2007A&A...462...21M

The luminosity function (LF) is a basic tool in the study of galaxy evolution since it constrains galaxy formation models. The earliest LF estimates in the IR and far-IR spectral ranges seem to suggest strong evolution. Deeper samples are needed to confirm these predictions. We have a useful IR data set, which provides a direct link between IRAS and ISO surveys, and the forthcoming deeper Spitzer Space Telescope and Akari cosmological surveys, to address this issue. This data set allows us to derive the 60µm local LF to sensitivity levels 10 times deeper than before, to investigate evolutionary effects up to a redshift of 0.37, and, using the 60/15µm bi-variate method, to analyze the poorly known 15µm local LF of galaxies. We exploited our ISOCAM observations of the IRAS Deep Survey (IDS) fields, to correct the 60µm fluxes for confusion effects and observational biases. We find indications of a significant incompleteness of the IDS sample, still one of the deepest far-IR selected galaxy samples, below ≃80mJy. We have reliable identifications and spectroscopic redshifts for 100% of a complete subsample comprising 56 sources with S(60µm)>80mJy. With our spectroscopic coverage we construct the 60µm LF for a sample complete down to 80mJy. This LF extends over three orders of magnitude in luminosity, from 9 up to more than 12 in log(L₆₀/L_☉). Despite the fact that the redshift range of our sample exceeds z=0.3, the V/V_max test gives <V/V_max > = 0.51±0.06, consistent with a uniform distribution of sources. A more direct test, whereby the LF was measured in each of four different redshift intervals, does not point out any signature of evolution. On the other hand, the rest-frame 15 µm local LF we derive, extends up to log(L₁₅/L_☉)=12 and predicts 10 times more sources at log(L₁₅/L_☉)=11 than before.