower limit on the 24 micron 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
uJy contribute 0.8 {+0.9/-0.4} nW m-2 sr-1 to the background, which provides an
estimate of the total 24 micron background of 2.7 {+1.1/-0.7} nW m-2 sr-1.

Astrophysics