  Most, if not all, disk galaxies have a thin (classical) disk and a thick
disk. In most models thick disks are thought to be a necessary consequence of
the disk formation and/or evolution of the galaxy. We present the results of a
study of the thick disk properties in a sample of carefully selected edge-on
galaxies with types ranging from T=3 to T=8. We fitted one-dimensional
luminosity profiles with physically motivated functions - the solutions of two
stellar and one gaseous isothermal coupled disks in equilibrium - which are
likely to yield more accurate results than other functions used in previous
studies. The images used for the fits come from the Spitzer Survey of Stellar
Structure in Galaxies (S4G). We found that thick disks are on average more
massive than previously reported, mostly due to the selected fitting function.
Typically, the thin and the thick disk have similar masses. We also found that
thick disks do not flare significantly within the observed range in
galactocentric radii and that the ratio of thick to thin disk scaleheights is
higher for galaxies of earlier types.
  Our results tend to favor an in situ origin for most of the stars in the
thick disk. In addition the thick disk may contain a significant amount of
stars coming from satellites accreted after the initial build-up of the galaxy
and an extra fraction of stars coming from the secular heating of the thin disk
by its own overdensities.
  Assigning thick disk light to the thin disk component may lead to an
underestimate of the overall stellar mass in galaxies, because of different
mass to light ratios in the two disk components. On the basis of our new
results, we estimate that disk stellar masses are between 10% and 50% higher
than previously thought and we suggest that thick disks are a reservoir of
"local missing baryons".
