The Stellar Populations of Starburst Galaxies Through near infrared spectroscopy

TL;DR

This study uses near-infrared spectroscopy to analyze the stellar populations in the central regions of four starburst galaxies, revealing older ages and a broader age spread than optical methods, and providing detailed stellar population models.

Contribution

First simultaneous fit of 15 NIR absorption features in starburst galaxies, revealing detailed stellar ages, metallicities, and improved understanding of stellar populations through NIR spectroscopy.

Findings

Stellar populations contain 20-56% ~1 Gyr old stars with TP-AGB stars.

Dominant stellar metallicity is solar, with worse fits for non-solar metallicities.

NIR estimates suggest older ages and larger age spreads than optical methods.

Abstract

We study the central (inner few hundred parsecs) stellar populations of four starburst galaxies (NGC34, NGC1614, NGC3310 and NGC7714) in the near-infrared (NIR), from 0.8 to 2.4microns, by fitting combinations of stellar population models of various ages and metallicities. The NIR spectra of these galaxies feature many absorption lines. For the first time, we fit simultaneously as much as 15 absorption features in the NIR. The observed spectra are best explained by stellar populations containing a sizable amount (20 to 56 % by mass) of ~1Gyr old stellar population with Thermally Pulsing-Asymptotic Giant Branch stars. We found that the metallicity of the stars which dominates the light is solar. Metallicities substantially different from solar give a worse fit. Though the ages and metallicities we estimate using the NIR spectroscopy are in agreement with values from the literature based on the UV/optical, we find older ages and a larger age spread. This may be due to the fact that the optical is mostly sensitive to the last episode of star formation, while the NIR better maintains the record of previous stellar generations. Another interesting result is that the reddening estimated from the whole NIR spectrum is considerably lower than that based on emission lines. Finally, we find a good agreement of the free emission line spectrum with photoionization models, using as input spectral energy distribution the synthetic composite template we derived as best-fit.