The Mid-Infrared Luminosities of Normal Galaxies over Cosmic Time

TL;DR

This study investigates the contribution of TP-AGB stars to the mid-infrared luminosity of galaxies across cosmic time, revealing their significant role and implications for star formation rate estimates.

Contribution

It introduces empirical models linking TP-AGB star emission to galaxy mid-IR luminosity and compares these models with observations from z=0 to z=2.

Findings

TP-AGB stars significantly contribute to galaxy mid-IR luminosity up to z=2.

Mid-IR emission can be used to trace star formation over ~1.5 Gyr.

Star formation rates at z=1 may be overestimated by factors of 1.5-6.

Abstract

Modern population synthesis models estimate that 50% of the restframe K-band light is produced by TP-AGB stars during the first Gyr of a stellar population, with a substantial fraction continuing to be produced by the TP-AGB over a Hubble time. Between 0.2 and 1.5 Gyr, intermediate mass stars evolve into TP-AGB C stars which, due to significant amounts of circumstellar dust, emit half their energy in the mid-IR. We combine these results using published mid-IR colors of Galactic TP-AGB M and C stars to construct simple models for exploring the contribution of the TP-AGB to 24micron data as a function of stellar population age. We compare these empirical models with an ensemble of galaxies in the CDFS from z=0 to z=2, and with high quality imaging in M81. Within the uncertainties, the TP-AGB appears responsible for a substantial fraction of the mid-IR luminosities of galaxies from z=0 to z=2, the maximum redshift to which we can test our hypothesis, while, at the same time, our models reproduce much of the detailed structure observed in mid-IR imaging of M81. The mid-IR is a good diagnostic of star formation over timescales of ~1.5 Gyr, but this implies that on-going star formation rates at z=1 may be overestimated by factors of ~1.5-6, depending on the nature of star formation events. Our results, if confirmed through subsequent work, have strong implications for the star formation rate density of the universe and the growth of stellar mass over time.