scholarly journals Buckling instability in tidally induced galactic bars

2019 ◽  
Vol 624 ◽  
pp. A37 ◽  
Author(s):  
Ewa L. Łokas
Keyword(s):  
Vertical Direction ◽  
Initial Structure ◽  
Mean Velocity ◽  
Stellar Orbits ◽  
Buckling Instability ◽  
Tidal Forces ◽  
The Galaxy ◽  
Disk Plane ◽  
Orbital Instability ◽  
Fire Hose

Strong galactic bars produced in simulations tend to undergo a period of buckling instability that weakens and thickens them and forms a boxy/peanut structure in their central parts. This theoretical prediction has been confirmed by identifying such morphologies in real galaxies. The nature and origin of this instability, however, remain poorly understood with some studies claiming that it is due to fire-hose instability while others relating it to vertical instability of stellar orbits supporting the bar. One of the channels for the formation of galactic bars is via the interaction of disky galaxies with perturbers of significant mass. Tidally induced bars offer a unique possibility of studying buckling instability because their formation can be controlled by changing the strength of the interaction while keeping the initial structure of the galaxy the same. We used a set of four simulations of flyby interactions where a galaxy on a prograde orbit forms a bar, which is stronger for stronger tidal forces. We studied their buckling by calculating different kinematic signatures, including profiles of the mean velocity in the vertical direction, as well as distortions of the bars out of the disk plane. Although our two strongest bars buckle most strongly, there is no direct relation between the ratio of vertical to horizontal velocity dispersion and the bar’s susceptibility to buckling, as required by the fire-hose instability interpretation. While our weakest bar buckles, a stronger one does not, its dispersion ratio remains low, and it grows to become the strongest of all at the end of evolution. Instead, we find that during buckling the resonance between the vertical and radial orbital frequencies becomes wide and therefore able to modify stellar orbits over a significant range of radii. We conclude that vertical orbital instability is the more plausible explanation for the origin of buckling.

scholarly journals Anatomy of a buckling galactic bar

2019 ◽  
Vol 629 ◽  
pp. A52 ◽  
Author(s):  
Ewa L. Łokas
Keyword(s):  
Outer Part ◽  
Vertical Direction ◽  
Dark Matter Halo ◽  
Stellar Disk ◽  
Stellar Orbits ◽  
Buckling Instability ◽  
Bending Waves ◽  
Before And After ◽  
The Galaxy ◽  
Circular Frequency

Using N-body simulations we study the buckling instability in a galactic bar forming in a Milky Way-like galaxy. The galaxy is initially composed of an axisymmetric, exponential stellar disk embedded in a spherical dark matter halo. The parameters of the model are chosen so that the galaxy is mildly unstable to bar formation and the evolution is followed for 10 Gyr. A strong bar forms slowly over the first few gigayears and buckles after 4.5 Gyr from the start of the simulation becoming much weaker and developing a pronounced boxy/peanut shape. We measure the properties of the bar at the time of buckling in terms of the mean acceleration, velocity, and distortion in the vertical direction. The maps of these quantities in face-on projections reveal characteristic quadrupole patterns which wind up over a short timescale. We also detect a secondary buckling event lasting much longer and occurring only in the outer part of the bar. We then study the orbital structure of the bar in periods before and after the first buckling. We find that most of the buckling orbits originate from x1 orbits supporting the bar. During buckling the ratio of the vertical to horizontal frequency of the stellar orbits decreases dramatically and after buckling the orbits obey a very tight relation between the vertical and circular frequency: 3ν = 4Ω. We propose that buckling is initiated by the vertical resonance of the x1 orbits creating the initial distortion of the bar that later evolves as kinematic bending waves.

scholarly journals The last breath of the Sagittarius dSph

2020 ◽  
Vol 497 (4) ◽  
pp. 4162-4182 ◽  
Author(s):  
Eugene Vasiliev ◽  
Vasily Belokurov
Keyword(s):  
Milky Way ◽  
Dwarf Galaxy ◽  
3D Structure ◽  
Major Axis ◽  
Final State ◽  
Tidal Forces ◽  
The Galaxy ◽  
The Mean ◽  
Red Clump ◽  
Astrometric Data

