Investigating the long-term evolution of galaxies: Noise, cuspy halos and bars (original) (raw)
Related papers
Bar-driven dark halo evolution: a resolution of the cusp-core controversy
The Astrophysical Journal, 2002
Simulations predict that the dark matter halos of galaxies should have central cusps, while those inferred from observed galaxies do not have cusps. We demonstrate, using both linear perturbation theory and n-body simulations, that a disk bar, which should be ubiquitous in forming galaxies, can produce cores in cuspy CDM dark matter profiles within five bar orbital times. Simulations of forming galaxies suggest that one of Milky Way size could have a 10 kpc primordial bar; this bar will remove the cusp out to ∼5 kpc in ∼1.5 gigayears, while the disk only loses ∼8% of its original angular momentum. An inner Lindblad-like resonance couples the rotating bar to orbits at all radii through the cusp, transferring the bar pattern angular momentum to the dark matter cusp, rapidly flattening it. This resonance disappears for profiles with cores and is responsible for a qualitative difference in bar driven halo evolution with and without a cusp. This bar induced evolution will have a profound effect on the structure and evolution of almost all galaxies. Hence, both to understand galaxy formation and evolution and to make predictions from theory it is necessary to resolve these dynamical processes. Unfortunately, correctly resolving these important dynamical processes in ab initio calculations of galaxy formation is a daunting task, requiring at least 4,000,000 halo particles using our SCF code, and probably requiring many times more particles when using noisier tree, direct summation, or grid based techniques, the usual methods employed in such calculations.
On the bar formation mechanism in galaxies with cuspy bulges
Monthly Notices of the Royal Astronomical Society, 2016
We show by numerical simulations that a purely stellar dynamical model composed of an exponential disc, a cuspy bulge, and a Navarro-Frenk-White halo with parameters relevant to the Milky Way is subject to bar formation. Taking into account the finite disc thickness, the bar formation can be explained by the usual bar instability, in spite of the presence of an inner Lindblad resonance, that is believed to damp any global modes. The effect of replacing the live halo and bulge by a fixed external axisymmetric potential (rigid models) is studied. It is shown that while the e-folding time of bar instability increases significantly (from 250 to 500 Myr), the bar pattern speed remains almost the same. For the latter, our average value of 55 km s −1 kpc −1 agrees with the assumption that the Hercules stream in the solar neighbourhood is an imprint of the bar-disc interaction at the outer Lindblad resonance of the bar. Vertical averaging of the radial force in the central disc region comparable to the characteristic scale length allows us to reproduce the bar pattern speed and the growth rate of the rigid models, using normal mode analysis of linear perturbation theory in a razor-thin disc. The strong increase of the e-folding time with decreasing disc mass predicted by the mode analysis suggests that bars in galaxies similar to the Milky Way have formed only recently.
Monthly Notices of the Royal Astronomical Society, 2013
We follow the formation and evolution of bars in N-body simulations of disc galaxies with gas and/or a triaxial halo. We find that both the relative gas fraction and the halo shape play a major role in the formation and evolution of the bar. In gas-rich simulations, the disc stays near-axisymmetric much longer than in gas-poor ones, and, when the bar starts growing, it does so at a much slower rate. Due to these two effects combined, large-scale bars form much later in gas-rich than in gas-poor discs. This can explain the observation that bars are in place earlier in massive red disc galaxies than in blue spirals. We also find that the morphological characteristics in the bar region are strongly influenced by the gas fraction. In particular, the bar at the end of the simulation is much weaker in gas-rich cases. The quality of our simulations is such as to allow us to discuss the question of bar longevity because the resonances are well resolved and the number of gas particles is sufficient to describe the gas flow adequately. In no case did we find a bar which was destroyed. Halo triaxiality has a dual influence on bar strength. In the very early stages of the simulation it induces bar formation to start earlier. On the other hand, during the later, secular evolution phase, triaxial haloes lead to considerably less increase of the bar strength than spherical ones. The shape of the halo evolves considerably with time. We confirm previous results of gas-less simulations that find that the inner part of an initially spherical halo can become elongated and develop a halo bar. However we also show that, on the contrary, in gas rich simulations, the inner parts of an initially triaxial halo can become rounder with time. The main body of initially triaxial haloes evolves towards sphericity, but in initially strongly triaxial cases it stops well short of becoming spherical. Part of the angular momentum absorbed by the halo generates considerable rotation of the halo particles that stay located relatively near the disc for long periods of time. Another part generates halo bulk rotation, which, contrary to that of the bar, increases with time but stays small. Thus, in our models there are two nonaxisymmetric components rotating with different pattern speeds, namely the halo and the bar, so that the resulting dynamics have strong similarities to the dynamics of double bar systems.
