SIMBAD references

We analyse spectral properties of 1671 galaxies from the Stromlo-APM Survey, selected to have 15{les}b_J{les}17.15 and having a mean redshift z=0.05. This is a representative local sample of field galaxies, so the global properties of the galaxy population provide a comparative point for analysis of more distant surveys. We measure Hα, [Oii] λ3727, [Sii] λλ6716, 6731, [Nii] λ6583 and [Oi] λ6300 equivalent widths and the D₄₀₀₀ break index. The 5-Å-resolution spectra use an 8-arcsec slit, which typically covers 40-50per cent of the galaxy area. We find no evidence for systematic trends depending on the fraction of galaxy covered by the slit, and further analysis suggests that our spectra are representative of integrated galaxy spectra.

We classify spectra according to their Hα emission, which is closely related to massive star formation. Overall we find that 61per cent of galaxies are Hα emitters with rest-frame equivalent widths EW(Hα) ≳2Å. The emission-line galaxy (ELG) fraction is smaller than seen in the Canada-France Redshift Survey (CFRS) at z=0.2, and is consistent with a rapid evolution of Hα luminosity density. The ELG fraction and EW(Hα) increase at fainter absolute magnitudes, smaller projected area and smaller D₄₀₀₀. In the local Universe, faint, small galaxies are dominated by star formation activity, while bright, large galaxies are more quiescent. This picture of the local Universe is quite different from that of the distant one; bright galaxies appear to show rapidly increasing activity as one moves further back in time.

We find that the ratio [Nii] λ6583/Hα is anticorrelated with EW(Hα), and that the value of 0.5 commonly used to remove the [Nii] contribution from blended Hα+[Nii] λλ6548, 6583 applies only for samples with an EW distribution similar to that seen at low redshift. We show that the [Oii], [Nii], [Sii] and Hα EWs are correlated, but with large dispersions (∼50per cent) owing to the diversity of galaxy contents sampled. Our [Oii]-Hα relation is similar to the one derived by Kennicutt, but is 10per cent higher at 1σ significance. We show that this relation is not valid for distant, strong [Oii] emitters with blue colours, which are more numerous than locally. This relation would overestimate the individual star formation rate by ∼50per cent for these kinds of galaxies. We find that, on average, luminous blue ELGs are likely to be enhanced in nitrogen abundance. This suggests that in faint, low-mass, late-type ELGs nitrogen is a primary element, whereas in brighter, more massive galaxies nitrogen comes from a secondary source. We also find that 4per cent of early-type galaxies show star formation activity; this fraction seems to increase at higher redshifts.