scholarly journals Resonance sweeping by a decelerating Galactic bar

2020 ◽  
Vol 500 (4) ◽  
pp. 4710-4729
Author(s):  
Rimpei Chiba ◽  
Jennifer K S Friske ◽  
Ralph Schönrich
Keyword(s):  
Quantitative Estimate ◽  
Dark Matter Halo ◽  
Dynamical Friction ◽  
Stellar Kinematics ◽  
Current Pattern ◽  
Stellar Disc ◽  
Quantitative Evidence ◽  
Particle Simulations ◽  
Pattern Speed ◽  
Radial Action

ABSTRACT We provide the first quantitative evidence for the deceleration of the Galactic bar from local stellar kinematics in agreement with dynamical friction by a typical dark matter halo. The kinematic response of the stellar disc to a decelerating bar is studied using secular perturbation theory and test particle simulations. We show that the velocity distribution at any point in the disc affected by a naturally slowing bar is qualitatively different from that perturbed by a steadily rotating bar with the same current pattern speed Ωp and amplitude. When the bar slows down, its resonances sweep through phase space, trapping, and dragging along a portion of previously free orbits. This enhances occupation on resonances, but also changes the distribution of stars within the resonance. Due to the accumulation of orbits near the boundary of the resonance, the decelerating bar model reproduces with its corotation resonance the offset and strength of the Hercules stream in the local vR-vφ plane and the double-peaked structure of mean vR in the Lz–φ plane. At resonances other than the corotation, resonant dragging by a slowing bar is associated with a continuing increase in radial action, leading to multiple resonance ridges in the action plane as identified in the Gaia data. This work shows models using a constant bar pattern speed likely lead to qualitatively wrong conclusions. Most importantly we provide a quantitative estimate of the current slowing rate of the bar $\dot{\Omega }_{\rm p}= (-4.5 \pm 1.4) \, {\rm km}\, {\rm s}^{-1}\, {\rm kpc}^{-1}\, {\rm Gyr}^{-1}$ with additional systematic uncertainty arising from unmodelled impacts of e.g. spiral arms.

scholarly journals Double X/Peanut Structures in Barred Galaxies – Insights from an N–body Simulation

2020 ◽  
Author(s):  
Bogdan C Ciambur ◽  
Francesca Fragkoudi ◽  
Sergey Khoperskov ◽  
Paola Di Matteo ◽  
Françoise Combes
Keyword(s):  
Dark Matter ◽  
Milky Way ◽  
Large Fraction ◽  
Formation Time ◽  
Dark Matter Halo ◽  
Barred Galaxies ◽  
Pattern Speed ◽  
Nearby Galaxies ◽  
Dynamical Instabilities ◽  
Bow Tie

Abstract Boxy, peanut– or X–shaped “bulges” are observed in a large fraction of barred galaxies viewed in, or close to, edge-on projection, as well as in the Milky Way. They are the product of dynamical instabilities occurring in stellar bars, which cause the latter to buckle and thicken vertically. Recent studies have found nearby galaxies that harbour two such features arising at different radial scales, in a nested configuration. In this paper we explore the formation of such double peanuts, using a collisionless N–body simulation of a pure disc evolving in isolation within a live dark matter halo, which we analyse in a completely analogous way to observations of real galaxies. In the simulation we find a stable double configuration consisting of two X/peanut structures associated to the same galactic bar – rotating with the same pattern speed – but with different morphology, formation time, and evolution. The inner, conventional peanut-shaped structure forms early via the buckling of the bar, and experiences little evolution once it stabilises. This feature is consistent in terms of size, strength and morphology, with peanut structures observed in nearby galaxies. The outer structure, however, displays a strong X, or “bow-tie”, morphology. It forms just after the inner peanut, and gradually extends in time (within 1 to 1.5 Gyr) to almost the end of the bar, a radial scale where ansae occur. We conclude that, although both structures form, and are dynamically coupled to, the same bar, they are supported by inherently different mechanisms.

scholarly journals Identifying resonances of the Galactic bar in Gaia DR2: I. Clues from action space

