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

2021 ◽  
Author(s):  
Wilma H Trick
Keyword(s):  
Galactic Center ◽  
Selection Effects ◽  
The Past ◽  
Angle Data ◽  
Angle Space ◽  
Particle Simulations ◽  
Pattern Speed ◽  
Orbit Orientation ◽  
Gaia Dr2 ◽  
Lindblad Resonance

Abstract The Milky Way disk exhibits intricate orbit substructure of still-debated dynamical origin. The angle variables (θφ, θR)—which are conjugates to the actions (Lz, JR), and describe a star’s location along its orbit—are a powerful diagnostic to identify l:m resonances via the orbit shape relation ΔθR/Δθφ = −m/l. In the past, angle signatures have been hidden by survey selection effects (SEs). Using test particle simulations of a barred galaxy, we demonstrate that Gaia should allow us to identify the Galactic bar’s Outer Lindblad Resonance (l = +1, m = 2, OLR) in angle space. We investigate strategies to overcome SEs. In the angle data of the Gaia DR2 RVS sample, we independently identify four candidates for the OLR and therefore for the pattern speed Ωbar. The strongest candidate, Ωbar ∼ 1.4Ω0, positions the OLR above the ‘Sirius’ moving group, agrees with measurements from the Galactic center, and might be supported by higher-order resonances around the ‘Hercules/Horn’. But it misses the classic orbit orientation flip, as discussed in the companion study on actions. The candidate Ωbar ∼ 1.2Ω0 was also suggested by the action-based study, has the OLR at the ‘Hat’, is consistent with slow bar models, but still affected by SEs. Weaker candidates are Ωbar = 1.6 and 1.74Ω0. In addition, we show that the stellar angles do not support the ‘Hercules/Horn’ being created by the OLR of a fast bar. We conclude that—to resolve if ‘Sirius’ or ‘Hat’ are related to the bar’s OLR—more complex dynamical explanations and more extended data with well-behaved SEs are required.

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 Tracing Hercules in Galactic azimuth with Gaia DR2

2019 ◽  
Vol 632 ◽  
pp. A107 ◽  
Author(s):  
G. Monari ◽  
B. Famaey ◽  
A. Siebert ◽  
O. Bienaymé ◽  
R. Ibata ◽  
...  
Keyword(s):  
Angular Momentum ◽  
Radial Velocity ◽  
Major Axis ◽  
The Other ◽  
Data Release ◽  
Stellar Velocity ◽  
Pattern Speed ◽  
Gaia Dr2 ◽  
Lindblad Resonance ◽  
Exact Origin

The second data release of the Gaia mission has revealed, in stellar velocity and action space, multiple ridges, the exact origin of which is still debated. Recently, we demonstrated that a large Galactic bar with pattern speed 39 km s−1 kpc−1 creates most of the observed ridges. Among these ridges, the Hercules moving group would then be associated with orbits trapped at the co-rotation resonance of the bar. Here we show that a distinctive prediction of such a model is that the angular momentum of Hercules at the Sun’s radius must significantly decrease with increasing Galactocentric azimuth (i.e. when getting closer to the major axis of the bar). We show that this dependence of the angular momentum of trapped orbits on the azimuth on the other hand does not happen close to the outer Lindblad resonance of a faster bar, unless the orbital distribution is still far from phase-mixed, namely for a bar perturbation younger than ∼2 Gyr. Using Gaia DR2 and Bayesian distances from the StarHorse code, and tracing the average Galactocentric radial velocity as a function of angular momentum and azimuth, we show that the Hercules angular momentum changes significantly with azimuth as expected for the co-rotation resonance of a dynamically old large bar.

scholarly journals Gaia DR2 data and the evolutionary status of eight high-velocity hot post-AGB candidates

2020 ◽  
Vol 72 (6) ◽  
Author(s):  
Mudumba Parthasarathy ◽  
Tadafumi Matsuno ◽  
Wako Aoki
Keyword(s):  
Effective Temperature ◽  
High Velocity ◽  
Circumstellar Dust ◽  
Evolutionary Status ◽  
Agb Stars ◽  
Horizontal Branch ◽  
Dust Shells ◽  
The Past ◽  
Gaia Dr2 ◽  
Branch Star

