2005MNRAS.357..449R

We present the optical identifications of a 95-µm ISOPHOT sample in the Lockman hole over an area of approximately half a deg². The Rodighiero et al. catalogue includes 36 sources, making up a complete flux-limited sample for S_95µm≥ 100 mJy. Reliable sources were detected, with decreasing but well-controlled completeness, down to S_95µm≃ 20 mJy. We have combined mid-infrared (IR) and radio catalogues in this area to identify the potential optical counterparts of the far-IR sources. We found 14 radio and 13 15-µm associations, 10 of which have both associations. For the 11 sources with spectroscopic redshift, we have performed a spectrophotometric analysis of the observed spectral energy distributions (SEDs). Four of these 95-µm sources have been classified as faint IR (FIR) galaxies (L_FIR < 1.e11 L_☉), six as luminous IR galaxies (LIRGs) and only one as an ultraluminous IR galaxy (ULIRG). We have discussed the redshift distribution of these objects, comparing our results with evolutionary model predictions 95 and 175 µm. Given their moderate distances (the bulk of the closest spectroscopically identified objects lying at z < 0.2), their luminosities and star formation rates (SFR; median value ∼ 10 M_☉/yr), the sources unveiled by ISOPHOT at 95 µm seem to correspond to the low redshift (z < 0.3) FIRBACK 175-µm population, composed of dusty, star-forming galaxies with moderate SFRs. We computed and compared different SFR estimators, and found that the SF derived from the bolometric IR luminosity is well correlated with that computed from the radio and mid-IR fluxes.