2020 ◽  
Vol 500 (2) ◽  
pp. 2645-2665
Author(s):  
Wilma H Trick ◽  
Francesca Fragkoudi ◽  
Jason A S Hunt ◽  
J Ted Mackereth ◽  
Simon D M White
Keyword(s):  
Action Space ◽  
Galactic Disc ◽  
Resonant Orbits ◽  
Particle Simulations ◽  
Pattern Speed ◽  
The Rich ◽  
First Time ◽  
Gaia Dr2 ◽  
Vertical Action ◽  
Lindblad Resonance

ABSTRACT Action space synthesizes the orbital information of stars and is well suited to analyse the rich kinematic substructure of the disc in the second Gaia data release's radial velocity sample. We revisit the strong perturbation induced in the Milky Way disc by an m = 2 bar, using test particle simulations and the actions (JR, Lz, Jz) estimated in an axisymmetric potential. These make three useful diagnostics cleanly visible. (1) We use the well-known characteristic flip from outward to inward motion at the outer Lindblad resonance (OLR; l = +1, m = 2), which occurs along the axisymmetric resonance line (ARL) in (Lz, JR), to identify in the Gaia action data three candidates for the bar’s OLR and pattern speed Ωbar: 1.85Ω0, 1.20Ω0, and 1.63Ω0 (with ∼0.1Ω0 systematic uncertainty). The Gaia data is therefore consistent with both slow and fast bar models in the literature, but disagrees with recent measurements of ∼1.45Ω0. (2) For the first time, we demonstrate that bar resonances – especially the OLR – cause a gradient in vertical action 〈Jz〉 with Lz around the ARL via ‘Jz-sorting’ of stars. This could contribute to the observed coupling of 〈vR〉 and 〈|vz|〉 in the Galactic disc. (3) We confirm prior results that the behaviour of resonant orbits is well approximated by scattering and oscillation in (Lz, JR) along a slope ΔJR/ΔLz = l/m centred on the l:m ARL. Overall, we demonstrate that axisymmetrically estimated actions are a powerful diagnostic tool even in non-axisymmetric systems.

scholarly journals Fast galaxy bars continue to challenge standard cosmology

2021 ◽  
Vol 508 (1) ◽  
pp. 926-939
Author(s):  
Mahmood Roshan ◽  
Neda Ghafourian ◽  
Tahere Kashfi ◽  
Indranil Banik ◽  
Moritz Haslbauer ◽  
...  
Keyword(s):  
Dark Matter ◽  
Cold Dark Matter ◽  
Dark Matter Halo ◽  
Dynamical Friction ◽  
Statistical Population ◽  
Barred Galaxies ◽  
Corotation Radius ◽  
Pattern Speeds ◽  
Hydrodynamical Simulations ◽  
Disc Galaxies

ABSTRACT Many observed disc galaxies harbour a central bar. In the standard cosmological paradigm, galactic bars should be slowed down by dynamical friction from the dark matter halo. This friction depends on the galaxy’s physical properties in a complex way, making it impossible to formulate analytically. Fortunately, cosmological hydrodynamical simulations provide an excellent statistical population of galaxies, letting us quantify how simulated galactic bars evolve within dark matter haloes. We measure bar strengths, lengths, and pattern speeds in barred galaxies in state-of-the-art cosmological hydrodynamical simulations of the IllustrisTNG and EAGLE projects, using techniques similar to those used observationally. We then compare our results with the largest available observational sample at redshift z = 0. We show that the tension between these simulations and observations in the ratio of corotation radius to bar length is 12.62σ (TNG50), 13.56σ (TNG100), 2.94σ (EAGLE50), and 9.69σ (EAGLE100), revealing for the first time that the significant tension reported previously persists in the recently released TNG50. The lower statistical tension in EAGLE50 is actually caused by it only having five galaxies suitable for our analysis, but all four simulations give similar statistics for the bar pattern speed distribution. In addition, the fraction of disc galaxies with bars is similar between TNG50 and TNG100, though somewhat above EAGLE100. The simulated bar fraction and its trend with stellar mass both differ greatly from observations. These dramatic disagreements cast serious doubt on whether galaxies actually have massive cold dark matter haloes, with their associated dynamical friction acting on galactic bars.

