Mid-UV Narrow-Band Indices of Evolved Simple Stellar Populations

TL;DR

This study analyzes mid-ultraviolet spectroscopic indices of simple stellar populations using high-resolution spectral libraries and compares the results with observations of Galactic globular clusters, revealing age and metallicity sensitivities.

Contribution

It introduces a new set of UV indices derived from synthetic spectra and compares them with observations, highlighting their potential to identify young, metal-rich stellar populations.

Findings

Most indices show slight age sensitivity for populations older than 2 Gyr.

Fe II 2332 and Fe II 2402 indices sharply increase within the first 2 Gyr at high metallicity.

Comparison with globular cluster data shows good agreement and confirms metallicity trends.

Abstract

We explore the properties of selected mid-ultraviolet (1900-3200 angstrom) spectroscopic indices of simple stellar populations (SSPs). We incorporate the high resolution UVBLUE stellar spectral library into an evolutionary population synthesis code, based on the most recent Padova isochrones. We analyze the trends of UV indices with respect to age and chemical composition. As a first test against observations, we compare our results with the empirical mid-UV spectral indices of Galactic globular clusters, observed with the International Ultraviolet Explorer (IUE). We find that synthetic indices exhibit a variety of properties, the main one being the slight age sensitivity of most of them for ages>2 Gyr. However, for high metallicity, two indices, Fe II 2332 and Fe II 2402, display a remarkably different pattern, with a sharp increase within the first two Gyr and, thereafter, a rapid decline. These indices clearly mark the presence of young (~1 Gyr) metal rich (Z > Z_sun) stellar populations. We complement existing UV indices of Galactic globular clusters with new measurements, and carefully identify a sub-sample of ten indices suitable for comparison with theoretical models. The comparison shows a fair agreement and, in particular, the strong trend of the indices with metallicity is well reproduced. We also discuss the main improvements that should be considered in future modelling concerning, among others, the effects of alpha-enhancement in the spectral energy distributions.