HII galaxies as standard candles: Evolutionary corrections

Over the past decade, the relation between the Balmer-line luminosity of HII galaxies and the velocity width of the emission lines, the L − σ relation, has been painstakingly calibrated as a cosmological distance indicator with seemingly spectacular results: the Hubble constant and the energy density of dark energy obtained using the L − σ indicator agree remarkably well with the values from canonical indicators. Since most of the luminosity of these young compact starburst galaxies is emitted by a few narrow emission lines, they can be observed with good precision up to redshifts of z ∼ 7 with JWST, making the L − σ indicator a potentially unique cosmological probe. However, the precision of the method remains too low to effectively constrain the relevant cosmological parameters, notably the equation of state of dark energy. The scatter of the L − σ relation is significantly larger than the random observational errors, so we do not have a good handle on the systematics of the method. In a previous paper, we posited that since the ionizing radiation of these young galaxies fades rapidly over timescales of only a few million years, age differences could be the main underlying cause of the scatter. In this paper, we explore several different ways of explaining the scatter of the correlation, but without success. We show that the majority of HII galaxies are powered by multiple starbursts of slightly different ages, and therefore that the equivalent widths are not reliable chronometers to correct the luminosities for evolution. Thus, it is not likely that the accuracy of the L − σ distance indicator can be improved in the near future. Since we do not fully understand either the systematics or the underlying physics of the L − σ relation, using large samples of distant HII galaxies may or may not improve the accuracy of the method.