The Spitzer Survey of Stellar Structure in Galaxies (S4G) is the largest available database of deep, homogeneous middle-infrared (mid-IR) images of galaxies of all types. The survey, which includes 2352 nearby galaxies, reveals galaxy morphology only
minimally affected by interstellar extinction. This paper presents an atlas and classifications of S4G galaxies in the Comprehensive de Vaucouleurs revised Hubble-Sandage (CVRHS) system. The CVRHS system follows the precepts of classical de Vaucouleurs (1959) morphology, modified to include recognition of other features such as inner, outer, and nuclear lenses, nuclear rings, bars, and disks, spheroidal galaxies, X patterns and box/peanut structures, OLR subclass outer rings and pseudorings, bar ansae and barlenses, parallel sequence late-types, thick disks, and embedded disks in 3D early-type systems. We show that our CVRHS classifications are internally consistent, and that nearly half of the S4G sample consists of extreme late-type systems (mostly bulgeless, pure disk galaxies) in the range Scd-Im. The most common family classification for mid-IR types S0/a to Sc is SA while that for types Scd to Sm is SB. The bars in these two type domains are very different in mid-IR structure and morphology. This paper examines the bar, ring, and type classification fractions in the sample, and also includes several montages of images highlighting the various kinds of stellar structures seen in mid-IR galaxy morphology.
Galaxy morphology has many structures that are suggestive of various processes or stages of secular evolution. Internal perturbations such as bars can drive secular evolution through gravity torques that move gas into the central regions and build up
a flattened, disk-like central bulge, or which may convert an open spiral pseudoring into a more closed ring. Interaction between individual components of a galaxy, such as between a bar and a dark halo, a bar and a central mass concentration, or between a perturbation and the basic state of a stellar disk, can also drive secular transformations. In this series of lectures, I review many aspects of galaxy morphology with a view to delineating some of the possible evolutionary pathways between different galaxy types.
Galaxy morphology is a product of how galaxies formed, how they interacted with their environment, how they were influenced by internal perturbations, AGN, and dark matter, and of their varied star formation histories. This article reviews the phenom
enology of galaxy morphology and classification with a view to delineating as many types as possible and how they relate to physical interpretations. The old classification systems are refined, and new types introduced, as the explosion in available morphological data has modified our views on the structure and evolution of galaxies.
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.
Using data from the Near-Infrared S0 Survey (NIRS0S) of nearby, early-type galaxies, we examine the distribution of bar strengths in S0 galaxies as compared to S0/a and Sa galaxies, and as compared to previously published bar strength data for Ohio S
tate University Bright Spiral Galaxy Survey (OSUBSGS) spiral galaxies. Bar strengths based on the gravitational torque method are derived from 2.2 micron Ks-band images for a statistical sample of 138 (98 S0, 40 S0/a,Sa) galaxies having a mean total blue magnitude <B_T> <= 12.5 and generally inclined less than 65 degrees. We find that S0 galaxies have weaker bars on average than spiral galaxies in general, even compared to their closest spiral counterparts, S0/a and Sa galaxies. The differences are significant and cannot be due entirely to uncertainties in the assumed vertical scale-heights or in the assumption of constant mass-to-light ratios. Part of the difference is likely due simply to the dilution of the bar torques by the higher mass bulges seen in S0s. If spiral galaxies accrete external gas, as advocated by Bournaud & Combes, then the fewer strong bars found among S0s imply a lack of gas accretion according to this theory. If S0s are stripped former spirals, or else are evolved from former spirals due to internal secular dynamical processes which deplete the gas as well as grow the bulges, then the weaker bars and the prevalence of lenses in S0 galaxies could further indicate that bar evolution continues to proceed during and even after gas depletion
