Mid-Infrared Galaxy Morphology from the Spitzer Survey of Stellar
  Structure in Galaxies (S^4G): The Imprint of the de Vaucouleurs Revised
  Hubble-Sandage Classification System at 3.6 microns
  Spitzer Space Telescope Infrared Array Camera (IRAC) imaging provides an
opportunity to study all known morphological types of galaxies in the mid-IR at
a depth significantly better than ground-based near-infrared and optical
images. The goal of this study is to examine the imprint of the de Vaucouleurs
classification volume in the 3.6 micron band, which is the best Spitzer
waveband for galactic stellar mass morphology owing to its depth and its
reddening-free sensitivity mainly to older stars. For this purpose, we have
prepared classification images for 207 galaxies from the Spitzer archive, most
of which are formally part of the Spitzer Survey of Stellar Structure in
Galaxies (S^4G), a Spitzer post-cryogenic ("warm") mission Exploration Science
Legacy Program survey of 2,331 galaxies closer than 40 Mpc. For the purposes of
morphology, the galaxies are interpreted as if the images are {\it blue light},
the historical waveband for classical galaxy classification studies. We find
that 3.6 micron classifications are well-correlated with blue-light
classifications, to the point where the essential features of many galaxies
look very similar in the two very different wavelength regimes. Drastic
differences are found only for the most dusty galaxies. Consistent with a
previous study by Eskridge et al. (2002), the main difference between blue
light and mid-IR types is an approximately 1 stage interval difference for S0/a
to Sbc or Sc galaxies, which tend to appear "earlier" in type at 3.6 microns
due to the slightly increased prominence of the bulge, the reduced effects of
extinction, and the reduced (but not completely eliminated) effect of the
extreme population I stellar component. We present an atlas of all of the 207
galaxies analyzed here, and bring attention to special features or galaxy types
that are particularly distinctive in the mid-IR.
