HII galaxies as standard candles: evolutionary corrections

TL;DR

HII galaxies' L-σ relation shows promise as a cosmological distance indicator, but current uncertainties and complex starburst histories limit its precision for constraining dark energy properties.

Contribution

This study investigates the causes of scatter in the L-σ relation, concluding that multiple starbursts and unreliable age indicators hinder improving its cosmological utility.

Findings

Scatter in the L-σ relation is not due to simple age effects.

Most HII galaxies are powered by multiple starbursts of different ages.

Using equivalent widths as age indicators is unreliable for correction.

Abstract

Over the past decade the relation between the Balmer-line luminosity of HII galaxies and the velocity width of the emission lines, the L - {\sigma} 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-{\sigma} 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 redshifts z ~ 7 with JWST, making the L - {\sigma} 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 - {\sigma} 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 time-scales 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 to explain 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 - {\sigma} distance indicator can be improved in the near future. Since we do not fully understand neither the systematics nor the underlying physics of the L - {\sigma} relation, using large samples of distant HII galaxies may or may not improve the accuracy of the method.