Strong CO Absorption Features in Massive ETGs

Abstract

Massive Early-Type Galaxies (ETG) in the local Universe are believed to be the most mature stage of galaxy evolution. Their stellar population content reveals the evolutionary history of these galaxies. However, while state-of-the-art Stellar Population Synthesis (SPS) models provide an accurate description of observed galaxy spectra in the optical range, the modelling in the Near-Infrared (NIR) is still in its infancy. Here we focus on NIR CO absorption features to show, in a systematic and comprehensive manner, that for massive ETGs, all CO indices, from H through to K band, are significantly stronger than currently predicted by SPS models. We explore and discuss several possible explanations of this "CO mismatch", including the effect of intermediate-age, AGB-dominated, stellar populations, high metallicity populations, non-solar abundance ratios and the initial mass function. While none of these effects is able to reconcile models and observations, we show that ad-hoc "empirical" corrections, taking into account the effect of CO-strong giant stars in the low-temperature regime, provide model predictions that are closer to the observations. Our analysis points to the effect of carbon abundance as the most likely explanation of NIR CO line-strengths, indicating possible routes for improving the SPS models in the NIR.

24 pages, 12 figures, accepted for publication in MNRAS

References in corpus (19)

• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• The Dartmouth Stellar Evolution Database
• Evolutionary Stellar Population Synthesis with MILES. Part I: The Base Models and a New Line Index System
• UV-extended E-MILES stellar population models: young components in massive early-type galaxies
• Population Synthesis in the Blue IV: Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations
• Stellar populations of early-type galaxies in different environments II. Ages and metallicities
• The Extended IRTF Spectral Library: Expanded coverage in metallicity, temperature, and surface gravity
• IMF and [Na/Fe] abundance ratios from optical and NIR Spectral Features in Early-type Galaxies
• A New Approach to the Study of Stellar Populations in Early-Type Galaxies: K-band Spectral Indices and an Application to the Fornax Cluster
• MILES extended: Stellar population synthesis models from the optical to the infrared
• Stellar population synthesis models between 2.5 and 5 μm based on the empirical IRTF stellar library
• A new stellar library in the region of the CO index at 2.3 $\mu$m. New index definition and empirical fitting functions
• Comparison of Stellar Population Model Predictions Using Optical and Infrared Spectroscopy
• Optical/NIR stellar absorption and emission-line indices from luminous infrared galaxies
• sMILES: A Library of Semi-Empirical MILES Stellar Spectra with Variable [$\alpha$/Fe] Abundances
• The puzzling interpretation of NIR indices: The case of NaI2.21
• Probing the Central Regions of Nearby Compact Elliptical Galaxies
• Young stellar population gradients in central cluster galaxies from NUV and optical spectroscopy
• The APOGEE Library of Infrared SSP Templates (A-LIST): High-Resolution Simple Stellar Population Spectral Models in the H-Band