ABSTRACT We use the astrometric and photometric data from Gaia Data Release 2 and line-of-sight velocities from various other surveys to study the 3D structure and kinematics of the Sagittarius dwarf galaxy. The combination of photometric and astrometric data makes it possible to obtain a very clean separation of Sgr member stars from the Milky Way foreground; our final catalogue contains 2.6 × 105 candidate members with magnitudes G < 18, more than half of them being red clump stars. We construct and analyse maps of the mean proper motion and its dispersion over the region ∼30 × 12 deg, which show a number of interesting features. The intrinsic 3D density distribution (orientation, thickness) is strongly constrained by kinematics; we find that the remnant is a prolate structure with the major axis pointing at ∼45° from the orbital velocity and extending up to ∼5 kpc, where it transitions into the stream. We perform a large suite of N-body simulations of a disrupting Sgr galaxy as it orbits the Milky Way over the past 2.5 Gyr, which are tailored to reproduce the observed properties of the remnant (not the stream). The richness of available constraints means that only a narrow range of parameters produce a final state consistent with observations. The total mass of the remnant is $\sim \!4\times 10^8\, \mathrm{M}_\odot$, of which roughly a quarter resides in stars. The galaxy is significantly out of equilibrium, and even its central density is below the limit required to withstand tidal forces. We conclude that the Sgr galaxy will likely be disrupted over the next Gyr.

scholarly journals Ionised gas structure of 100 kpc in an over-dense region of the galaxy group COSMOS-Gr30 at z ~ 0.7

2018 ◽  
Vol 609 ◽  
pp. A40 ◽  
Author(s):  
B. Epinat ◽  
T. Contini ◽  
H. Finley ◽  
L. A. Boogaard ◽  
A. Guérou ◽  
...  
Keyword(s):  
Physical Properties ◽  
High Velocity ◽  
Active Galactic Nucleus ◽  
Dense Region ◽  
Tidal Forces ◽  
Shock Models ◽  
The Galaxy ◽  
Group Members ◽  
Low Mass ◽  
Massive Galaxy

We report the discovery of a 104 kpc2 gaseous structure detected in [O ii]λλ3727, 3729 in an over-dense region of the COSMOS-Gr30 galaxy group at z ~ 0.725 with deep MUSE Guaranteed Time Observations. We estimate the total amount of diffuse ionised gas to be of the order of (~5 ± 3) × 1010 M⊙ and explore its physical properties to understand its origin and the source(s) of the ionisation. The MUSE data allow the identification of a dozen group members that are embedded in this structure through emission and absorption lines. We extracted spectra from small apertures defined for both the diffuse ionised gas and the galaxies. We investigated the kinematics and ionisation properties of the various galaxies and extended gas regions through line diagnostics (R23, O32, and [O iii]/Hβ) that are available within the MUSE wavelength range. We compared these diagnostics to photo-ionisation models and shock models. The structure is divided into two kinematically distinct sub-structures. The most extended sub-structure of ionised gas is likely rotating around a massive galaxy and displays filamentary patterns that link some galaxies. The second sub-structure links another massive galaxy that hosts an active galactic nucleus (AGN) to a low-mass galaxy, but it also extends orthogonally to the AGN host disc over ~ 35 kpc. This extent is likely ionised by the AGN itself. The location of small diffuse regions in the R23 vs. O32 diagram is compatible with photo-ionisation. However, the location of three of these regions in this diagram (low O32, high R23) can also be explained by shocks, which is supported by their high velocity dispersions. One edge-on galaxy shares the same properties and may be a source of shocks. Regardless of the hypothesis, the extended gas seems to be non-primordial. We favour a scenario where the gas has been extracted from galaxies by tidal forces and AGN triggered by interactions between at least the two sub-structures.

scholarly journals Population Synthesis of Old Neutron Stars in the Galaxy

2000 ◽  
Vol 195 ◽  
pp. 181-188
Author(s):  
S. B. Popov ◽  
M. Colpi ◽  
A. Treves ◽  
R. Turolla ◽  
V. M. Lipunov ◽  
...  
Keyword(s):  
Neutron Stars ◽  
Ambient Medium ◽  
Dynamical Evolution ◽  
Low Velocity ◽  
Mean Velocity ◽  
Secular Evolution ◽  
Sky Survey ◽  
The Galaxy ◽  
The Mean ◽  
Statistical Sample

The paucity of old, isolated accreting neutron stars in ROSAT observations is used to derive a lower limit on the mean velocity of neutron stars at birth. The secular evolution of the population is simulated following the paths of a statistical sample of stars for different values of the initial kick velocity, drawn from an isotropic, Gaussian distribution with mean velocity 0 ≤ 〈V〉 ≤ 550 km s−1. The spin-down, induced by dipole losses and the interaction with the ambient medium, is tracked together with the dynamical evolution in the Galactic potential, allowing for the determination of the fraction of stars which are, at present, in each of the four possible stages: Ejector, Propeller, Accretor, and Georotator. Taking from the ROSAT All-Sky Survey an upper limit of ~ 10 accreting neutron stars within ~ 140 pc from the Sun, we infer a lower bound for the mean kick velocity, 〈V〉 ≳ 200–300 km s−1. The same conclusion is reached for both a constant (B ~ 1012 G) and an exponentially decaying magnetic field with a timescale ~ 109 yr. Present results, moreover, constrain the fraction of low-velocity stars which could have escaped pulsar statistics to ≲ 1%.

