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Selection effects

The main ELG candidate selection criteria applied here are a blue continuum (near $\lambda$ 4000 Å) and the presence of strong or moderate [O III] $\lambda\lambda$4959,5007 Å emission lines recognized in digitized prism spectra with magnitudes in the range B = $16\fm0 - 19\fm5$. Figure 5 shows the equivalent widths of the [O III]$\lambda$5007 Å line versus the apparent and absolute blue magnitudes. It is evident from Fig. 5a that the detection limit of the HSS for BCGs based on this strongest emission line is EW([O III]$\lambda$5007 Å) $\le$ 15 Å. There is a clear trend of increase of equivalent widths (EW) with apparent magnitude, and even more noticeable anticorrelation between EWs and absolute magnitudes, if not only BCGs but all ELGs are taken into account. Similar trends were noticed for the UM ELGs by Salzer et al. ([1989a], [1989b]).

  

Figure: a,b Logarithm of the equivalent width of [O III]$\lambda$5007 Å emission line versus blue apparent ( a) and absolute magnitudes ( b) for HSS ELGs. BCG are plotted as filled circles and other types as empty circles. The solid line joins averages of equivalent widths taken over 1 mag apparent magnitude intervals ( a) and over 2 mag absolute magnitude intervals ( b)

In analogy to the UM ELGs, the anticorrelation evident in Fig. 5b is caused by two effects. First, because of the brightness limit imposed by the plates we miss low-luminosity ELGs with weak emission lines. The galaxies would populate the lower left corner of the Fig. 5b. Second, the region of avoidance in the upper right corner is most likely caused by metallicity effects. In general HII regions in bright galaxies have larger heavy element abundance, and, therefore, they are cooler with lower EW([O III]$\lambda$5007 Å).