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

Abstract

We analyse spectral properties of 1671 galaxies from the Stromlo–APM Survey, selected to have 15bJ17.15 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‐arcsec slit, which typically covers 40–50 per 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 61 per cent of galaxies are Hα emitters with rest‐frame equivalent widths EW(Hα) ≳2 Å. 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 [N ii] λ6583/Hα is anticorrelated 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 the [O ii], [N ii], [S ii] and Hα EWs are correlated, but with large dispersions (∼50 per cent) owing to the diversity of galaxy contents sampled. Our [O ii]–Hα relation is similar to the one derived by Kennicutt, but is 10 per cent higher at 1σ 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 would overestimate the individual star formation rate by ∼50 per 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 4 per cent of early‐type galaxies show star formation activity; this fraction seems to increase at higher redshifts.

An Emission‐Line Search for Star‐forming Dwarf Galaxies toward Abell 851

“…Collisions between the same free electrons and singly ionized oxygen atoms produce the excited states that decay via the [O II] jj3726, 3729 transitions. It is therefore not surprising that the [O II] and Ha Ñuxes are correlated (e.g., Gallagher et al 1989 ;Kennicutt 1992a ;Tresse et al 1999). The empirical relation among the bluest local galaxies shows a scatter of a factor of 3 about the mean [O II] versus Hb relation, (Gallagher et al 1989 (Osterbrock 1989).…”

“…Throughout this paper we adopt as a representative extinction A B \ 1.0 correction for dwarf irregular galaxies (e.g., Martin 1997), and the reddened Ha Ñux is predicted to be This 1.17F *O II+ . approximation will systematically underestimate the SFRs of spiral galaxies because their Ðtted ratio is F *O II+ /F Ha slightly lower (Kennicutt 1992a ;Tresse et al 1999).…”

“…1996 ;Hammer et al 1997 ;Balogh et al 1997 ;Hogg et al 1998 ;Tresse et al 1999 ;C. Gronwall 2000, private communication).…”

“…The Ha luminosity is assumed to be attenuated by a factor of 1.75, corresponding to (Seaton 1979). For comparison, A B \ 1.0 the local Ha luminosity functions from Gallego et al (1995) and Tresse et al (1999) are shown. All results are scaled to km s~1 Mpc~1, H 0 \ 70 and…”

“…The major limitation of this method is the limited accuracy of [O II] as a tracer of the number of young massive stars. Fortunately, the [O II] Ñux from nearby galaxies correlates with their Ha luminosity, which is a direct measure of the SFR (Gallagher, Bushouse, & Hunter 1989 ;Kennicutt 1992a ;Tresse et al 1999). The scatter in this relation and uncertainty regarding the amount of attenuation from dust in the galaxies limit how accurately we can estimate SFRs for the objects we Ðnd, but the method does identify a much larger sample of candidates for follow-up studies than has previously been available.…”