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Register a new userAge distribution of stars in boxy/peanut/X-shaped bulges formed without bar buckling
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Authors
Junichi Baba NAOJ
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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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From a sample of 84 local barred, moderately inclined disc galaxies, we determine the fraction which host boxy or peanut-shaped (B/P) bulges (the vertically thickened inner parts of bars). We find that the frequency of B/P bulges in barred galaxies is a very strong function of stellar mass: 79% of the bars in galaxies with log (M_{star}/M_{sun}) >~ 10.4 have B/P bulges, while only 12% of those in lower-mass galaxies do. (We find a similar dependence in data published by Yoshino & Yamauchi 2015 for edge-on galaxies.) There are also strong trends with other galaxy parameters -- e.g., Hubble type: 77% of S0-Sbc bars, but only 15% of Sc-Sd bars, have B/P bulges -- but these appear to be side effects of the correlations of these parameters with stellar mass. In particular, despite indications from models that a high gas content can suppress bar buckling, we find no evidence that the (atomic) gas mass ratio M_{atomic}/M_{star} affects the presence of B/P bulges, once the stellar-mass dependence is controlled for. The semi-major axes of B/P bulges range from one-quarter to three-quarters of the full bar size, with a mean of R_{box}/L_{bar} = 0.42 +/- 0.09 and R_{box}/a_{max} = 0.53 +/- 0.12 (where R_{box} is the size of the B/P bulge and a_{max} and L_{bar} are lower and upper limits on the size of the bar).
Vertically thickened bars, observed in the form of boxy/peanut (B/P) bulges, are found in the majority of massive barred disc galaxies in the local Universe, including our own. B/P bulges indicate that their host bars have suffered violent bending instabilities driven by anisotropic velocity distributions. We investigate for the first time how the frequency of B/P bulges in barred galaxies evolves from $z = 1$ to $zapprox 0$, using a large sample of non-edge-on galaxies with masses $M_{star} > 10^{10}:M_{odot}$, selected from the HST COSMOS survey. We find the observed fraction increases from $0^{+3.6}_{-0.0}%$ at $z = 1$ to $37.8^{+5.4}_{-5.1}%$ at $z = 0.2$. We account for problems identifying B/P bulges in galaxies with low inclinations and unfavourable bar orientations, and due to redshift-dependent observational biases with the help of a sample from the Sloan Digital Sky Survey, matched in resolution, rest-frame band, signal-to-noise ratio and stellar mass and analysed in the same fashion. From this, we estimate that the true fraction of barred galaxies with B/P bulges increases from $sim 10%$ at $z approx 1$ to $sim 70%$ at $z = 0$. In agreement with previous results for nearby galaxies, we find a strong dependence of the presence of a B/P bulge on galaxy stellar mass. This trend is observed in both local and high-redshift galaxies, indicating that it is an important indicator of vertical instabilities across a large fraction of the age of the Universe. We propose that galaxy formation processes regulate the thickness of galaxy discs, which in turn affect which galaxies experience violent bending instabilities of the bar.
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.
We introduce the study of box/peanut (B/P) bulges in the action space of the initial axisymmetric system. We explore where populations with different actions end up once a bar forms and a B/P bulge develops. We find that the density bimodality due to the B/P bulge (the X-shape) is better traced by populations with low radial, JR,0, or vertical, Jz,0, actions, or high azimuthal action, J{phi},0. Generally populations separated by JR,0 have a greater variation in bar strength and vertical heating than those separated by Jz,0. While the bar substantially weakens the initial vertical gradient of Jz,0, it also drives a strikingly monotonic vertical profile of JR,0. We then use these results to guide us in assigning metallicity to star particles in a pure N-body model. Because stellar metallicity in unbarred galaxies depends on age as well as radial and vertical positions, the initial actions are particularly well suited for assigning metallicities. We argue that assigning metallicities based on single actions, or on positions, results in metallicity distributions inconsistent with those observed in real galaxies. We therefore use all three actions to assign metallicity to an N-body model by comparing with the actions of a star-forming, unbarred simulation. The resulting metallicity distribution is pinched on the vertical axis, has a realistic vertical gradient and has a stronger X-shape in metal-rich populations, as found in real 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.
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