The Milky Way bar/bulge in proper motions: a 3D view from VIRAC &amp; Gaia

Clarke, Jonathan P.; Wegg, Christopher; Gerhard, Ortwin; Smith, Leigh C.; Lucas, P. W.; Wylie, Shola M.

doi:10.48550/arxiv.1903.02003

Cited by 6 publications

(8 citation statements)

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“…We remind the reader that all of these fields reside in a relatively small area of the bulge, within ∼ 1.5 • of the minor axis and within ∼ 3 • of the plane. Our measured dispersion gradient also agrees with the recent study of Clarke et al (2019) who analyzed PM data from the Vista Variables in the Via Lactea (VVV) and the Gaia DR2 survey. It's important to emphasize the dependence of σ ℓ on b and σ b on ℓ and is apparent when comparing dispersed fields against the lowest longitude field in the set by far (Stanek at l = 0.25).…”

Section: Anisotropy Ratiosupporting

confidence: 91%

Comparing Observed Stellar Kinematics and Surface Densities in a Low-latitude Bulge Field to Galactic Population Synthesis Models

Terry

Barry

Bennett

et al. 2020

ApJ

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We present an analysis of Galactic bulge stars from Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) observations of the Stanek window (l,b=[0.25,-2.15]) from two epochs approximately two years apart. This dataset is adjacent to the provisional Wide-field Infrared Survey Telescope (WFIRST) microlensing field. Proper motions are measured for approximately 115,000 stars down to 28th mag in V band and 25th mag in I band, with accuracies of 0.5 mas yr −1 (20 km s −1 ) at I ≈ 21. A cut on the longitudinal proper motion µ l allows us to separate disk and bulge populations and produce bulge-only star counts that are corrected for photometric completeness and efficiency of the proper-motion cut. The kinematic dispersions and surface density in the field are compared to the nearby SWEEPS sight-line, finding a marginally larger than expected gradient in stellar density. The observed bulge star counts and kinematics are further compared to the Besançon, Galaxia, and GalMod Galactic population synthesis models. We find that most of the models underpredict low-mass bulge stars by ∼33% below the main-sequence turnoff, and upwards of ∼70% at redder J and H wavebands. While considering inaccuracies in the Galactic models, we give implications for the exoplanet yield from the WFIRST microlensing mission.

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Section: Anisotropy Ratiosupporting

confidence: 91%

Comparing Observed Stellar Kinematics and Surface Densities in a Low-latitude Bulge Field to Galactic Population Synthesis Models

Terry

Barry

Bennett

et al. 2020

ApJ

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“…Recently, using an update of the Tremaine & Weinberg (1984) method -similar to that of Debattista et al (2002) -on proper motion data from a combination of multiepoch data from the VVV survey and Gaia DR2, Sanders et al (2019) also found a pattern speed of Ω b = 41 ± 3 km s −1 kpc −1 . Simultaneously, Clarke et al (2019) also derived line-of-sight integrated and distance-resolved maps of mean proper motions and dispersions from the VVV Infrared Astrometric Catalogue combined with data from Gaia DR2, and found excellent agreement with a bar pattern speed of Ω b = 37.5 km s −1 kpc −1 . Studying in details all the dynamical effects of such a slowly rotating bar on the local velocity field is thus extremely timely.…”

Section: Introductionmentioning

confidence: 75%

Signatures of the resonances of a large Galactic bar in local velocity space

Monari

Famaey

Siebert

et al. 2019

A&A

113

148

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The second data release of the Gaia mission has revealed a very rich structure in local velocity space. In terms of in-plane motions, this rich structure is also seen as multiple ridges in the actions of the axisymmetric background potential of the Galaxy. These ridges are probably related to a combination of effects from ongoing phase-mixing and resonances from the spiral arms and the bar. We have recently developed a method to capture the behaviour of the stellar phase-space distribution function at a resonance, by reexpressing it in terms of a new set of canonical actions and angles variables valid in the resonant region. Here, by properly treating the distribution function at resonances, and by using a realistic model for a slowly rotating large Galactic bar with pattern speed Ω b = 39 km s −1 kpc −1 , we show that no less than six ridges in local action space can be related to resonances with the bar. Two of these at low angular momentum correspond to the corotation resonance, and can be associated to the Hercules moving group in local velocity space. Another one at high angular momentum corresponds to the outer Lindblad resonance, and can tentatively be associated to the velocity structure seen as an arch at high azimuthal velocities in Gaia data. The other ridges are associated to the 3:1, 4:1 and 6:1 resonances. The latter can be associated to the so-called 'horn' of the local velocity distribution. While it is clear that effects from spiral arms and incomplete phase-mixing related to external perturbations also play a role in shaping the complex kinematics revealed by Gaia data, the present work demonstrates that, contrary to common misconceptions, the bar alone can create multiple prominent ridges in velocity and action space.

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“…Recently, the proper motion is measured from the VVV survey data, the VIRAC catalogue (Smith et al 2018), with a median uncertainty of 0.67 mas yr −1 for stars with 11 < K s < 14 mag. The absolute proper motion is also measured using the Gaia reference frame (Clarke et al 2019;. Ultimately, near-infrared (NIR) astrometric space missions, such as Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE; Gouda 2012) 2 and GaiaNIR (Hobbs et al 2016(Hobbs et al , 2019, will provide the accurate measurement of the transverse velocity of the NSD stars, and are expected to identify the age of NSD and hence the formation time of the Galacitc bar decisively.…”

Section: Discussionmentioning

confidence: 99%

Age dating the Galactic bar with the nuclear stellar disc

Baba

Kawata

2020

Monthly Notices of the Royal Astronomical Society

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We perform an N-body/hydrodynamics simulation of an isolated Milky Way-like galaxy and demonstrate that the formation epoch of the galactic bar can be identified from the age distribution of the nuclear stellar disc (NSD). The bar formation triggers the gas inflow in the bar region, and the gas inflow reaches the central subkpc region followed by an intense star formation for ∼ 1 Gyr. The star formation in the central sub-kpc region forms the thinner, kinematically cooler and rotating NSD component. Consequently, the formation epoch of the galactic bar becomes the oldest limit of the NSD stellar populations, which tells us the formation epoch of the bar. We also discuss that a challenge to measure the age distribution of the NSD in the Milky Way is contamination from the other stellar components, such as a classical bulge component, whose contamination may not be negligible in the central region. We demonstrate that transverse velocities of tracer stars, which will be measured by the near-infrared space astrometry mission, JASMINE, are crucial to kinematically distinguish the NSD from the other stellar component.

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The Milky Way bar/bulge in proper motions: a 3D view from VIRAC & Gaia

Cited by 6 publications

References 0 publications

Comparing Observed Stellar Kinematics and Surface Densities in a Low-latitude Bulge Field to Galactic Population Synthesis Models

Comparing Observed Stellar Kinematics and Surface Densities in a Low-latitude Bulge Field to Galactic Population Synthesis Models

Signatures of the resonances of a large Galactic bar in local velocity space

Age dating the Galactic bar with the nuclear stellar disc

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