scholarly journals Detection of a slow H i bar in the dwarf irregular galaxy DDO 168

2019 ◽  
Vol 488 (4) ◽  
pp. 4942-4951
Author(s):  
Narendra Nath Patra ◽  
Chanda J Jog
Keyword(s):  
Dark Matter ◽  
Time Scale ◽  
Dark Matter Halo ◽  
Irregular Galaxy ◽  
Stringent Constraint ◽  
Dynamical Time ◽  
Dimensionless Ratio ◽  
Pattern Speed ◽  
Pattern Speeds ◽  
First Time

Abstract We examine the H i total intensity maps of the VLA LITTLE-THINGS galaxies and identify an H i bar in the dwarf irregular galaxy DDO 168 which has a dense and compact dark matter halo that dominates at all radii. This is only the third galaxy found to host an H i bar. Using the H i kinematic data, we apply the Tremaine–Weinberg method to estimate the pattern speed of the bar. The H i bar is found to have an average pattern speed of 23.3 ± 5.9 $\rm km \, s^{-1} \, kpc^{-1}$. Interestingly, for the first time, we find that the observed pattern speeds of the bar in the two kinematic halves are different. We identify the origin of this difference to be the kinematic asymmetry. This observed offset in the pattern speed serves to put a stringent constraint on the lifetime of the bar set by the winding time-scale. The lifetime of the bar is found to be 5.3 × 108 yr, which is two times the dynamical time-scale of the disc. We also find the H i bar in DDO 168 to be a weak bar with a strength of 0.2. If H i bar being weak can be easily disturbed, this could possibly explain why it is extremely rare to observe H i bars in galaxies. We estimate the bar radius to be 1 kpc and the dimensionless ratio, RL/Rb to be ≥2.1 indicating a ‘slow’ bar in DDO 168. Our results confirm the proposition that the dynamical friction with the halo slows down a rotating bar in a galaxy dominated by dark matter halo from inner radii.

scholarly journals NIHAO XXIV: rotation- or pressure-supported systems? Simulated Ultra Diffuse Galaxies show a broad distribution in their stellar kinematics

2020 ◽  
Vol 497 (4) ◽  
pp. 4282-4292
Author(s):  
Salvador Cardona-Barrero ◽  
Arianna Di Cintio ◽  
Christopher B A Brook ◽  
Tomas Ruiz-Lara ◽  
Michael A Beasley ◽  
...  
Keyword(s):  
Galaxy Evolution ◽  
Dark Matter Halo ◽  
Stellar Kinematics ◽  
Broad Distribution ◽  
Hydrodynamical Simulation ◽  
Low Surface Brightness ◽  
Astrophysical Objects ◽  
Comprehensive Survey ◽  
Gas Accretion ◽  
State Pressure

ABSTRACT In recent years, a new window on galaxy evolution opened, thanks to the increasing discovery of galaxies with a low-surface brightness, such as Ultra Diffuse Galaxies (UDGs). The formation mechanism of these systems is still a much debated question and so are their kinematical properties. In this work, we address this topic by analysing the stellar kinematics of isolated UDGs formed in the hydrodynamical simulation suite Numerical Investigation of a Hundred Astrophysical Objects (NIHAO). We construct projected line-of-sight velocity and velocity dispersion maps to compute the projected specific angular momentum, λR, to characterize the kinematical support of the stars in these galaxies. We found that UDGs cover a broad distribution, ranging from dispersion to rotation-supported galaxies, with similar abundances in both regimes. The degree of rotation support of simulated UDGs correlates with several properties such as galaxy morphology, higher H i fractions, and larger effective radii with respect to the dispersion-supported group, while the dark matter halo spin and mass accretion history are similar among the two populations. We demonstrate that the alignment of the infalling baryons into the protogalaxy at early z is the principal driver of the z = 0 stellar kinematic state: pressure-supported isolated UDGs form via misaligned gas accretion while rotation-supported ones build up their baryons in an ordered manner. Accounting for random inclination effects, we predict that a comprehensive survey will find nearly half of field UDGs to have rotationally supported stellar discs, when selecting UDGs with effective radius larger than 1 kpc.

