Spectral Analysis of the Stromlo-APM Survey I. Spectral Properties of Galaxies

We analyze spectral properties of 1671 galaxies from the Stromlo-APM survey, selected to have 15 ≤ bJ ≤ 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, [O II] λ3727, [S II] λλ6716, 6731, [N II] λ6583 and [O I] λ6300 equivalent widths and the D4000 break index. The 5 Å resolution spectra use an 8 slit, which typically covers 40– 50% 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 61% of galaxies are Hα emitters with restframe equivalent widths EW(Hα) >∼ 2 Å. The emission-line galaxy (ELG) fraction is smaller than seen in the CFRS at z = 0.2 (Tresse et al. 1996) and is consistent with a rapid evolution of Hα luminosity density (Tresse & Maddox 1998). 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 the distant one, where bright galaxies appear to show rapidly-increasing activity back in time. We find that the ratio [N II] λ6583/Hα is anti-correlated with EW(Hα), and that the value of 0.5 commonly used to remove the [N II] contribution from blended Hα + [N II] λλ6548, 6583 applies only for samples with an EW distribution similar to that seen at low redshift. We show that [O II], [N II], [S II] and Hα EWs are correlated, but with large dispersions (∼ 50%) due to the diversity of galaxy contents sampled. Our [O II]–Hα relation is similar to the one derived by Kennicutt (1992), but is a factor 10% higher at 1σ significance. We show that this relation is not valid for distant, strong [O II] emitters with blue colors, which are more numerous than locally. This relation would overestimate the individual star formation rate (SFR) by ∼ 50% for these kind of galaxies. We find that on average luminous blue ELG are likely to be enhanced in nitrogen abundance. This suggests that in faint, low-mass, late-type ELG, nitrogen is a primary element, whereas in more bright, more massive galaxies nitrogen it comes from a secondary source. We also find 4% of early-type galaxies which show star formation activity; this fraction seems to increase at higher redshifts.