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ABSTRACT. The Spitzer Survey of Stellar Structure in Galaxies (S 4 G) is an Exploration Science Legacy Program approved for the Spitzer post-cryogenic mission. It is a volume-, magnitude-, and size-limited (d < 40 Mpc, jbj > 30°, m Bcorr < 15:5, and D 25 > 1 0 ) survey of 2331 galaxies using the Infrared Array Camera (IRAC) at 3.6 and 4.5 μm. Each galaxy is observed for 240 s and mapped to ≥1:5 × D 25 . The final mosaicked images have a typical 1σ rms noise level of 0.0072 and 0:0093 MJy sr À1 at 3.6 and 4.5 μm, respectively. Our azimuthally averaged surface brightness profile typically traces isophotes at μ 3:6μm ðABÞð1σÞ ∼ 27 mag arcsec À2 , equivalent to a stellar mass surface density of ∼1 M ⊙ pc À2 . S 4 G thus provides an unprecedented data set for the study of the distribution of mass and stellar structures in the local universe. This large, unbiased, and extremely deep sample of all Hubble types from dwarfs to spirals to ellipticals will allow for detailed structural studies, not only as a function of stellar mass, but also as a function of the local environment. The data from this survey will serve as a vital testbed for cosmological simulations predicting the stellar mass properties of present-day galaxies. This article introduces the survey and describes the sample selection, the significance of the 3.6 and 4.5 μm bands for this study, and the data collection and survey strategies. We describe the S 4 G data analysis pipeline and present measurements for a first set of galaxies, observed in both the cryogenic and warm mission phases of Spitzer. For every galaxy we tabulate the galaxy diameter, position angle, axial ratio, inclination at μ 3:6μm ðABÞ ¼ 25:5, and 26:5 mag arcsec À2 (equivalent to ≈μ B ðABÞ ¼ 27:2 and 28:2 mag arcsec À2 , respectively). These measurements will form the initial S 4 G catalog of galaxy properties. We also measure the total magnitude and the azimuthally averaged radial profiles of ellipticity, position angle, surface brightness, and color. Finally, using the galaxy-fitting code GALFIT, we deconstruct each galaxy into its main constituent stellar components: the bulge/spheroid, disk, bar, and nuclear point source, where necessary. Together, these data products will provide a comprehensive and definitive catalog of stellar structures, mass, and properties of galaxies in the nearby universe and will enable a variety of scientific investigations, some of which are highlighted in this introductory S 4 G survey paper.

The Spitzer Survey of Stellar Structure in Galaxies (S 4 G) 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 S 4 G 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 S 4 G 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.

Photometric scaling relations are studied for S0 galaxies and compared with those obtained for spirals. New two-dimensional multi-component decompositions are presented for 122 early-type disc galaxies, using deep K s -band images. Combining them with our previous decompositions, the final sample consists of 175 galaxies (Near-Infrared Survey of S0s, NIRS0S: 117 S0s + 22 S0/a and 36 Sa galaxies). As a comparison sample we use the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS) of nearly 200 spirals, for which similar multi-component decompositions have previously been made by us. The improved statistics, deep images and the homogeneous decomposition method used allow us to reevaluate the parameters of the bulges and discs. For spirals we largely confirm previous results, which are compared with those obtained for S0s. Our main results are as follows. disc), μ 0 (0)] of the S0 galaxies in NIRS0S are similar to those obtained for spirals in the OSUBSGS. Overall, our results support the view that spiral galaxies with bulges brighter than −20 mag in the K band can evolve directly into S0s, due to stripping of gas followed by truncated star formation.

Properties of bars and bulges in the Hubble sequence are discussed, based on an analysis of 216 disc galaxies of S0‐Sm types (S0s from the Near‐Infrared S0 Survey and spirals from the Ohio State University Bright Spiral Galaxy Survey). For this purpose we have collected, and completed when necessary, the various analyses we have previously made separately for early‐ and late‐type galaxies. We find strong evidence of pseudo‐bulges in all Hubble types. Pseudo‐bulges are disc‐like structures formed by secular evolutionary processes in galaxies. Similar to spirals, the early‐type disc galaxies (S0‐S0/a) have on average relatively exponential bulges with Sersic index n < 2, and 56 per cent of them show disc‐like fine structures in the region of the bulge. For some of the galaxies there is also kinematic evidence of pseudo‐bulges. If S0‐S0/a galaxies were once spirals, stripped of their gas, then redistributed gas and star formation in the disc would be a natural explanation for all pseudo‐bulges in the Hubble sequence. However, it is difficult to explain how the bulges of S0 galaxies, which typically include about 30 per cent of the total galaxy mass, were formed by secularly induced central star formation. A more likely explanation is that pseudo‐bulges in barred early‐type galaxies are a combination of secularly induced star formation and the central steepening of the old stellar distribution. Bulges in non‐barred early‐type galaxies could be either classical merger‐built bulges, or pseudo‐bulges formed by similar processes as in barred galaxies, but in response to massive ovals or lenses (70 per cent of S0‐S0/a galaxies have ovals/lenses). Observational support for the outlined picture comes from the fact that bars in early‐type galaxies seem more evolved: their bars are long and massive and frequently (40 per cent) have ansae morphologies. In this scenario it would be possible also to explain why barred early‐type galaxies (preferentially pseudobulges) have slightly smaller B/T flux ratios than the non‐barred early‐type galaxies (mostly classical bulges).

