Spectral analysis of the Stromlo-APM survey - I. Spectral properties of galaxies

We analyse spectral properties of 1671 galaxies from the Stromlo-APM Survey , selected to have 15 less than or equal to b(J) less than or equal to 17.1 5 and having a mean redshift z = 0.05. This is a representative local sampl e of field galaxies, so the global properties of the galaxy population prov ide a comparative point for analysis of more distant surveys. We measure H alpha, [O II] lambda 3727, [S II] lambda lambda 6716, 6731, [N II] lambda 6 583 and [O I] lambda 6300 equivalent widths and the D-4000 break index. The 5-Angstrom-resolution spectra use an 8-arcsec slit, which typically covers 40-50 per cent of the galaxy area. We find no evidence for systematic tren ds depending on the fraction of galaxy covered by the slit, and further ana lysis suggests that our spectra are representative of integrated galaxy spe ctra. We classify spectra according to their H alpha emission, which is closely r elated to massive star formation. Overall we find that 61 per cent of galax ies are H alpha emitters with rest-frame equivalent widths EW(H alpha) grea ter than or similar to 2 Angstrom. 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 H alpha luminosity density. T he ELG fraction and EW(H alpha) increase at fainter absolute magnitudes, sm aller projected area and smaller D-4000. In the local Universe, faint, smal l galaxies are dominated by star formation activity, while bright, large ga laxies are more quiescent. This picture of the local Universe is quite diff erent 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 [N II] lambda 6583/H alpha is anticorrelated with EW (H alpha), and that the value of 0.5 commonly used to remove the [N II] con tribution from blended H alpha + [N II] lambda lambda 6548, 6583 applies on ly for samples with an EW distribution similar to that seen at low redshift . We show that the [O II], [N II], [S II] and H alpha EWs are correlated, b ut with large dispersions (similar to 50 per cent) owing to the diversity o f galaxy contents sampled. Our [O II]-H alpha relation is similar to the on e derived by Kennicutt, but is 10 per cent higher at 1 sigma significance. We show that this relation is not valid for distant, strong [O II] emitters with blue colours, which are more numerous than locally. This relation wou ld overestimate the individual star formation rate by similar to 50 per cen t 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 fai nt, low-mass, late-type ELGs nitrogen is a primary element, whereas in brig hter, more massive galaxies nitrogen comes from a secondary source. We also find that 4 per cent of early-type galaxies show star formation activity; this fraction seems to increase at higher redshifts.