A New Approach to the Study of Stellar Populations in Early-Type Galaxies: K-band Spectral Indices and an Application to the Fornax Cluster

TL;DR

This study measures K-band spectral features in early-type galaxies in the Fornax cluster, revealing correlations with metallicity, stellar population differences, and the effectiveness of near-IR diagnostics in identifying young stellar components.

Contribution

It introduces new K-band spectral measurements and demonstrates their utility in analyzing stellar populations and detecting young stars in early-type galaxies.

Findings

K-band Na I index correlates with metallicity and velocity dispersion.

Fe features saturate at high velocity dispersions, Na I continues to increase.

Galaxies with young stars show distinct near-IR signatures.

Abstract

New measurements of K-band spectral features are presented for eleven early-type galaxies in the nearby Fornax galaxy cluster. Based on these measurements, the following conclusions have been reached: (1) in galaxies with no signatures of a young stellar component, the K-band Na I index is highly correlated with both the optical metallicity indicator [MgFe]' and central velocity dispersion; (2) in the same galaxies, the K-band Fe features saturate in galaxies with sigma > 150 km/s while Na I (and [MgFe]') continues to increase; (3) [Si/Fe] (and possibly [Na/Fe]) is larger in all observed Fornax galaxies than in Galactic open clusters with near-solar metallicity; (4) in various near-IR diagnostic diagrams, galaxies with signatures of a young stellar component (strong Hbeta, weak [MgFe]') are clearly separated from galaxies with purely old stellar populations; furthermore, this separation is consistent with the presence of an increased number of M-giant stars (most likely to be thermally pulsating AGB stars); (5) the near-IR diagrams discussed here seem as efficient for detecting putatively young stellar components in early-type galaxies as the more commonly used age/metallicity diagnostic plots using optical indices (e.g Hbeta vs. [MgFe]').