Optical imaging for the Spitzer Survey of Stellar Structure in Galaxies.
  Data release and notes on interacting galaxies
  (Abridged) The Spitzer Survey for Stellar Structure in Galaxies (S4G) and its
more recently approved extension will lead to a set of 3.6 and 4.5 micron
images for 2829 galaxies, which can be used to study many different aspects of
the structure and evolution of local galaxies. We collected and re-processed
optical images in five bands from the Sloan Digital Sky Survey for 1657
galaxies, which are publicly released with the publication of this paper. We
observed, in only the g-band, an additional 111 S4G galaxies in the northern
hemisphere with the 2.5 m Liverpool Telescope, so that optical imaging is
released for 1768 galaxies, or for 62% of the S4G sample. We visually checked
all images. We noted interactions and close companions in our optical data set
and in the S4G sample, confirming them by determining the galaxies' radial
velocities and magnitudes in the NASA-IPAC Extragalactic Database. We find that
17% of the S4G galaxies (21% of those brighter than 13.5 mag) have a close
companion (within a radius of five times the diameter of the sample galaxy, a
recession velocity within 200km/s and not more than 3 mag fainter) and that
around 5% of the bright part of the S4G sample show significant morphological
evidence of an ongoing interaction. This confirms and further supports previous
estimates of these fractions. The over 8000 science images described in this
paper, the re-processed Sloan Digital Sky Survey ones, the new Liverpool
Telescope images, the set of 29 false-colour pictures, and the catalogue of
companion and interacting galaxies, are all publicly released for general use
for scientific, illustrative, or public outreach purposes.