scholarly journals On the stellar kinematics and mass of the Virgo ultradiffuse galaxy VCC 1287

2020 ◽  
Vol 495 (3) ◽  
pp. 2582-2598 ◽  
Author(s):  
Jonah S Gannon ◽  
Duncan A Forbes ◽  
Aaron J Romanowsky ◽  
Anna Ferré-Mateu ◽  
Warrick J Couch ◽  
...  
Keyword(s):  
Virgo Cluster ◽  
Dark Matter Halo ◽  
Mass Relation ◽  
Stellar Kinematics ◽  
Tidal Effects ◽  
Dynamical Mass ◽  
Normal Galaxies ◽  
Stellar Feedback ◽  
Stellar Velocity ◽  
Good Agreement

ABSTRACT Here, we present a kinematical analysis of the Virgo cluster ultradiffuse galaxy (UDG) VCC 1287 based on data taken with the Keck Cosmic Web Imager (KCWI). We confirm VCC 1287’s association both with the Virgo cluster and its globular cluster (GC) system, measuring a recessional velocity of 1116 ± 2 km s−1. We measure a stellar velocity dispersion (19 ± 6 km s−1) and infer both a dynamical mass ($1.11^{+0.81}_{-0.81} \times 10^{9} \ \mathrm{M_{\odot }}$) and mass-to-light ratio (M/L) ($13^{+11}_{-11}$) within the half-light radius (4.4 kpc). This places VCC 1287 slightly above the well-established relation for normal galaxies, with a higher M/L for its dynamical mass than normal galaxies. We use our dynamical mass, and an estimate of GC system richness, to place VCC 1287 on the GC number–dynamical mass relation, finding good agreement with a sample of normal galaxies. Based on a total halo mass derived from GC counts, we then infer that VCC 1287 likely resides in a cored or low-concentration dark matter halo. Based on the comparison of our measurements to predictions from simulations, we find that strong stellar feedback and/or tidal effects are plausibly the dominant mechanisms in the formation of VCC 1287. Finally, we compare our measurement of the dynamical mass with those for other UDGs. These dynamical mass estimates suggest relatively massive haloes and a failed galaxy origin for at least some UDGs.

scholarly journals Spatially Resolved Stellar Kinematics of the Ultra-diffuse Galaxy Dragonfly 44. I. Observations, Kinematics, and Cold Dark Matter Halo Fits

2019 ◽  
Vol 880 (2) ◽  
pp. 91 ◽  
Author(s):  
Pieter van Dokkum ◽  
Asher Wasserman ◽  
Shany Danieli ◽  
Roberto Abraham ◽  
Jean Brodie ◽  
...  
Keyword(s):  
Dark Matter ◽  
Cold Dark Matter ◽  
Dark Matter Halo ◽  
Stellar Kinematics ◽  
Spatially Resolved

scholarly journals Dust and the observed dark matter content of galaxies

2004 ◽  
Vol 220 ◽  
pp. 343-344 ◽  
Author(s):  
Maarten Baes ◽  
Herwig Dejonghe ◽  
Jonathan I. Davies
Keyword(s):  
Dark Matter ◽  
Interstellar Dust ◽  
Optical Rotation ◽  
Elliptical Galaxies ◽  
Matter Content ◽  
Dark Matter Halo ◽  
Stellar Kinematics ◽  
The Galaxy ◽  
Dust Attenuation ◽  
Radiative Transfer Modeling

Using detailed Monte Carlo radiative transfer modeling, we examine the effects of absorption and scattering by interstellar dust on the observed kinematics of galaxies. Our modeling results have a direct impact on the derivation of the properties of dark matter haloes around both elliptical and spiral galaxies. We find that interstellar dust has a very significant effect on the observed stellar kinematics of elliptical galaxies, in the way that it mimics the presence of a dark matter halo. Taking dust into account in kinematical modeling procedures can reduce or even eliminate the need for dark matter at a few effective radii. Dust profoundly affects the optical rotation curve and stellar kinematics of edge-on disc galaxies. This effect, however, is significantly reduced when the galaxy is more than a few degrees from strictly edge-on. These results demonstrate that dust attenuation cannot be invoked as a possible mechanism to reconcile the discrepancies between the observed shallow slopes of LSB galaxy rotation curves and the dark matter cusps found in CDM cosmological simulations.

