Stellar population synthesis models between 2.5 and 5 {\mu}m based on the empirical IRTF stellar library

TL;DR

This paper introduces the first empirical stellar population models in the 2.5-5 μm infrared range, based on the IRTF stellar library, to better understand galaxy properties and star formation histories.

Contribution

It provides new empirical models for the infrared range using the IRTF library, covering various ages, metallicities, and initial mass functions, and compares them with observed galaxy colours.

Findings

Good agreement with observed early-type galaxy colours for older, massive galaxies.

Colours show slight dependence on metallicity and age, indicating robustness of the models.

Mass-to-light ratio in the infrared is less sensitive to age and metallicity than optical bands.

Abstract

We present the first single-burst stellar population models in the infrared wavelength range between 2.5 and 5 {\mu}m which are exclusively based on empirical stellar spectra. Our models take as input 180 spectra from the stellar IRTF (Infrared Telescope Facility) library. Our final single-burst stellar population models are calculated based on two different sets of isochrones and various types of initial mass functions of different slopes, ages larger than 1 Gyr and metallicities between [Fe/H] = -0.70 and 0.26. They are made available online to the scientific community on the MILES web page. We analyse the behaviour of the Spitzer [3.6]-[4.5] colour calculated from our single stellar population models and find only slight dependences on both metallicity and age. When comparing to the colours of observed early-type galaxies, we find a good agreement for older, more massive galaxies that resemble a single-burst population. Younger, less massive and more metal-poor galaxies show redder colours with respect to our models. This mismatch can be explained by a more extended star formation history of these galaxies which includes a metal-poor or/and young population. Moreover, the colours derived from our models agree very well with most other models available in this wavelength range. We confirm that the mass-to-light ratio determined in the Spitzer [3.6] {\mu}m band changes much less as a function of both age and metallicity than in the optical bands.