Nonlinear Effects in Models of the Galaxy. I. Midplane Stellar Orbits in the Presence of Three‐dimensional Spiral Arms

2003 ◽  
Vol 582 (1) ◽  
pp. 230-245 ◽  
Author(s):  
Barbara Pichardo ◽  
Marco Martos ◽  
Edmundo Moreno ◽  
Julia Espresate
Keyword(s):  
Three Dimensional ◽  
Nonlinear Effects ◽  
Spiral Arms ◽  
Stellar Orbits ◽  
The Galaxy

scholarly journals Neutral hydrogen gas within and around NGC 1316

2019 ◽  
Vol 628 ◽  
pp. A122 ◽  
Author(s):  
P. Serra ◽  
F. M. Maccagni ◽  
D. Kleiner ◽  
W. J. G. de Blok ◽  
J. H. van Gorkom ◽  
...  
Keyword(s):  
Large Scale ◽  
Molecular Form ◽  
Neutral Hydrogen ◽  
Hydrogen Gas ◽  
Neutral Gas ◽  
Tidal Forces ◽  
The Galaxy ◽  
Long Tails ◽  
Stellar Body ◽  
Fornax Cluster

We present MeerKAT observations of neutral hydrogen gas (H I) in the nearby merger remnant NGC 1316 (Fornax A), the brightest member of a galaxy group which is falling into the Fornax cluster. We find H I on a variety of scales, from the galaxy centre to its large-scale environment. For the first time we detect H I at large radii (70–150 kpc in projection), mostly distributed on two long tails associated with the galaxy. Gas in the tails dominates the H I mass of NGC 1316: 7 × 108 M⊙– 14 times more than in previous observations. The total H I mass is comparable to the amount of neutral gas found inside the stellar body, mostly in molecular form. The H I tails are associated with faint optical tidal features thought to be the remnant of a galaxy merger occurred a few billion years ago. They demonstrate that the merger was gas-rich. During the merger, tidal forces pulled some gas and stars out to large radii, where we now detect them in the form of optical tails and, thanks to our new data, H I tails; while torques caused the remaining gas to flow towards the centre of the remnant, where it was converted into molecular gas and fuelled the starburst revealed by the galaxy’s stellar populations. Several of the observed properties of NGC 1316 can be reproduced by a ∼10:1 merger between a dominant, gas-poor early-type galaxy and a smaller, gas-rich spiral occurred 1–3 Gyr ago, likely followed by subsequent accretion of satellite galaxies.

scholarly journals The distance to the Galactic Center

2012 ◽  
Vol 8 (S289) ◽  
pp. 29-35 ◽  
Author(s):  
Stefan Gillessen ◽  
Frank Eisenhauer ◽  
Tobias K. Fritz ◽  
Oliver Pfuhl ◽  
Thomas Ott ◽  
...  
Keyword(s):  
Black Hole ◽  
Galactic Center ◽  
Distance Estimate ◽  
Massive Black Hole ◽  
Geometric Distance ◽  
Stellar Orbits ◽  
Fundamental Parameters ◽  
The Galaxy ◽  
Error Bars ◽  
Definition Of

AbstractOne of the Milky Way's fundamental parameters is the distance of the Sun from the Galactic Center, R0. This article reviews the various ways of estimating R0, placing special emphasis on methods that have become possible recently. In particular, we focus on the geometric distance estimate made possible thanks to observations of individual stellar orbits around the massive black hole at the center of the Galaxy. The specific issues of concern there are the degeneracies with other parameters, most importantly the mass of the black hole and the definition of the reference frame. The current uncertainty is nevertheless only a few percent, with error bars shrinking every year.

scholarly journals Local Stellar Kinematics from RAVE data – V. Kinematic Investigation of the Galaxy with Red Clump Stars

2014 ◽  
Vol 31 ◽  
Author(s):  
S. Karaali ◽  
S. Bilir ◽  
S. Ak ◽  
E. Yaz Gökçe ◽  
Ö. Önal ◽  
...  
Keyword(s):  
Velocity Component ◽  
Galactic Plane ◽  
Radial Distance ◽  
Vertical Direction ◽  
Space Velocity ◽  
Galactic Centre ◽  
Galactic Rotation ◽  
Stellar Kinematics ◽  
The Galaxy ◽  
Red Clump

