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Aspects of spectral classification, chemistry and metallicity

We already discussed in section 4.1.1, that the ELGs are classified according to their positions in the line ratio diagnostic diagrams.

A comparison of the positions of the new HSS ELGs in these diagrams shows that our classification appears to be self-consistent for all BCGs, and, therefore, is rather reliable.

It is interesting to compare the positions in diagnostic diagram of the new HSS BCGs with those of the well studied BCGs from the SBS sample (Izotov et al. [1994], [1997b]; Thuan et al. [1995]). Most of the SBS BCGs populate the region with [N II] $\lambda\,6583$ Å/H$\alpha$ < -1.2 (Fig. 4). About 20 % of the HSS BCGs are located in the same region. The real fraction of such BCGs with high excitation HII regions will be quantified after better quality spectroscopic observations. At least 2 HSS BCGs with high excitation HII regions are present in the SBS list ( $\delta = 49^\circ - 50^\circ$) and they are independently rediscovered in the present study. In particular, our line flux ratios for HS 1249+4919 (log(O/H)+12 = 7.72) are very close to those from Thuan et al. ([1995]).

  

Figure: Histograms of apparent a and absolute b photographic magnitudes for three BCGs samples: new HSS BCGs from this paper, BCGs from the SBS sample (Pustilnik et al. [1995]) and Case BCGs from Ugryumov et al. ([1998]), Ugryumov ([1997]). The arrows indicate the mean magnitude for each sample

The region with [N II] $\lambda\,6583$ Å/H$\alpha$ < -2 contains the BCGs with the lowest metallicities. We discover two new HSS BCGs located in this region. Hence low metallicity objects can be selected by our search method.