Deconstructing double-barred galaxies in 2D and 3D. I. Classical nature
  of the dominant bulges
  We present here a thorough photometric analysis of double-barred galaxies,
consisting of i) two-dimensional photometric decompositions including a bulge,
inner bar, outer bar, and (truncated) disc; and ii) three-dimensional
statistical deprojections to derive the intrinsic shape of bulges, inner bars,
and outer bars. This is the first time the combination of both techniques is
applied to a sample of double-barred galaxies. It represents a step forward
with respect to previous works, which are based on properties of the integrated
light through ellipse fitting and unsharp masking. In this first paper of a
series of two, we analyse the nature of the dominant bulges within
double-barred systems by using several photometric diagnostics, namely S\'ersic
index, Kormendy relation, colours, and the better suited intrinsic flattening.
Our results indicate that almost all bulges in our sample are classical,
whereas only 2 out of the 17 galaxies under study appear as potential
candidates to host secularly-formed disc-like bulges. Such result poses the
possibility that having a central hot structure may be a requirement for inner
bar formation.