Dark Halo Shapes and the Fate of Stellar Bars
Astrophysical Journal, 2002
We investigate the stability of trajectories in barred galaxies with mildly triaxial halos by means of Liapunov exponents. This method is perfectly suitable for time-dependent 3D potentials where surfaces of sections and other simple diagnostics are not applicable. We find that when halos are centrally-concentrated most trajectories starting near the plane containing the bar become chaotic. Moreover, the shape of many of the remaining regular trajectories do not match the bar density distribution, being too round. Therefore, time-independent self-consistent solutions are highly unlikely to be found. When the non-rotating non-axisymmetric perturbation in the potential reaches 10%, almost all trajectories integrated are chaotic and have large Liapunov exponents. No regular trajectories aligned with the bar have been found. Hence, if the evolution of the density figure is directly related to the characteristic timescale of orbital instability, bar dissolution would take place on a timescale of few dynamical times. The slowly rotating non-axisymmetric contribution to the potential required for the onset of widespread chaotic behavior is remarkably small. Systems consisting of centrally-concentrated axisymmetric halos and stellar bars thus appear to be structurally unstable, and small (1%) deviations from perfect axisymmetry should result in a bar dissolution on a timescale significantly smaller than the Hubble time. Since halos found in CDM simulations of large scale structure are both centrally-concentrated and triaxial it is unlikely that stellar bars embedded in such halos would form and survive unless the halos are modified during the formation of the baryonic component.
03 06 37 4 v 1 1 8 Ju n 20 03 Bar-Induced Evolution of Dark Matter Cusps
2003
The evolution of a stellar bar transforms not only the galactic disk, but also the host dark matter halo. We present high resolution, fully self-consistent N-body simulations that clearly demonstrate that dark matter halo central density cusps flatten as the bar torques the halo. This effect is independent of the bar formation mode and occurs even for rather short bars. The halo and bar evolution is mediated by resonant interactions between orbits in the halo and the bar pattern speed, as predicted by linear Hamiltonian perturbation theory. The bar lengthens and slows as it loses angular momentum, a process that occurs even in rather warm disks. We demonstrate that the bar and halo response can be critically underestimated for experiments that are unable to resolve the relevant resonant dynamics; this occurs when the phase space in the resonant region is under sampled or plagued by noise. Subject headings: galaxies: spiral, galaxies: kinematics and dynamics, galaxies: structure, met...
Bar instabilities in disk galaxies: the role of the triaxial halo
Astronomy and Astrophysics, 1999
We investigate the growth of bar instabilities inside a stellar-gaseous disk using a smooth particle hydrodynamics code. We carried out several simulations embedding the disk inside dark matter haloes with different masses, shapes and different dynamical states. We aim to focus on the role of a non axisymmetric halo and its evolution on the bar triggering. Our approach allows us to follow, for the first time in a selfconsistent way, the effects of a live halo on the disk evolution. We point out that a massive halo not yet relaxed has a major role in triggering such an instability. Moreover, a relaxed halo with the same mass as that of the disk cannot avoid bar instability lasting at least 1 Gyr.
