Definition of our sample. Among the 209 northern galaxies (filled symbols) found in NED between 0.2 < z < 0.6 and detected at 60 μm   by IRAS, we selected the most luminous ones (log L_FIR/L_⊙ > 12.45, as indicated by the horizontal line). By comparison, the ULIRGs in the sample of Solomon et al. (1997) are plotted as open triangles. The circles indicate detections, non-detections are marked by a cross. Sources that have neither a circle nor a cross could not be observed due to weather conditions.