Lick-index entanglement and biased diagnostic of stellar populations in galaxies

TL;DR

This paper critically examines the statistical and operational properties of the Lick-index system, proposing improvements for more accurate stellar population diagnostics in galaxies, including bias correction and index redefinition.

Contribution

It introduces a revised approach to Lick indices, addressing non-Gaussian effects, index overlaps, and emission contamination, enhancing the reliability of stellar population estimates.

Findings

Spectral S/N >= 5 px^{-1} needed for reliable indices

Revised Hbeta scale suggests 20-30% younger galaxy ages

Fe4531 index is more sensitive to Titanium than Iron

Abstract

The Lick-index spectrophotometric system is investigated in its inherent statistical and operational properties to ease a more appropriate use for astrophysical studies. Non-Gaussian effects in the index standardization procedure suggest that a minimum S/N ratio has to be reached by spectral data, such as S/N >= 5 px^{-1} for a spectral resolution R~2000. In addition, index (re-)definition in terms of narrow-band "color" should be preferred over the classical pseudo-equivalent width scheme. The overlapping wavelength range among different indices is also an issue, as it may lead the latter ones to correlate, beyond any strictly physical relationship. The nested configuration of the Fe5335, Fe5270 indices, and the so-called "Mg complex" (including Mg1, Mg2 and Mgb) is analysed, in this context, by assessing the implied bias when joining entangled features into "global" diagnostic meta-indices, like the perused [MgFe] metallicity tracer. The perturbing effect of [OIII](5007) and [NI](5199) forbidden gas emission on Fe5015 and Mgb absorption features is considered, and an updated correction scheme is proposed when using [OIII](5007) as a proxy to appraise Hbeta residual emission. When applied to present-day elliptical galaxy population, the revised Hbeta scale leads, on average, to 20-30% younger age estimates. Finally, the misleading role of the christening element in Lick-based chemical analyses is illustrated for the striking case of Fe4531. In fact, while Iron is nominally the main contributor to the observed feature in high-resolution spectra, we have shown that the Fe4531 index actually maximizes its responsiveness to Titanium abundance.