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The future of the field of secular evolution in galaxies is in no small measure infrared. The Herschel satellite has just started producing spectacular data in nearby galaxies, the James Webb Space Telescope (JWST) is being prepared for launch, and the Spitzer Space Telescope (SST; Werner et al. 2004) is still producing wonderful data, even now its coolant has been depleted and the telescope is `warm'.

The Spitzer Survey of Stellar Structure in Galaxies (S4G; Sheth et al. 2010) is designed to be the definitive survey of the distribution of stellar structure in the nearby Universe. Over the two years of the warm mission of the SST, the S4G will observe the stellar mass distribution in a volume-, magnitude- and size-limited (d < 40 Mpc, mb < 15.5, D25 > 1 arcmin) sample of 2,331 galaxies using the Infrared Array Camera (IRAC; Fazio et al. 2004) at 3.6 and 4.5 micron. This survey will provide an unprecedented set of imaging data which will be used to study the stellar structure in galaxies. Among many other topics, the S4G data will allow the study of how outer disks and halos are formed, how the formation and evolution of galactic structures are affected by galaxy interactions, or which structural parameters govern secular galaxy evolution. The large sample, ranging from dwarfs to spirals to ellipticals will allow for such structural studies both as a function of stellar mass and as a function of environment, vital to test cosmological simulations predicting the mass properties of present day galaxies.

The S4G survey was awarded 637.2 hours of time on the Spitzer telescope, and the observations are in progress. It is expected that the survey will be completed by mid-2011. The data are being analysed using dedicated pipeline software mostly developed by the S4G consortium, which will perform a basic reduction, sky subtraction and mosaicing of the images, will produce masks of foreground stars, will measure basic parameters of all galaxies, and will deliver multi-component decompositions (see Sheth et al. 2010 for a detailed description).

As an example of the data quality, including depth of imaging and field of view related to the size of the galaxy, that will be delivered by the S4G for all survey galaxies, we present in Fig. 9 a 3.6 µm image of the well-known galaxy M51. The image is reproduced from a paper by Buta et al. (2010b), which discusses the morphology of the first 207 S4G galaxies. Not only does the image show the intricate outer tidal structure caused by the interaction of M51 and its companion NGC 5194 (see Salo & Laurikainen 2000), it also illustrates how a high-quality dust-free vision of a galaxy can lead to a better insight into its nature. On the basis of this new image, Buta et al. (2010b) were able to revise the morphological classification of M51 from SA(s)bc pec (RC3) to SAB(rs,nr)bc, but much more spectacular is the revision of the type of the companion of M51, NGC 5195, which was classified as an I0 pec galaxy in de Vaucouleurs et al. (1991), but which Buta et al. (2010b) could classify as SAB(r)0/a pec. The bar and ring giving rise to this new classification can be easily seen in Fig. 9.

Figure 9

Figure 9. Spitzer 3.6 µm S4G image of the galaxy M51 (NGC 5194, right) and its companion NGC 5195. From Buta et al. (2010b).

The combination of deep, wide-field, mid-infrared imaging and a large sample offered by the S4G will allow significant progress on most of the issues discussed earlier in this paper. For instance, the survey will allow a comprehensive and definitive study of bar and spiral arm properties across a wide range of host galaxy parameters. The new images will allow a more complete study of the host galaxies of not only nuclear, but also inner and outer rings, including the interesting cases of unbarred ring hosts. The combination of the Spitzer images with ancillary data, for instance GALEX UV imaging, Halpha imaging, atomic and molecular gas observations, and kinematic observations offers almost limitless opportunities for scientific progress. The S4G should thus lead to a much more complete understanding of galaxy evolution, and of the precise role of secular evolution.

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