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Register a new userIs the late buckling stage inevitable in the bar life?
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By means of self-consistent numerical simulations we investigated the dynamical impact of classical bulges on the growth of the secondary buckling of a bar. Overall we considered 14 models with different disc and bulge parameters. We obtained that a bulge with a quite modest mass $B/D=0.1$ leads to completely symmetrical evolution of the bar almost independently of the initial stellar disc parameters and even can damp the first bending. At the same time, the bars in all our bulgeless models suffer from the short primary and prolonged secondary buckling. Given the smallness of the mass suppressing secondary buckling, we conclude that a classical bulge along with the gas central concentration may be the main culprits for the rarity of bars with ongoing buckling in the local Universe.
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Using a single N-body simulation ($N=0.14times 10^9$) we explore the formation, evolution and spatial variation of the phase-space spirals similar to those recently discovered by Antoja et al. in the Milky Way disk, with Gaia DR2. For the first time in the literature, we use a self-consistent N-body simulation of an isolated Milky Way-type galaxy to show that the phase-space spirals develop naturally from vertical oscillations driven by the buckling of the stellar bar. We claim that the physical mechanism standing behind the observed incomplete phase-space mixing process can be internal and not necessarily due to the perturbation induced by a massive satellite. In our model, the bending oscillations propagate outwards and produce axisymmetric variations of the mean vertical coordinate and of the vertical velocity component. As a consequence, the phase-space wrapping results in the formation of patterns with various morphology across the disk, depending on the bar orientation, distance to the galactic center and time elapsed since the bar buckling. Once bending waves appear, they are supported for a long time via disk self-gravity. The underlying physical mechanism implies the link between in-plane and vertical motion that leads directly to phase-space structures whose amplitude and shape are in remarkable agreement with those of the phase-space spirals observed in the Milky Way disk. In our isolated galaxy simulation, phase-space spirals are still distinguishable, at the solar neighbourhood, 3 Gyr after the buckling phase. The long-lived character of the phase-space spirals generated by the bar buckling instability cast doubts on the timing argument used so far to get back at the time of the onset of the perturbation: phase-space spirals may have been caused by perturbations originated several Gyrs ago, and not as recent as suggested so far.
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
Studies of the ages, abundances, and motions of individual stars in the Milky Way provide one of the best ways to study the evolution of disk galaxies over cosmic time. The formation of the Milky Ways barred inner region in particular is a crucial piece of the puzzle of disk galaxy evolution. Using data from APOGEE and Gaia, we present maps of the kinematics, elemental abundances, and age of the Milky Way bulge and disk that show the barred structure of the inner Milky Way in unprecedented detail. The kinematic maps allow a direct, purely kinematic determination of the bars pattern speed of 41+/-3 km/s/kpc and of its shape and radial profile. We find the bars age, metallicity, and abundance ratios to be the same as those of the oldest stars in the disk that are formed in its turbulent beginnings, while stars in the bulge outside of the bar are younger and more metal-rich. This implies that the bar likely formed ~8 Gyr ago, when the decrease in turbulence in the gas disk allowed a thin disk to form that quickly became bar-unstable. The bars formation therefore stands as a crucial epoch in the evolution of the Milky Way, a picture that is in line with the evolutionary path that emerges from observations of the gas kinematics in external disk galaxies over the last ~10 Gyr.
We present new high-resolution ALMA (13CO J=1-0 and J= 2-1) and CARMA (12CO and 13CO J=1-0) observations of two Luminous Infrared Galaxies (LIRGs): Arp 55 and NGC 2623. The new data are complementary to published and archival Submillimeter Array observations of 12CO J=2-1 and J=3-2. We perform a Bayesian likelihood non-local thermodynamic equilibrium analysis to constrain the molecular gas physical conditions such as temperature, column and volume densities and the [12CO]/[13CO] abundance ratio. For Arp 55, an early/intermediate staged merger, the line measurements are consistent with cold (~10-20 K), dense (>10$^{3.5}$ cm$^{-3}$) molecular gas. For NGC 2623, the molecular gas is warmer (~110 K) and less dense (~10$^{2.7}$ cm$^{-3}$). Since Arp 55 is an early/intermediate stage merger while NGC 2623 is a merger remnant, the difference in physical conditions may be an indicator of merger stage. Comparing the temperature and volume density of several LIRGs shows that the molecular gas, averaged over ~kpc scale, of advanced mergers is in general warmer and less dense than early/intermediate stage mergers. We also find that the [12CO]/[13CO] abundance ratio of NGC 2623 is unusually high (>250) when compared to the Milky Way; however, it follows a trend seen with other LIRGs in literature. This high [12CO]/[13CO] value is very likely due to stellar nucleosynthesis enrichment of the interstellar medium. On the other hand, Arp 55 has a more Galactic [12CO]/[13CO] value with the most probable [12CO]/[13CO] value being 20-30. We measure the CO-to-H2 conversion factor, $alpha_{CO}$, to be ~0.1 and ~0.7 (3x10$^{-4}$/x$_{CO}$) M$_{odot}$ (K km s$^{-1}$ pc$^{2}$)$^{-1}$ for Arp 55 and NGC 2623, respectively. Since Arp 55 is an early/intermediate , this suggests that the transition from a Galactic conversion factor to a LIRG value happens at an even earlier merger stage.
This text contains the main message of my previous review cite{Bednyakov:2015uoa} on the dark matter problem and supports resent paper cite{Froborg:2020tdh}. True dark matter particles possess an exclusive galactic signature --- the annual modulation, which is accessible today via direct dark matter detection only. One has no another way to prove the true nature of any dark matter candidate.
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