The Spitzer Survey of Stellar Structure in Galaxies (S4G) is a volume, magnitude, and size-limited survey of 2352 nearby galaxies with deep imaging at 3.6 and 4.5um. In this paper we describe our surface photometry pipeline and showcase the associate
d data products that we have released to the community. We also identify the physical mechanisms leading to different levels of central stellar mass concentration for galaxies with the same total stellar mass. Finally, we derive the local stellar mass-size relation at 3.6um for galaxies of different morphologies. Our radial profiles reach stellar mass surface densities below 1 Msun pc-2. Given the negligible impact of dust and the almost constant mass-to-light ratio at these wavelengths, these profiles constitute an accurate inventory of the radial distribution of stellar mass in nearby galaxies. From these profiles we have also derived global properties such as asymptotic magnitudes (and the corresponding stellar masses), isophotal sizes and shapes, and concentration indices. These and other data products from our various pipelines (science-ready mosaics, object masks, 2D image decompositions, and stellar mass maps), can be publicly accessed at IRSA (http://irsa.ipac.caltech.edu/data/SPITZER/S4G/).
The Spitzer Survey of Stellar Structure in Galaxies S^4G is an Exploration Science Legacy Program approved for the Spitzer post-cryogenic mission. It is a volume-, magnitude-, and size-limited (d < 40 Mpc, |b| > 30 degrees, m_(Bcorr) < 15.5, D25>1) s
urvey of 2,331 galaxies using IRAC at 3.6 and 4.5 microns. Each galaxy is observed for 240 s and mapped to > 1.5 x D25. The final mosaicked images have a typical 1 sigma rms noise level of 0.0072 and 0.0093 MJy / sr at 3.6 and 4.5 microns, respectively. Our azimuthally-averaged surface brightness profile typically traces isophotes at mu_3.6 (AB) (1 sigma) ~ 27 mag arcsec^-2, equivalent to a stellar mass surface density of ~ 1 Msun pc^-2. S^4G thus provides an unprecedented data set for the study of the distribution of mass and stellar structures in the local Universe. This paper introduces the survey, the data analysis pipeline and measurements for a first set of galaxies, observed in both the cryogenic and warm mission phase of Spitzer. For every galaxy we tabulate the galaxy diameter, position angle, axial ratio, inclination at mu_3.6 (AB) = 25.5 and 26.5 mag arcsec^-2 (equivalent to ~ mu_B (AB) =27.2 and 28.2 mag arcsec^-2, respectively). These measurements will form the initial S^4G 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, we deconstruct each galaxy using GALFIT 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.
Spitzer IRAC observations of two fields in the XUV-disk of M83 have been recently obtained,3R_{HII} away from the center of the galaxy (R_{HII)=6.6 kpc).GALEX UV images have shown the two fields to host in-situ recent star formation.The IRAC images a
re used in conjunction with GALEX data and new HI imaging from THINGS to constrain stellar masses and ages of the UV clumps in the fields,and to relate the local recent star formation to the reservoir of available gas. multi wavelength photometry in the UV and mid-IR bands of 136 UV clumps(spatial resolution >220pc) identified in the two target fields, together with model fitting of the stellar UV-MIR SED,suggest that the clumps cover a range of ages between a few Myr and >1Gyr with a median value around <100Myr,and have masses in the range 10^3-3*10^6M, with a peak ~10^4.7M.The range of observed ages,for which only a small fraction of the mass in stars appears to have formed in the past ~10Myr, agrees with the dearth of Ha emission observed in these outer fiel ds. At the location of our IRAC fields, the HI map shows localized enhancement and clumping of atomic gas. A comparison of the observed star formation with the gas reservoir shows that the UV clumps follow the Schmidt--Kennicutt scaling law of star formation,and that star formation is occurring in regions with gas dens ities at approximately (within a factor of a few) the critical density value de -rived according to the Toomre Q gravitational stability criterion. The signifi cant 8 micron excess in several of the clumps (16% of the total by number accou nting for ~67% of the 8 micron flux)) provides evidence for the existence of dust in these remote fields, in agreement with results for other galaxies. Furt hermore, we observe a relatively small excess of emission at 4.5 micron in the clumps...