The role of galaxy formation in the structure and dynamics of dark matter halos
2009
The structure and dynamics of dark matter halos, as predicted by the hierarchical clustering scenario, are at odds with the properties inferred from the observations at galactic scales. My Thesis addresses this problem by taking an evolutionary approach. I analysed in detail the many and different observational evidences of a discrepancy the predicted halo equilibrium state and the one inferred from the measurable properties of disk galaxies, as well as of the scaling relations existing between the angular momentum, geometry and mass distribution of the luminous and dark components, and realized that they all seem to point towards the same conclusion: the baryons hosted inside the halo, by collapsing and assembling to form the galaxy, perturb the halo equilibrium structure and made it evolve into new configurations. From the theoretical point of view, the behaviour of dark matter halos as collisionless systems of particles makes their equilibrium structure and mass distribution extremely sensitive to perturbations of their inner dynamics. The galaxy formation occurring inside the halos is a tremendous event, and the dynamical coupling between the baryons and the dark matter during the protogalaxy collapse represents a perturbation of the halo dynamical structure large enough to trigger a halo evolution, according to the relative mass and angular momentum of the two components. My conclusion is that the structure and dynamics of dark matter halos, as well as the origin of the connection between the halo and galaxy properties, are to be understood in in terms of a joint evolution of the baryonic and dark components, originating at the epoch of the collapse and formation of the galaxy.
The structure and dynamical evolution of dark matter haloes
Monthly Notices of the Royal Astronomical Society, 1997
We use N -body simulations to investigate the structure and dynamical evolution of dark matter halos in clusters of galaxies. Our sample consists of nine massive halos from an Einstein-De Sitter universe with scale free power spectrum and spectral index n = −1. Halos are resolved by 20000 particles each, on average, and have a dynamical resolution of 20-25 kpc, as shown by extensive tests. Large scale tidal fields are included up to a scale L = 150 Mpc using background particles. We find that the halo formation process can be characterized by the alternation of two dynamical configurations: a merging phase and a relaxation phase, defined by their signature on the evolution of the total mass and root mean square (rms) velocity. Halos spend on average one third of their evolution in the merging phase and two thirds in the relaxation phase. Using this definition, we study the density profiles and show how they change during the halo dynamical history. In particular, we find that the average density profiles of our halos are fitted by the Navarro, Frenk & White (1995) analytical model with an rms residual of 17% between the virial radius R v and 0.01R v . The Hernquist (1990) analytical density profiles fits the same halos with an rms residual of 26%. The trend with mass of the scale radius of these fits is marginally consistent with that found by : compared to their results our halos are more centrally concentrated, and the relation between scale radius and halo mass is slightly steeper. We find a moderately large scatter in this relation, due both to dynamical evolution within halos and to fluctuations in the halo population. We analyze the dynamical equilibrium of our halos using the Jeans' equation, and find that on average they are approximately in equilibrium within their virial radius. Finally, we find that the projected mass profiles of our simulated halos are in very good agreement with the profiles of three rich galaxy clusters derived from strong and weak gravitational lensing observations.
Evidence against cuspy dark matter haloes in large galaxies
Monthly Notices of the Royal Astronomical Society, 2017
We develop and apply new techniques to uncover systematic effects in galaxy rotation curves (RCs). Considering that an ideal dark matter (DM) profile should yield RCs that have no bias towards any particular radius, we find that the Burkert DM profile satisfies the test, while the Navarro-Frenk-While (NFW) profile is better at fitting the region between one and two disc scale lengths than the inner disc scale length region. Our sample indicates that this behaviour happens to more than 75 per cent of the galaxies fitted with an NFW halo. Also, this tendency is not weakened by considering large galaxies, for instance those with M * 10 10 M. Besides the tests on the homogeneity of the fits, we also use a sample of 62 galaxies of diverse types to test the quality of the overall fit of each galaxy, and to search for correlations with stellar mass, gas mass and the disc scale length. In particular, we find that only 13 galaxies are better fitted by the NFW halo, and that even for the galaxies with M * 10 10 M , the Burkert profile either fits as well as, or better than, the NFW profile. This result is relevant since different baryonic effects important for the smaller galaxies, like supernova feedback and dynamical friction from baryonic clumps, indicate that at such large stellar masses the NFW profile should be preferred over the Burkert profile. Hence, our results suggest either there is a new baryonic effect or a change to the DM physics is required.