scholarly journals The tale of the tail – disentangling the high transverse velocity stars in Gaia DR2

2020 ◽  
Vol 492 (3) ◽  
pp. 3816-3828 ◽  
Author(s):  
João A S Amarante ◽  
Martin C Smith ◽  
Corrado Boeche
Keyword(s):  
Transverse Velocity ◽  
Kinematic Model ◽  
Double Sequence ◽  
Good Match ◽  
Galactic Disc ◽  
Thick Disc ◽  
Resonant Orbits ◽  
Halo Stars ◽  
Stellar Halo ◽  
Gaia Dr2

ABSTRACT Although the stellar halo accounts for just ∼1 per cent of the total stellar mass of the Milky Way, the kinematics of halo stars encode valuable information about the origins and evolution of our Galaxy. It has been shown that the high transverse velocity stars in Gaia DR2 reveal a double sequence in the Hertzsprung–Russell (HR) diagram, indicating a bifurcation in the local stellar halo within 1 kpc. We fit these stars by updating the popular Besançon/Galaxia model, incorporating the latest observational results for the stellar halo and an improved kinematic description for the thick disc from Schönrich & Binney (2012). We are able to obtain a good match to the Gaia data and provide new constraints on the properties of the Galactic disc and stellar halo. In particular, we show that the kinematically defined thick-disc contribution to this high velocity tail is $\approx 13{{\ \rm per\ cent}}$. We look in greater detail using chemistry from LAMOST DR5, identifying a population of retrograde stars with thick-disc chemistry. Our thick-disc kinematic model cannot account for this population and so we conclude there is likely to be a contribution from heated or accreted stars in the Solar Neighbourhood. We also investigate proposed dynamical substructures in this sample, concluding that they are probably due to resonant orbits rather than accreted populations. Finally, we provide new insights on the nature of the two sequences and their relation with past accretion events and the primordial Galactic disc.

The high transverse velocity stars in Gaia-LAMOST

2019 ◽  
Vol 14 (S353) ◽  
pp. 59-60
Author(s):  
João A. S. Amarante ◽  
Martin C. Smith ◽  
Corrado Boeche
Keyword(s):  
High Velocity ◽  
Milky Way ◽  
Transverse Velocity ◽  
Double Sequence ◽  
Good Match ◽  
Thick Disc ◽  
Halo Stars ◽  
Stellar Halo ◽  
Gaia Dr2 ◽  
Hr Diagram

AbstractAlthough the stellar halo accounts for just ∼1% of the total stellar mass of the Milky Way, the kinematics of halo stars can tell us a lot about the origins and evolution of our Galaxy. It has been shown that the high transverse velocity stars in Gaia DR2 reveal a double sequence in the Hertzsprung-Russell (HR) diagram, indicating a duality in the local halo within 1 kpc. We fit these stars by updating the popular Besançon/Galaxia model, incorporating the latest observational results for the stellar halo. We are able to obtain a good match to the Gaia data and provide new constraints on the properties of the disc and halo. In particular, we show that the thick disc contribution to this high velocity tail is small, but not negligible, and likely has an influence on the red sequence of the HR diagram.

scholarly journals The Milky Way has no in-situ halo other than the heated thick disc

2019 ◽  
Vol 632 ◽  
pp. A4 ◽  
Author(s):  
P. Di Matteo ◽  
M. Haywood ◽  
M. D. Lehnert ◽  
D. Katz ◽  
S. Khoperskov ◽  
...  
Keyword(s):  
Galactic Halo ◽  
The Sun ◽  
Galactic Disc ◽  
Elemental Abundances ◽  
Thick Disc ◽  
Halo Stars ◽  
Stellar Halo ◽  
Halo Population ◽  
Astrometric Data

Previous studies based on the analysis of Gaia DR2 data have revealed that accreted stars, possibly originating from a single progenitor satellite, are a significant component of the halo of our Galaxy, potentially constituting most of the halo stars at [Fe/H] <  −1 within a few kpc from the Sun and beyond. In this paper, we couple astrometric data from Gaia DR2 with elemental abundances from APOGEE DR14 to characterise the kinematics and chemistry of in-situ and accreted populations up to [Fe/H] ∼ −2. Accreted stars appear to significantly impact the galactic chemo–kinematic relations, not only at [Fe/H] <  −1, but also at metallicities typical of the thick and metal-poor thin discs. They constitute about 60% of all stars at [Fe/H] <  −1, the remaining 40% being made of (metal-weak) thick-disc stars. We find that the stellar kinematic fossil record shows the imprint left by this accretion event, which heated the old galactic disc. We are able to age-date this kinematic imprint, showing that the accretion occurred between nine and 11 Gyr ago, and that it led to the last significant heating of the galactic disc. An important fraction of stars with abundances typical of the (metal-rich) thick disc, and heated by this interaction, is now found in the galactic halo. Indeed, about half of the kinematically defined halo at few kpc from the Sun is composed of metal-rich thick-disc stars. Moreover, we suggest that this metal-rich thick-disc component dominates the stellar halo of the inner Galaxy. The new picture that emerges from this study is one where the standard, non-rotating in-situ halo population, the collapsed halo, seems to be more elusive than ever.

