The relic galaxy NGC~1277 rules out intermediate-age stellar populations origin of CO-strong absorptions in massive early-type galaxies

TL;DR

This study compares the CO absorption features of the relic galaxy NGC 1277 with typical massive ETGs, demonstrating that intermediate-age stars are unlikely to be the cause of strong CO features in these galaxies.

Contribution

It provides evidence against the hypothesis that intermediate-age stellar populations cause strong CO absorptions in massive ETGs by comparing relic and typical galaxies.

Findings

CO lines in NGC 1277 and ETGs are similarly strong

Strong CO features are not explained by stellar population models

Intermediate-age stars are unlikely the cause of CO absorption strength

Abstract

Massive Early-Type Galaxies (ETGs) show several strong CO absorption features in their H- and K-band spectra that cannot be explained by state-of-the-art stellar population models. For many years, the disagreement has been attributed to the presence of intermediate-age stellar components that are dominated by stars in the Asymptotic Giant Branch (AGB) phase. However, no robust evidence of this scenario has been provided so far. One way to test this claim is by comparison of CO indices for ETGs and for relic galaxies. Lacking the intermediate-age stellar populations, relic galaxies provide us with a unique opportunity to address the origin of strong CO absorptions in ETGs. Here, we utilize the prototype relic galaxy NGC 1277 and compare the CO absorption features of this galaxy with the ones of a representative sample of massive ETGs. We show that the CO lines in both systems have similar strengths, significantly stronger than the predictions of stellar population synthesis models. We conclude that intermediate-age stellar populations in massive ETGs are not the culprit of the strong CO absorptions.