The SLUGGS Survey: stellar masses and effective radii of early-type galaxies from Spitzer Space Telescope 3.6$\mu$m imaging

TL;DR

This study uses Spitzer 3.6μm imaging to accurately measure stellar masses and effective radii of early-type galaxies, revealing that optical surveys underestimate sizes of massive galaxies.

Contribution

It introduces a homogeneous method for deriving stellar masses and sizes of early-type galaxies using 3.6μm imaging, improving upon optical and 2MASS measurements.

Findings

3.6μm stellar masses agree with 2.2μm masses

Optical imaging underestimates sizes of massive galaxies

Method can be applied to other early-type galaxy samples

Abstract

Galaxy starlight at 3.6$\mu$m is an excellent tracer of stellar mass. Here we use the latest 3.6$\mu$m imaging from the Spitzer Space Telescope to measure the total stellar mass and effective radii in a homogeneous way for a sample of galaxies from the SLUGGS survey. These galaxies are representative of nearby early-type galaxies in the stellar mass range of 10 $<$ log M$_{\ast}$/M$_{\odot}$ $<$ 11.7, and our methodology can be applied to other samples of early-type galaxies. We model each galaxy in 2D and estimate its total asymptotic magnitude from a 1D curve-of-growth. Magnitudes are converted into stellar masses using a 3.6$\mu$m mass-to-light ratio from the latest stellar population models of R\"ock et al., assuming a Kroupa IMF. We apply a ratio based on each galaxy's mean mass-weighted stellar age within one effective radius (the mass-to-light ratio is insensitive to galaxy metallicity for the generally old stellar ages and high metallicities found in massive early-type galaxies). Our 3.6$\mu$m stellar masses agree well with masses derived from 2.2$\mu$m data. From the 1D surface brightness profile we fit a single Sersic law, excluding the very central regions. We measure the effective radius, Sersic n parameter and effective surface brightness for each galaxy. We find that galaxy sizes derived from shallow optical imaging and the 2MASS survey tend to underestimate the true size of the largest, most massive galaxies in our sample. We adopt the 3.6$\mu$m stellar masses and effective radii for the SLUGGS survey galaxies.