We use the Spitzer Wide-area InfraRed Extragalactic Legacy Survey ( SWIRE ) to explore the specific star-formation activity of galaxies and their evolution near the peak of the cosmic far-infrared background at 70 and 160µm .
We use a stacking analysis to determine the mean far-infrared properties of well defined subsets of galaxies at flux levels well below the far-infrared catalogue detection limits of SWIRE and other Spitzer surveys .
We tabulate the contribution of different subsets of galaxies to the far-infrared background at 70 \micron and 160 \micron .
These long wavelengths provide a good constraint on the bolometric obscured emission .
The large area provides good constraints at low z and in finer redshift bins than previous work .
At all redshifts we find that the specific far-infrared luminosity decreases with increasing mass , following a trend L _ { FIR } / M _ { * } \propto M _ { * } ^ { \beta } with \beta = -0.38 \pm 0.14 .
This is a more continuous change than expected from the ( 23 ) semi-analytic model suggesting modifications to the feedback prescriptions .
We see an increase in the specific far-infrared luminosity by about a factor of \sim 100 from 0 < z < 2 and find that the specific far infrared luminosity evolves as ( 1 + z ) ^ { \alpha } with \alpha = 4.4 \pm 0.3 for galaxies with 10.5 < \log _ { 10 } M _ { * } / M _ { \odot } \leq 12 .
This is considerably steeper than the ( 23 ) semi-analytic model ( \alpha \sim 2.5 ) .
When separating galaxies into early and late types on the basis of the optical/IR spectral energy distributions we find that the decrease in specific far-infrared luminosity with stellar mass is stronger in early type galaxies ( \beta \sim - 0.46 ) , while late type galaxies exhibit a flatter trend ( \beta \sim - 0.15 ) .
The evolution is strong for both classes but stronger for the early type galaxies .
The early types show a trend of decreasing strength of evolution as we move from lower to higher masses while the evolution of the late type galaxies has little dependence on stellar mass .
We suggest that in late-type galaxies we are seeing a consistently declining specific star-formation rate \alpha = 3.36 \pm 0.16 through a common phenomenon e.g .
exhaustion of gas supply i.e .
not systematically dependent on the local properties of the galaxy .