Top panel: rest-frame equivalent width of the He II λ1640 line as a function of the average S/N per resolution element of the spectra in the two regions 1420–1500 Å and 1700–1800 Å for the 39 galaxies with detected He II in emission. The blue circles, red diamonds, black triangles, and green asterisks represent the reliable emitters with narrow He II lines, the reliable emitters with broad He II lines, the possible emitters, and the objects classified as AGN, respectively. Bottom panel: histogram of the average S/N at 1420–1500 Å and 1700–1800 Å for all 277 galaxies in the sample (long dashed line), for the DEEP and UDEEP spectra (dotted and dot-dashed, respectively), and for the 39 galaxies with He II in emission (continuous filled histogram).