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Register a new userBarlenses and X-shape features compared: different manifestations of Boxy/Peanut bulges
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Morphological characteristics of the vertically thick inner bar components are studied. At high galaxy inclinations they manifest as Boxy/Peanut/X-shape features, and near to face-on view as barlenses. Using the Spitzer Survey of Stellar Structure in Galaxies (S4G) and the Near-IR S0 galaxy Survey (NIRS0S), we compared the properties of 88 X-shape features, 85 barlenses, and the photometric bulges of 41 non-barred galaxies. Sizes and minor-to-major axis ratios (b/a) of these structures are compared, and interpreted by means of synthetic images using N-body simulation models. Barlenses and their parent galaxies are also divided into different sub-groups. The synthetic images are analyzed in a similar manner as the observations. This is the first time that the observed properties of barlenses and X-shape features are compared, over a large range of galaxy inclinations. Our analysis are consistent with the idea that barlenses and X-shape features are physically the same phenomenon. However, which of the two features is observed depends, not only on galaxy inclination, but also on its central flux concentration. The observed nearly round face-on barlens morphology is expected when at least a few percents of the disk mass is in a central component, within a region much smaller than the size of the barlens itself. We also discuss that the large range of stellar population ages obtained for the photometric bulges in the literature, are consistent with our interpretation.
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We study the colors and orientations of structures in low and intermediate inclination barred galaxies. We test the hypothesis that barlenses, roundish central components embedded in bars, could form a part of the bar in a similar manner to boxy/peanut bulges in the edge-on view. A sample of 79 barlens galaxies was selected from the S$^4$G and the NIRS0S surveys. The sizes, ellipticities, and orientations of barlenses were measured and used to define the barlens regions in the color measurements. The orientations of barlenses were studied with respect to those of the thin bars and the line-of-nodes of the disks. For 47 galaxies color maps were constructed using the SDSS images in five optical bands, u, g, r, i, and z. Colors of bars, barlenses, disks, and central regions of the galaxies were measured using two different approaches and color-color diagrams sensitive to metallicity, stellar surface gravity, and short lived stars were constructed. Color differences between the structure components were calculated for each individual galaxy, and presented in histogram form. We find that the colors of barlenses are very similar to those of the surrounding bars, indicating that most probably they form part of the bar. We also find that barlenses have orientations closer to the disk line-of-nodes than to the thin bars, which is consistent with the idea that they are vertically thick, in a similar manner as the boxy/peanut structures in more inclined galaxies. Typically, the colors of barlenses are similar to those of normal E/S0 galaxies. Galaxy by galaxy studies show that in spiral galaxies very dusty barlenses also exist, along with barlenses with rejuvenated stellar populations. The central regions of galaxies are found to be on average redder than bars or barlenses, although galaxies with bluer central peaks also exist.
Bars have a complex three-dimensional shape. In particular their inner part is vertically much thicker than the parts further out. Viewed edge-on, the thick part of the bar is what is commonly known as a boxy-, peanut- or X- bulge and viewed face-on it is referred to as a barlens. These components are due to disc and bar instabilities and are composed of disc material. I review here their formation, evolution and dynamics, using simulations, orbital structure theory and comparisons to observations.
Boxy/peanut bulges in disc galaxies have been associated to stellar bars. We analyse their properties in a large sample of $N$-body simulations, using different methods to measure their strength, shape and possible asymmetry, and then inter-compare the results. Some of these methods can be applied to both simulations and observations. In particular, we seek correlations between bar and peanut properties, which, when applied to real galaxies, will give information on bars in edge-on galaxies, and on peanuts in face-on galaxies.
We present SAURON integral-field observations of a sample of 12 mid to high-inclination disk galaxies, to unveil hidden bars on the basis of their kinematics, i.e., the correlation between velocity and h3 profiles, and to establish their degree of cylindrical rotation. For the latter, we introduce a method to quantify cylindrical rotation that is robust against inner disk components. We confirm high-levels of cylindrical rotation in boxy/peanut bulges, but also observe this feature in a few galaxies with rounder bulges. We suggest that these are also barred galaxies with end-on orientations. Re-analysing published data for our own Galaxy using this new method, we determine that the Milky Way bulge is cylindrically rotating at the same level as the strongest barred galaxy in our sample. Finally, we use self-consistent three-dimensional N-body simulations of bar-unstable disks to study the dependence of cylindrical rotation on the bars orientation and host galaxy inclination.
Some of barred galaxies, including the Milky Way, host a boxy/peanut/X-shaped bulge (BPX-shaped bulge). Previous studiessuggested that the BPX-shaped bulge can either be developed by bar buckling or by vertical inner Lindblad resonance (vILR)heating without buckling. In this paper, we study the observable consequence of an BPX-shaped bulge built up quickly after barformation via vILR heating without buckling, using anN-body/hydrodynamics simulation of an isolated Milky Way-like galaxy.We found that the BPX-shaped bulge is dominated by stars born prior to bar formation. This is because the bar suppresses starformation, except for the nuclear stellar disc (NSD) region and its tips. The stars formed near the bar ends have higher Jacobienergy, and when these stars lose their angular momentum, their radial action increases to conserve Jacobi energy. This preventsthem from reaching the vILR to be heated to the BPX region. By contrast, the NSD forms after the bar formation. From thissimulation and general considerations, we expect that the age distributions of the NSD and BPX-shaped bulge formed withoutbar buckling do not overlap each other. Then, the transition age between these components betrays the formation time of the bar, and is testable in future observations of the Milky Way and extra-galactic barred galaxies
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