The<i>Spitzer</i>Survey of Stellar Structure in Galaxies

The Spitzer Survey of Stellar Structure in Galaxies (S^4G) is an Exploration Science Legacy Program&#13;\napproved for the Spitzer post–cryogenic mission. It is a volume-, magnitude-, and size-limited (d &lt; 40 Mpc,&#13;\n|b|&gt; 30°,m_(Bcorr) &lt; 15:5, and D_(25) &gt; 1') survey of 2331 galaxies using the Infrared Array Camera (IRAC) at 3.6 and&#13;\n4.5 μm. Each galaxy is observed for 240 s and mapped to ≥ 1:5 × D_(25). The final mosaicked images have a typical&#13;\n1 σ rms noise level of 0.0072 and 0:0093 MJy sr^-1 at 3.6 and 4.5 μm, respectively. Our azimuthally averaged surface&#13;\nbrightness profile typically traces isophotes at μ_(3.6μm (AB)(1σ) ~ 27 mag arcsec^(-2), equivalent to a stellar mass surface&#13;\ndensity of ~1 M_⊙pc^(-2). S^4G thus provides an unprecedented data set for the study of the distribution of mass&#13;\nand stellar structures in the local universe. This large, unbiased, and extremely deep sample of all Hubble types from&#13;\ndwarfs to spirals to ellipticals will allow for detailed structural studies, not only as a function of stellar mass, but also&#13;\nas a function of the local environment. The data from this survey will serve as a vital testbed for cosmological&#13;\nsimulations predicting the stellar mass properties of present-day galaxies. This article introduces the survey and&#13;\ndescribes the sample selection, the significance of the 3.6 and 4.5 μm bands for this study, and the data collection&#13;\nand survey strategies. We describe the S^4G data analysis pipeline and present measurements for a first set of galaxies,&#13;\nobserved in both the cryogenic and warm mission phases of Spitzer. For every galaxy we tabulate the galaxy&#13;\ndiameter, position angle, axial ratio, inclination at μ_(3.6μm)(AB) = 25:5, and 26:5 mag arcsec^(-2) (equivalent to&#13;\n≈μ_B(AB) = 27:2 and 28:2 mag arcsec^(-2), respectively). These measurements will form the initial S^4G catalog&#13;\nof galaxy properties. We also measure the total magnitude and the azimuthally averaged radial profiles of ellipticity,&#13;\nposition angle, surface brightness, and color. Finally, using the galaxy-fitting code GALFIT, we deconstruct each&#13;\ngalaxy into its main constituent stellar components: the bulge/spheroid, disk, bar, and nuclear point source, where&#13;\nnecessary. Together, these data products will provide a comprehensive and definitive catalog of stellar structures,&#13;\nmass, and properties of galaxies in the nearby universe and will enable a variety of scientific investigations, some of&#13;\nwhich are highlighted in this introductory S^4G survey paper.