Face-on structure of barlenses and boxy bars: an insight from spectral dynamics

Abstract

Based on the spectral analysis of individual orbits of stars from different N-body models, we show that the face-on morphology of the so-called ‘face-on peanut’ bars (boxy bars) and barlenses is supported by different types of orbits. For ‘face-on peanut’ bars, the so-called boxy orbits come to the fore, and they are responsible for the unusual morphology of the bar in the central regions. In the models with compact bulges, the bars show a barlens morphology in their central parts. We found that the barlens supporting orbits come in two types, one of which gives a square-like shape and the other have a rosette-like shape in the frame co-rotating with the bar. Such a shape is typical for orbits around stable loop orbits in nearly axisymmetric potentials only slightly distorted by the bar. They were already known from some of the previous studies but their role in barlens shaping was barely investigated. Although quite simple, the rosette-like orbits are found to be the main building block of a barlens in our models. The detailed analysis of all bar orbits also allowed us to distinguish the x2 orbital family and isolate the structure supported by orbits trapped around this family. The x2 family is well-known, but, apparently, for the first time in N-body models we have revealed the structure it supports by means of spectral dynamics and highlight its contribution to the barlens. We found that the x2 family population increases with an increase in central matter concentration.

First direct identification of the barlens vertical structure in galaxy models

“…At the same time, Salo & Laurikainen (2017) conclude that the introduction of a small and compact classical bulge in the N-body model drastically changes the bar morphology and leads to the formation of a central barlens in a face-on view. In the present paper, we analyse one of the models by Smirnov et al (2021) with a small compact bulge (labelled as a BL model in their paper), which demonstrates a barlens in its structure.…”

“…In Smirnov et al (2021), the BL model was disassembled into several orbital groups to analyse the face-on morphology of an overall bar. We assume that each individual group is characterised by a unique set of frequency ratios f R / f x and f R / f y .…”

“…Another major problem is that the thick part of the bar is a complex structure. Based on the spectral analysis of individual orbits of stars from different N-body models, Smirnov et al (2021) dissected the modelled bars into separate components that are supported by completely different orbital groups. They identified at least four orbital components: the narrow elongated bar, the face-on boxy bar, and two components constituting the barlens.…”

“…All these components may have their unique vertical structure and a prominent and large face-on component does not have to be thick in the edge-on view. Moreover, Smirnov et al (2021) determined that these components differ rather little in their linear sizes, but they have a rather different morphology. Thus, the coincidence of relative sizes also does not serve as an argument in favour of the structure coincidence.…”

“…Dynamical arguments are required to reliably separate the components from each other. Smirnov et al (2021) focus on the face-on morphology of bars. In this letter, we consider an N-body model with a barlens (Sect.…”