We present the IR luminosity function derived from ultra-deep 70 micron
imaging of the GOODS-North field. The 70 micron observations are longward of
the PAH and silicate features which complicate work in the MIR. We derive
far-infrared luminosities for the 143 sources with S_{70} > 2 mJy (S/N > 3
\sigma). The majority (81%) of the sources have spectroscopic redshifts, and
photometric redshifts are calculated for the remainder. The IR luminosity
function at four redshifts (z ~ 0.28, 0.48, 0.78, and 0.97) is derived and
compared to the local one. There is considerable degeneracy between luminosity
and density evolution. If the evolving luminosity function is described as
\rho(L, z) = (1 + z)^q \rho(L/(1 + z)^p, 0), we find q = -2.19p + 6.09. In the
case of pure luminosity evolution, we find a best fit of p =
2.78^{+0.34}_{-0.32}. This is consistent with the results from 24 micron and
1.4 GHz studies. Our results confirm the emerging picture of strong evolution
in LIRGs and ULIRGs at 0.4 < z < 1.1, but we find no evidence of significant
evolution in the sub-LIRG (L < 10^{11} L_{\odot}) population for z < 0.4.Comment: accepted by ApJL, 5 page
