The L - σ relation for HII galaxies in green

The correlation between emission-line luminosity (L) and profile-width (sigma) 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 alpha or H beta 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-sigma relation, defined using [OIII]5007 luminosities, is significantly more sensitive than H beta to the effects of age and the physical conditions of the nebulae, which more than off sets 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 beta luminosities using the Hubble constant H-0 to quantify the possible systematic effects. We find the mixed L(H beta) - sigma([OIII]) relation to be at least as powerful as the canonical L-sigma 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-sigma 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 beta, 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(-4.5)(+5.0) 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.