Boxy/Peanut/X-shape bulges: steep inner rotation curve leads to barlens
  face-on morphology
  We use stellar dynamical bulge/disk/halo simulations to study whether
barlenses (lens-like structures embedded in the narrow bar component) are just
the face-on counterparts of Boxy/Peanut/X-shapes (B/P/X) seen in edge-on bars,
or if some additional physical parameter affects that morphology. A range of
bulge-to-disk mass and size ratios are explored: our nominal parameters
($B/D=0.08$, $r_{\rm eff}/h_r=0.07$, disk comprising 2/3 of total force at
$2.2h_r$) correspond to typical MW mass galaxies. In all models a bar with
pronounced B/P/X forms in a few Gyrs, visible in edge-on view. However, the
pure barlens morphology forms only in models with sufficiently steep inner
rotation curves, $dV_{cir}/dr\gtrsim5V_{max}/h_r$, achieved when including a
small classical bulge with $B/D\gtrsim0.02$ and $r_{\rm eff}/h_r\lesssim0.1$.
For shallower slopes the central structure still resembles a barlens, but shows
a clear X-signature even in low inclinations. Similar result holds for
bulgeless simulations, where the central slope is modified by changing the halo
concentration. The predicted sensitivity on inner rotation curve is consistent
with the slopes estimated from gravitational potentials calculated from the
3.6$\mu$m images, for the observed barlens and X-shape galaxies in the Spitzer
Survey of Stellar Structure in Galaxies (S$^4$G). For inclinations $<60^\circ$
the galaxies with barlenses have on average twice steeper inner rotation curves
than galaxies with X-shapes: the limiting slope is $\sim250$km/s/kpc. Among
barred galaxies, those with barlenses have both the strongest bars and the
largest relative excess of inner surface density, both in barlens region
($\lesssim0.5h_r$) and near the center ($\lesssim0.1h_r$); this provides
evidence for bar-driven secular evolution in galaxies.
