A multi-wavelength study of the IRAS Deep Survey galaxy sample. II. The far-IR properties

Abstract

Context: 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.
Aims: This data set allows us to derive the 60 μm local LF to sensitivity levels 10 times deeper than before, to investigate evolutionary effects up to a redshift of 0.37, and, using the 60/15 μm bi-variate method, to analyze the poorly known 15 μm local LF of galaxies.
Methods: We exploited our ISOCAM observations of the IRAS Deep Survey (IDS) fields, to correct the 60~μm 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 ≃80~mJy. We have reliable identifications and spectroscopic redshifts for 100% of a complete subsample comprising 56 sources with S(60~μ m)> 80 mJy.
Results: With our spectroscopic coverage we construct the 60 μm LF for a sample complete down to 80 mJy. 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.