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