Abstract From Gaia DR2 data of eight high-velocity hot post-AGB candidates, LS 3593, LSE 148, LS 5107, HD 172324, HD 214539, LS IV −12 111, LS III +52 24, and LS 3099, we found that six of them have accurate parallaxes which made it possible to derive their distances, absolute visual magnitudes (MV) and luminosity (log L/L⊙). All the stars except LS 5107 have an accurate effective temperature (Teff) in the literature. Some of these stars are metal poor, and some of them do not have circumstellar dust shells. In the past, the distances of some stars were estimated to be 6 kpc, which we find to be incorrect. The accurate Gaia DR2 parallaxes show that they are relatively nearby, post-AGB stars. When compared with post-AGB evolutionary tracks we find their initial masses to be in the range 1 M⊙ to 2 M⊙. We find the luminosity of LSE 148 to be significantly lower than that of post-AGB stars, suggesting that this is a post-horizontal-branch star or post-early-AGB star. LS 3593 and LS 5107 are new high-velocity hot post-AGB stars from Gaia DR2.

scholarly journals On a mechanism that structures galaxies

1979 ◽  
Vol 84 ◽  
pp. 157-158
Author(s):  
D. Lynden-Bell
Keyword(s):  
Angular Momentum ◽  
Differential Rotation ◽  
Spiral Structure ◽  
Rotation Curves ◽  
Eccentric Orbits ◽  
The Galaxy ◽  
Central Regions ◽  
Pattern Speed ◽  
Stellar Orbit ◽  
Lindblad Resonance

By considering the interaction of a single stellar orbit with a weak cos 2Φ potential it is shown that in the central regions of galaxies with slowly rising rotation curves, the elongations of the orbits will align along any potential valley and oscillate about it. This effect is more pronounced for elongated orbits. In such regions any pair of orbits will naturally align under their mutual gravity and so a bar will form. The gravity of this bar will drive a spiral structure in the outer parts of the galaxy where differential rotation is too strong to allow the orbits to be caught by the bar. The spiral structure carries a torque which slowly drains angular momentum from the bar, gradually making its outline more eccentric and slowing its pattern speed. In the outer parts of the bar only the more eccentric orbits align with the potential valley; the rounder ones form a ring or lens about the bar. As the pattern speed slows down, the corotation resonance and outer Lindblad resonance, which receive the angular momentun, move outwards. The evolution of the system is eventually slowed down by the weakness of these outer resonances where the material is rather sparse.

scholarly journals 11.15. The effect of a central massive black hole on the gas fueling

1998 ◽  
Vol 184 ◽  
pp. 485-486
Author(s):  
H. Fukuda ◽  
A. Habe ◽  
K. Wada
Keyword(s):  
Black Hole ◽  
Numerical Simulations ◽  
Gravitational Potential ◽  
Galactic Center ◽  
The Self ◽  
Massive Black Hole ◽  
Nuclear Regions ◽  
Self Gravity ◽  
Lindblad Resonance ◽  
Inner Lindblad Resonance

Nuclear activities in galaxies, such as nuclear starbursts or AGNs, are supposed to be induced by gas fueling into nuclear regions of galaxies. Non-axisymmetric gravitational potential caused by a stellar bar is a convincing mechanism for triggering gas fueling (Phinney 1994). However, numerical simulations have shown that the bar can not force the gas to accrete toward the galactic center beyond the inner Lindblad resonance (ILR). As a mechanism to overcome the ILR barrier, the double barred structure (Friedli & Martinet 1993), or the self-gravity of gas (Wada & Habe 1992, 1995; Elmegreen 1994) are proposed.

scholarly journals 6.5. Hydrodynamical simulations as probes for the structure of the galactic center

1998 ◽  
Vol 184 ◽  
pp. 271-272
Author(s):  
K. Wada ◽  
T. Minezaki ◽  
K. Sakamoto ◽  
H. Fukuda
Keyword(s):  
Numerical Modeling ◽  
Numerical Simulations ◽  
Mass Distribution ◽  
Galactic Center ◽  
Dynamical Behavior ◽  
Interstellar Gas ◽  
Interstellar Gas In Galaxies ◽  
Dynamical Parameters ◽  
Pattern Speed ◽  
Hydrodynamical Simulations

Numerical modeling of the interstellar gas in galaxies is an effective approach to infer galactic gravitational structure. This is because the dynamical behavior of gas is very sensitive to the background gravitational potential. Since the dynamical resonances depend closely on the mass distribution and the pattern speed of the non-axisymmetric component, it is possible to determine these dynamical parameters by comparison of numerical simulations and gas observations.

scholarly journals Presence and Absence of Outer Rings in Barred Galaxies

2000 ◽  
Vol 174 ◽  
pp. 330-333
Author(s):  
P. Rautiainen ◽  
H. Salo
Keyword(s):  
Major Axis ◽  
Spiral Arms ◽  
Barred Galaxies ◽  
Near Ir ◽  
Closed Orbits ◽  
Detailed Classification ◽  
Different Types ◽  
Particle Simulations ◽  
Barred Spiral Galaxies ◽  
Lindblad Resonance

