The L−σ relation for HII galaxies in green

The correlation between emission-line luminosity ( L ) and profile-width ( σ ) for HII galaxies provides a powerful method to measure the distances to galaxies over a wide range of redshifts. In this paper, we use SDSS spectrophotometry to explore the systematics of the correlation using the [OIII]5007 lines instead of H α or H β to measure luminosities and line widths. We also examine possible systematic effects involved in measuring the profile-widths and the luminosities through different apertures. We find that the green L − σ relation, defined using [OIII]5007 luminosities, is significantly more sensitive than H β to the effects of age and the physical conditions of the nebulae, which more than offsets the advantage of the higher strength of the [OIII]5007 lines. We then explore the possibility of mixing [OIII]5007 profile-widths with SDSS H β luminosities using the Hubble constant H 0 to quantify the possible systematic effects. We find the mixed L (H β ) − σ [OIII] relation to be at least as powerful as the canonical L − σ relation as a distance estimator, and we show that evolutionary corrections do not change the slope and the scatter of the correlation and, therefore, do not bias the L − σ distance indicator at high redshifts. Locally, however, the luminosities of the giant HII regions that provide the zero-point calibrators are sensitive to evolutionary corrections and may bias the Hubble constant if their mean ages, as measured by the equivalent widths of H β , are significantly different from the mean age of the HII galaxies. Using a small sample of 16 ad-hoc zero point calibrators we obtain a value of H 0 = 66.4 + 5.0 -4.5 km s -1 Mpc -1 for the Hubble constant, which is fully consistent with the best modern determinations, and which is not biased by evolutionary corrections.