Below is a brief description of the enhanced products delivered by the Spitzer Survey of Stellar Structure in Galaxies (S4G). We refer the reader to the overview paper (Sheth et al. 2010, PASP, 122, 1397) for a more detailed description of the survey strategy and scientific goals. More technical details of Pipeline 1 will be presented in Regan et al. 2013 (in prep), and of Pipelines 2 and 3 in Munoz-Mateos et al. 2013 (in prep).


Pipeline 1 takes as an input the original Basic Calibrated Data (BCD) fits files and produces science-ready mosaics. For each galaxy we deliver the following products:

XXX.phot.1.fits, XXX.phot.2.fits: Science-ready mosaics of the galaxy in the IRAC 1 and 2 bands. The images are calibrated in units of MJy/sr, and have a pixel scale of 0.75 arcsec/pixel. The background has *not* been subtracted; our best estimates of the background level and noise are quoted in the P3 table.

XXX.phot.1_wt.fits, XXX.phot.2_wt.fits: Weight maps showing the spatial coverage of each mosaic. The value at a given pixel is 10 times the number of frames that cover that pixel. The weight maps have the same pixel scale as the science images, and each one is spatially aligned to the corresponding science image at the same band.


Pipeline 2 creates masks to exclude foreground stars and background galaxies from any subsequent analysis. Masks are first generated automatically with SExtractor; they are then visually checked and edited by hand, masking extra sources missed by SExtractor and unmasking any regions of the galaxy that may have been incorrectly identified at first.

For each galaxy we provide two fits files, one for each band (XXX.1.final_mask.fits, XXX.2.final_mask.fits), that contain our final edited masks. Masked pixels have non-zero values.


Pipeline 3 performs surface photometry on the IRAC 1 and 2 images of each galaxy. We first measure the background level, the local pixel-to-pixel noise and the large scale noise in a set of boxes placed around the galaxy. The coordinates of the galaxy center are then determined using the imcenter task in Iraf. We then use the ellipse task to obtain different sets of radial profiles:

(1) XXX.1fr6a_noclean_fin.dat, XXX.2fr6a_noclean_fin.dat:

These fits have a radial increment of 6 arcseconds along the semimajor axis ('6a'). The center is kept fixed, but the ellipticity and position angle are left as free parameters ('fr'). As with the P1 and P2 files, the numbers 1 and 2 refer to the IRAC 1 and 2 bands. These fits with a coarse 6 arcsec resolution are used to determine the outer size, ellipticity and position angle at 25.5 and 26.5 AB mag arcsec^-2 at each band, in an RC3-like fashion.

(2) XXX.1fr2a_noclean_fin.dat, XXX.2fr2a_noclean_fin.dat:

Same as the previous profiles (fixed center, free ellipticity and PA) but using a smaller radial increment of 2 arcseconds. These profiles can be used to measure structures like bars with a finer detail.

(3) XXX.1fx2a_noclean_fin.dat, XXX.2fx2a_noclean_fin.dat:

These profiles also have a 2 arcsecond radial resolution, but all geometrical parameters are kept fixed and equal to those of the 25.5 outer isophotes derived in (1). These profiles are ideal to measure disk scale-lengths.

All these profiles are ASCII tables that follow the structure and naming convention of the ellipse task in Iraf. The content of each column is detailed here.

Global quantities such as our improved central coordinates, bakground level and noise and outer galaxy sizes and shapes can be found in the corresponding P3 catalog.