The unusual vertical mass distribution of NGC 4013 seen through the
  Spitzer Survey of Stellar Structure in Galaxies (S4G)
  NGC 4013 is a nearby Sb edge-on galaxy known for its "prodigious" HI warp and
its "giant" tidal stream. Previous work on this unusual object shows that it
cannot be fitted satisfactorily by a canonical thin+thick disk structure. We
have produced a new decomposition of NGC 4013, considering three stellar
flattened components (thin+thick disk plus an extra and more extended
component) and one gaseous disk. All four components are considered to be
gravitationally coupled and isothermal. To do so, we have used the 3.6micron
images from the Spitzer Survey of Stellar Structure in Galaxies (S4G). We find
evidence for NGC4013 indeed having a thin and a thick disk and an extra
flattened component. This smooth and extended component (scaleheight z_EC kpc)
could be interpreted as a thick disk or as a squashed ellipsoidal halo and
contains ~20% of the total mass of all three stellar components. We argue it is
unlikely to be related to the ongoing merger or due to the off-plane stars from
a warp in the other two disk components. Instead, we favor a scenario in which
the thick disk and the extended component were formed in a two-stage process,
in which an initially thick disk has been dynamically heated by a merger soon
enough in the galaxy history to have a new thick disk formed within it.