scholarly journals Galactic seismology: the evolving “phase spiral” after the Sagittarius dwarf impact

2021 ◽  
Author(s):  
Joss Bland-Hawthorn ◽  
Thor Tepper-García
Keyword(s):  
High Resolution ◽  
Total Mass ◽  
Phase Plane ◽  
Density Wave ◽  
Analytic Model ◽  
Spiral Pattern ◽  
Galactic Disc ◽  
Stellar Disc ◽  
Pattern Speed ◽  
Bending Wave

Abstract In 2018, the ESA Gaia satellite discovered a remarkable spiral pattern (“phase spiral”) in the z − Vz phase plane throughout the solar neighbourhood, where z and Vz are the displacement and velocity of a star perpendicular to the Galactic disc. In response to Binney & Schönrich’s analytic model of a disc-crossing satellite to explain the Gaia data, we carry out a high-resolution, N-body simulation (N ≈ 108 particles) of an impulsive mass (2 × 1010 M⊙) that interacts with a cold stellar disc at a single transit point. The disc response is complex since the impulse triggers a superposition of two distinct bisymmetric (m = 2) modes − a density wave and a corrugated bending wave − that wrap up at different rates. Stars in the faster density wave wrap up with time T according to φD(R, T) = (ΩD(R) + Ωo) T where φD describes the spiral pattern and ΩD = Ω(R) − κ(R)/2, where κ is the epicyclic frequency. While the pattern speed Ωo is small, it is non-zero. The slower bending wave wraps up according to ΩB ≈ ΩD/2 producing a corrugated wave. The bunching effect of the density wave triggers the phase spiral as it rolls up and down on the bending wave (“rollercoaster” model). The phase spiral emerges slowly about ΔT ≈ 400 Myr after impact. It appears to be a long-lived, disc-wide phenomenon that continues to evolve over most of the 2 Gyr simulation. Thus, given Sagittarius’ (Sgr) low total mass today (Mtot ∼ 3 × 108 M⊙ within 10 kpc diameter), we believe the phase spiral was excited by the disc-crossing dwarf some 1 − 2 Gyr before the recent transit. For this to be true, Sgr must be losing mass at 0.5-1 dex per orbit loop.

scholarly journals The twisted dark matter halo of the Milky Way

2020 ◽  
Author(s):  
Shi Shao ◽  
Marius Cautun ◽  
Alis Deason ◽  
Carlos S Frenk
Keyword(s):  
Dark Matter ◽  
Milky Way ◽  
Orbital Plane ◽  
Minor Axis ◽  
Dark Matter Halo ◽  
Misalignment Angle ◽  
Stellar Disc ◽  
Confidence Levels ◽  
The Common ◽  
Inner Parts

Abstract We analyse systems analogous to the Milky Way (MW) in the eagle cosmological hydrodynamics simulation in order to deduce the likely structure of the MW’s dark matter halo. We identify MW-mass haloes in the simulation whose satellite galaxies have similar kinematics and spatial distribution to those of the bright satellites of the MW, specifically systems in which the majority of the satellites (8 out of 11) have nearly co-planar orbits that are also perpendicular to the central stellar disc. We find that the normal to the common orbital plane of the co-planar satellites is well aligned with the minor axis of the host dark matter halo, with a median misalignment angle of only 17.3○. Based on this result, we infer that the minor axis of the Galactic dark matter halo points towards (l, b) = (182○, −2○), with an angular uncertainty at the 68 and 95 percentile confidence levels of 22○ and 43○ respectively. Thus, the inferred minor axis of the MW halo lies in the plane of the stellar disc. The halo, however, is not homologous and its flattening and orientation vary with radius. The inner parts of the halo are rounder than the outer parts and well aligned with the stellar disc (that is the minor axis of the halo is perpendicular to the disc). Further out, the halo twists and the minor axis changes direction by 90○. This twist occurs over a very narrow radial range and reflects variations in the filamentary network along which mass was accreted into the MW.

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