The secular growth of bars revealed by flat (peak + shoulders) density profiles

TL;DR

This paper introduces an automated method to detect flat density profiles in galactic bars, revealing that shoulders in these profiles are indicators of secular bar growth and are linked to specific orbital structures.

Contribution

The study develops a non-parametric algorithm to identify flat profiles and demonstrates that shoulders are signatures of bar growth, independent of bar age or other features.

Findings

Flat profiles indicate secular bar growth.

Shoulders are linked to looped x1 orbits.

Bar growth correlates with shoulder strength.

Abstract

The major-axis density profiles of bars are known to be either exponential or 'flat'. We develop an automated non-parametric algorithm to detect flat profiles and apply it to a suite of simulations (with and without gas). We demonstrate that flat profiles are a manifestation of a bar's secular growth, producing a 'shoulder' region (an overdensity above an exponential) in its outskirts. Shoulders are not present when bars form, but develop as the bar grows. If the bar does not grow, shoulders do not form. Shoulders are often accompanied by box/peanut bulges, but develop separately from them and are independent tracers of a bar's growth. They can be observed at a wide range of viewing orientations with only their slope varying significantly with inclination. We present evidence that shoulders are produced by looped x1 orbits. Since the growth rate of the bar moderately correlates with the growth rate of the shoulder strength, these orbits are probably recently trapped. Shoulders therefore are evidence of bar growth. The properties of the shoulders do not, however, establish the age of a bar, because secondary buckling or strong spirals may destroy shoulders, and also because shoulders do not form if the bar does not grow much. In particular, our results show that an exponential profile is not necessarily an indication of a young bar.