Boxy/Peanut/X-shape bulges: steep inner rotation curve leads to barlens face-on morphology

TL;DR

This study uses stellar dynamical simulations to determine that steep inner rotation curves, influenced by small classical bulges, lead to face-on barlens morphology, linking it to the physical parameters of galaxies.

Contribution

It demonstrates that steep inner rotation curves are essential for forming barlens structures, revealing the physical conditions that differentiate barlenses from X-shaped bulges.

Findings

Steep inner rotation curves correlate with barlens formation.

Galaxies with barlenses have twice steeper inner rotation curves than those with X-shapes.

Barlenses are associated with stronger bars and higher inner surface density.

Abstract

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.