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

Abstract

The strong spectral features near 2.2 μm in early-type galaxies remain relatively unexplored. Yet, they open a tightly focused window on the coolest giant stars in these galaxies—a window that can be used to explore both age and metallicity effects. Here, new measurements of K-band spectral features are presented for 11 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 the central velocity dispersion σ; (2) in the same galaxies, the K-band Fe features saturate in galaxies with σ > 150 km s−1, 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 Hβ, 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 Na I versus σ or ⟨Fe I⟩ versus σ 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., Hβ vs. [MgFe]'). The combination of these spectral indices near 2.2 μm with high spatial resolution spectroscopy from ground-based or space-based observatories promises to provide new insights into the nature of stellar populations in the central regions of distant early-type galaxies.