We have estimated the bulge-to-total (B/T) light ratios in the K s band for a sample of 24 S0, S0/a and Sa galaxies by applying a two-dimensional multicomponent decomposition method. For the disc an exponential function is used, the bulges are fitted by a Sérsic R 1/n function and the bars and ovals are described either by a Sérsic or a Ferrers function. In order to avoid non-physical solutions, preliminary characterization of the structural components is made by inspecting the radial profiles of the orientation parameters and the low azimuthal wavenumber Fourier amplitudes and phases. In order to identify also the inner structures, unsharp masks were created: previously undetected inner spiral arms were found in NGC 1415 and marginally in NGC 3941. Most importantly, we found that S0s have a mean B/T K ratio of 0.24 ± 0.11, which is significantly smaller than the mean B/T R = 0.6 generally reported in the literature. Also, the surface brightness profiles of the bulges in S0s were found to be more exponentiallike than generally assumed, the mean shape parameter of the bulge being n = 2.1 ± 0.7. We did not find examples of barred S0s lacking the disc component, but we found some galaxies (NGC 718, 1452 and 4608) having a non-exponential disc in the bar region. To our knowledge, our study is the first attempt to apply a multicomponent decomposition method for a moderately sized sample of early-type disc galaxies.

We present the most complete atlas of nuclear rings to date. We include 113 rings found in 107 galaxies, six of which are elliptical galaxies, five are highly inclined disc galaxies, 18 are unbarred disc galaxies, and 78 are barred disc galaxies. Star-forming nuclear rings occur in 20% of disc galaxies with types between T=-3 and T=7. We aim to explore possible relationships between the size and morphology of the rings and various galactic parameters. We produce colour index and structure maps, as well as Halpha and Paalpha continuum-subtracted images from HST archival data. We derive ellipticity profiles from H-band 2MASS images in order to detect bars and find their metric parameters. We measure the non-axisymmetric torque parameter, Qg, and search for correlations between bar, ring metric parameters, and Qg. Our atlas of nuclear rings includes star-forming and dust rings. Nuclear rings span a range from a few tens of parsecs to a few kiloparsecs in radius. Star-forming nuclear rings can be found in a wide range of morphological types, from S0 to Sd, with a peak in the distribution between Sab and Sb, and without strong preference for barred galaxies. Dust nuclear rings are found in elliptical and S0 galaxies. For barred galaxies, the maximum radius that a nuclear ring can reach is a quarter of the bar radius. We found a nearly random distribution of PA offsets between nuclear rings and bars. There is some evidence that nuclear ring ellipticity is limited by bar ellipticity. We confirm that the maximum relative size of a star-forming nuclear ring is inversely proportional to the non-axisymmetric torque parameter, Qg, and that the origin of nuclear rings, even the ones in non-barred hosts, are closely linked to the existence of dynamical resonances.Comment: Accepted for publication in MNRAS. A full resolution version of the manuscript with high resolution figures can be found at http://www.iac.es/folleto/research/preprints

We present an atlas of K s -band images of 206 early-type galaxies, including 160 S0-S0/a galaxies, 12 ellipticals and 33 Sa galaxies (+ one later type). The majority of the atlas galaxies belong to a magnitude-limited (m B ≤ 12.5 mag) sample of 185 Near-InfraRed S0 Survey galaxies. To ensure that misclassified S0s are not omitted, 25 ellipticals from the Third Reference Catalogue of Bright Galaxies classified as S0s in the Carnegie Atlas were included in the sample. The observations were carried out using 3-4 m class telescopes with subarcsecond pixel resolution (∼0.25 arcsec), and were obtained in good seeing conditions (full width at half-maximum ∼1 arcsec). The images are 2-3 mag deeper than Two-Micron All-Sky Survey images, allowing the detection of faint outer discs in S0s. Both visual and photometric classifications are made, largely following the classification criteria of de Vaucouleurs. Special attention is paid to the classification of lenses, which are coded in a more systematic manner than in any of the previous studies. A new lens type, called a 'barlens', is introduced, possibly forming part of the bar itself. Also, boxy/peanut/x-shaped structures are identified in many barred galaxies, even though the galaxies are not seen edge-on, indicating that vertical thickening is not enough to explain these structures. Photometric classification includes detection of exponential outer discs or other structures not directly visible in the images, but becoming clear in unsharp masking or residual images in decompositions. In our photometric classification, nuclear bars are assigned for 15 galaxies, which are overshadowed by bulges in visual classification. The mean Hubble stage in the near-infrared is found to be similar to that in the optical. We give dimensions of structure components, and radial profiles of the position angles and ellipticities, and show deviations from perfect elliptical isophotes. Shells and ripples, generally assumed to be manifestations of recent mergers, are detected only in six galaxies. However, multiple lenses appear in as much as 25 per cent of the atlas galaxies, which is a challenge to the hierarchical evolutionary picture of galaxies. Such models need to explain how the lenses were formed and then survived in multiple merger events that galaxies may have suffered during their lifetimes.