# Deconstructing double-barred galaxies in 2D and 3D. II. Two distinct groups of inner bars

## TL;DR

This study investigates the 3D shapes and properties of inner and outer bars in double-barred galaxies, revealing two distinct groups of inner bars with different lengths and ellipticities, and discussing their formation scenarios.

## Contribution

It provides a detailed photometric and 3D analysis of double-barred galaxies, identifying bimodal inner bar properties and exploring their possible formation mechanisms.

## Key findings

- Two distinct groups of inner bars with different lengths and ellipticities.
- Inner bars do not correlate with host galaxy or bulge properties.
- Inner bar formation may involve slow dynamical assembly over several Gyr.

## Abstract

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.