Deconstructing double-barred galaxies in 2D and 3D. II. Two distinct
  groups of inner bars
  The intrinsic photometric properties of inner and outer stellar bars within
17 double-barred galaxies are thoroughly studied through a photometric analysis
consisting of: i) two-dimensional multi-component photometric decompositions,
and ii) three-dimensional statistical deprojections for measuring the
thickening of bars, thus retrieving their 3D shape. The results are compared
with previous measurements obtained with the widely used analysis of integrated
light. Large-scale bars in single- and double-barred systems show similar
sizes, and inner bars may be longer than outer bars in different galaxies. We
find two distinct groups of inner bars attending to their in-plane length and
ellipticity, resulting in a bimodal behaviour for the inner/outer bar length
ratio. Such bimodality is related neither to the properties of the host galaxy
nor the dominant bulge, and it does not show a counterpart in the dimension off
the disc plane. The group of long inner bars lays at the lower end of the outer
bar length vs. ellipticity correlation, whereas the short inner bars are out of
that relation. We suggest that this behaviour could be due to either a
different nature of the inner discs from which the inner bars are dynamically
formed, or a different assembly stage for the inner bars. This last possibility
would imply that the dynamical assembly of inner bars is a slow process taking
several Gyr to happen. We have also explored whether all large-scale bars are
prone to develop an inner bar at some stage of their lives, possibility we
cannot fully confirm or discard.