Many barred spiral galaxies have an outer ring (R) or a pseudoring (R') in their outer disk. R. Buta (e.g. Buta 1995) has developed a detailed classification for different types of outer rings. Two main families of outer rings are recognized. In R1 family the major axis of the ring is perpendicular to the bar and the spiral arms wind 180° before meeting the other arm. In R2 family the major axis of the ring is parallel to the bar and the spiral arms wind 270°.The outer rings are usually considered to be related to the outer Lindblad resonance (OLR) of a rotating stellar bar. The ring shapes correspond to families of closed orbits in the vicinity of this resonance (e.g. Contopoulos & Grosbøl 1989), and also the sizes of the outer rings relative to the bar component fit to this suggestion. Furthermore, test-particle simulations which use analytical bar potentials can produce different types of outer rings near the OLR (Schwarz 1981; Byrd et al. 1994). We have obtained similar results when we modelled ringed galaxy IC 4214 by constructing the gravitational potential from near-IR observations (Buta et al. 1999; Salo et al. 1999).

scholarly journals Mini-spiral as source of material for Sgr A* in bright state

2012 ◽  
Vol 8 (S290) ◽  
pp. 199-200 ◽  
Author(s):  
Bozena Czerny ◽  
Vladimír Karas ◽  
Devaky Kunneriath ◽  
Tapas K. Das
Keyword(s):  
Black Hole ◽  
Angular Momentum ◽  
Molecular Clouds ◽  
Galactic Center ◽  
Supermassive Black Hole ◽  
The Past ◽  
Accretion Process ◽  
Ring Mode ◽  
Sgr A ◽  
Bright Phase

AbstractThe question of the origin of the gas supplying the accretion process is pertinent especially in the context of enhanced activity of Galactic Center during the past few hundred years, seen now as echo from the surrounding molecular clouds, and the currently observed new cloud approaching Sgr A*. We discuss the so-called Galactic Center mini-spiral as a possible source of material feeding the supermassive black hole on a 0.1 parsec scale. The collisions between individual clumps reduce their angular momentum. and set some of the clumps on a plunging trajectory.We conclude that the amount of material contained in the mini-spiral is sufficient to sustain the luminosity of Sgr A* at the required level. The accretion episodes of relatively dense gas from the mini-spiral passing through a transient ring mode at ~ 104 Rg provide a viable scenario for the bright phase of Galactic Center.

scholarly journals What can Fermi LAT observation of the Galactic Centre tell us about its active past?

2016 ◽  
Vol 12 (S324) ◽  
pp. 115-118
Author(s):  
Gabrijela Zaharijas ◽  
Jovana Petrović ◽  
Pasquale Serpico
Keyword(s):  
Black Hole ◽  
Gamma Ray ◽  
Galactic Center ◽  
High Energy ◽  
Galactic Centre ◽  
Massive Black Hole ◽  
The Past ◽  
High Energy Electrons ◽  
Inverse Compton ◽  
Past Activity

AbstractThe Fermi-LAT gamma-ray data in the inner Galaxy region show several prominent features possibly related to the past activity of the Milky Way’s super massive black hole. At a large, 50 deg scale, the Fermi LAT revealed symmetric hour glass structures with hard energy spectra extending up to 100 GeV (and dubbed ‘the Fermi bubbles’). More recently and closer to the Galactic centre, at the 10 deg scale, several groups have claimed evidence for excess gamma-ray emission that appears symmetric around the Galactic center and has an energy spectrum peaking at few GeVs. We explore here the possibility that this emission originates in inverse Compton emission from high-energy electrons produced in a short duration, burst-like event injecting 1052 − 1053 erg, roughly 106 yrs ago. Several lines of evidence suggest that a series of ‘burst like’ events happened in the vicinity of our black hole in the past and gamma-ray observations may offer a new view of that scenario.

scholarly journals Searching for close approaches of the stars pairs in the galactic disk with Gaia EDR3 data

2021 ◽  
Author(s):  
M. D. Sizova ◽  
◽  
S. V. Vereshchagin ◽  
Keyword(s):  
Galactic Center ◽  
Orbital Motion ◽  
Galactic Disk ◽  
Small Bodies ◽  
The Past ◽  
The Future

Using calculations of the orbital motion around the Galactic center we searched for approaching stars on 5 Myr in the past and the future. We provide stars that approached up to 1 pc to each other. Considered process is important for researching the interstellar small bodies origin.

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