AbstractWe investigated the space velocity components of 6 610 red clump (RC) stars in terms of vertical distance, Galactocentric radial distance and Galactic longitude. Stellar velocity vectors are corrected for differential rotation of the Galaxy which is taken into account using photometric distances of RC stars. The space velocity components estimated for the sample stars above and below the Galactic plane are compatible only for the space velocity component in the direction to the Galactic rotation of the thin disc stars. The space velocity component in the direction to the Galactic rotation (Vlsr) shows a smooth variation relative to the mean Galactocentric radial distance (Rm), while it attains its maximum at the Galactic plane. The space velocity components in the direction to the Galactic centre (Ulsr) and in the vertical direction (Wlsr) show almost flat distributions relative to Rm, with small changes in their trends at Rm ~ 7.5 kpc. Ulsr values estimated for the RC stars in quadrant 180° < l ⩽ 270° are larger than the ones in quadrants 0° < l ⩽ 90° and 270° < l ⩽ 360°. The smooth distribution of the space velocity dispersions reveals that the thin and thick discs are kinematically continuous components of the Galaxy. Based on the Wlsr space velocity components estimated in the quadrants 0° < l ⩽ 90° and 270° < l ⩽ 360°, in the inward direction relative to the Sun, we showed that RC stars above the Galactic plane move towards the North Galactic Pole, whereas those below the Galactic plane move in the opposite direction. In the case of quadrant 180° < l ⩽ 270°, their behaviour is different, i.e. the RC stars above and below the Galactic plane move towards the Galactic plane. We stated that the Galactic long bar is the probable origin of many, but not all, of the detected features.

Simulation of a propelled wake with moderate excess momentum in a stratified fluid

2011 ◽  
Vol 692 ◽  
pp. 28-52 ◽  
Author(s):  
Matthew B. de Stadler ◽  
Sutanu Sarkar
Keyword(s):  
Kinetic Energy ◽  
Velocity Profile ◽  
Turbulent Kinetic Energy ◽  
Vertical Direction ◽  
Qualitative Change ◽  
Stratified Fluid ◽  
Mean Velocity ◽  
Moderate Amount ◽  
Excess Momentum ◽  
The Mean

AbstractDirect numerical simulation is used to simulate the turbulent wake behind an accelerating axisymmetric self-propelled body in a stratified fluid. Acceleration is modelled by adding a velocity profile corresponding to net thrust to a self-propelled velocity profile resulting in a wake with excess momentum. The effect of a small to moderate amount of excess momentum on the initially momentumless self-propelled wake is investigated to evaluate if the addition of excess momentum leads to a large qualitative change in wake dynamics. Both the amount and shape of excess momentum are varied. Increasing the amount of excess momentum and/or decreasing the radial extent of excess momentum was found to increase the defect velocity, mean kinetic energy, shear in the velocity gradient and the wake width. The increased shear in the mean profile resulted in increased production of turbulent kinetic energy leading to an increase in turbulent kinetic energy and its dissipation. Slightly larger vorticity structures were observed in the late wake with excess momentum although the differences between vorticity structures in the self-propelled and 40 % excess momentum cases was significantly smaller than suggested by previous experiments. Buoyancy was found to preserve the doubly inflected velocity profile in the vertical direction, and similarity for the mean velocity and turbulent kinetic energy was found to occur in both horizontal and vertical directions. While quantitative differences were observed between cases with and without excess momentum, qualitatively similar evolution was found to occur.

scholarly journals First direct identification of the barlens vertical structure in galaxy models

2021 ◽  
Vol 648 ◽  
pp. L4
Author(s):  
Iliya S. Tikhonenko ◽  
Anton A. Smirnov ◽  
Natalia Ya. Sotnikova
Keyword(s):  
Vertical Structure ◽  
Vertical Direction ◽  
Direct Identification ◽  
Flat Structure ◽  
Body Model ◽  
Thick Part ◽  
Different Types ◽  
The Galaxy ◽  
The Face ◽  
First Time

Applying spectral dynamics methods to one typical N-body model with a barlens, we dissect the modelled bar into separate components supported by completely different types of orbits. We identify at least four components: a narrow elongated bar, a boxy bar, and two components contributing to the barlens. We analyse the vertical structure of all components that make up the thick part of the bar, which has a boxy/peanut shape (B/P bulge). We show that the ‘peanut’ shape is mainly due to the orbits that assemble the boxy part of the face-on bar. We associate the X-shape with the narrow and elongated bar. The wider part of the barlens with square-like isophotes contributes to the boxy shape of the B/P bulge when we observe the galaxy edge-on. However, the part of the barlens with rounded isophotes in the face-on view is a rather flat structure in the vertical direction without any significant off-centre protrusions. Thus, for the first time, we demonstrate that the rounded face-on barlens cannot be entirely associated with the B/P bulge.

Sign in / Sign up

Export Citation Format

Share Document