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 Co-formation of the thin and thick discs revealed by APOGEE-DR16 and Gaia-DR2

2020 ◽  
Author(s):  
Leandro Beraldo e Silva ◽  
Victor P Debattista ◽  
David Nidever ◽  
João A S Amarante ◽  
Bethany Garver
Keyword(s):  
Solar Neighbourhood ◽  
Full Sample ◽  
Thick Disc ◽  
Thin Disc ◽  
Vertical Scale ◽  
Stellar Halo ◽  
Sph Simulation ◽  
Gaia Dr2 ◽  
Cross Match ◽  
Age Estimates

Abstract Since thin disc stars are younger than thick disc stars on average, the thin disc is predicted by some models to start forming after the thick disc had formed, around 10 Gyr ago. Accordingly, no significant old thin disc population should exist. Using 6-D coordinates from Gaia-DR2 and age estimates from Sanders & Das (2018), we select ∼24000 old stars (${\tau &gt; 10{\, \rm{Gyr}}}$, with uncertainties $\lesssim 15\%$) within $2{\, \rm{kpc}}$ from the Sun (full sample). A cross-match with APOGEE-DR16 (∼1000 stars) reveals comparable fractions of old chemically defined thin/thick disc stars. We show that the full sample pericenter radius (rper) distribution has three peaks, one associated with the stellar halo and the other two having contributions from the thin/thick discs. Using a high-resolution N-body+SPH simulation, we demonstrate that one peak, at ${r_\rm{per}}\approx 7.1{\, \rm{kpc}}$, is produced by stars from both discs which were born in the inner Galaxy and migrated to the Solar Neighbourhood. In the Solar Neighbourhood, ∼1/2 (∼1/3) of the old thin (thick) disc stars are classified as migrators. Our results suggest that thin/thick discs are affected differently by radial migration inasmuch as they have different eccentricity distributions, regardless of vertical scale heights. We interpret the existence of a significant old thin disc population as evidence for an early co-formation of thin/thick discs, arguing that clump instabilities in the early disc offer a compelling explanation for the observed trends.

scholarly journals Signatures of resonance and phase mixing in the Galactic disc

2019 ◽  
Vol 490 (1) ◽  
pp. 1026-1043 ◽  
Author(s):  
Jason A S Hunt ◽  
Mathew W Bub ◽  
Jo Bovy ◽  
J Ted Mackereth ◽  
Wilma H Trick ◽  
...  
Keyword(s):  
Spiral Structure ◽  
Solar Neighbourhood ◽  
Good Match ◽  
Galactic Disc ◽  
Phase Mixing ◽  
Angle Space ◽  
Pattern Speeds ◽  
Gaia Dr2

ABSTRACT Gaia DR2 has provided an unprecedented wealth of information about the kinematics of stars in the Solar neighbourhood, and has highlighted the degree of features in the Galactic disc. We confront the data with a range of bar and spiral models in both action-angle space, and the RG–vϕ plane. We find that the phase mixing induced by transient spiral structure creates ridges and arches in the local kinematics which are consistent with the Gaia data. We are able to produce a qualitatively good match to the data when combined with a bar with a variety of pattern speeds, and show that it is non-trivial to decouple the effects of the bar and the spiral structure.

scholarly journals The escape speed curve of the Galaxy obtained from Gaia DR2 implies a heavy Milky Way

2018 ◽  
Vol 616 ◽  
pp. L9 ◽  
Author(s):  
G. Monari ◽  
B. Famaey ◽  
I. Carrillo ◽  
T. Piffl ◽  
M. Steinmetz ◽  
...  
Keyword(s):  
Mass Concentration ◽  
Milky Way ◽  
Galactic Centre ◽  
High Concentration ◽  
Circular Velocity ◽  
Halo Stars ◽  
Dark Matter Mass ◽  
The Galaxy ◽  
Escape Speed ◽  
Gaia Dr2

We measure the escape speed curve of the Milky Way based on the analysis of the velocity distribution of ~2850 counter-rotating halo stars from the Gaia Data Release 2. The distances were estimated through the StarHorse code, and only stars with distance errors smaller than 10% were used in the study. The escape speed curve is measured at Galactocentric radii ranging from ~5 kpc to ~10.5 kpc. The local Galactic escape at the Sun’s position is estimated to be ve(r⊙) = 580 ± 63 km s−1, and it rises towards the Galactic centre. Defined as the minimum speed required to reach three virial radii, our estimate of the escape speed as a function of radius implies for a Navarro–Frenk–White profile and local circular velocity of 240 km s−1 a dark matter mass M200 = 1.28−0.50+0.68 × 1012 M⊙ and a high concentration c200 = 11.09−1.79+2.94. Assuming the mass-concentration relation of ΛCDM, we obtain M200 = 1.55−0.51+0.64 × 1012 M⊙ and c200 = 7.93−0.27+0.33 for a local circular velocity of 228 km s−1.

scholarly journals The gravitational force field of the Galaxy measured from the kinematics of RR Lyrae in Gaia

2019 ◽  
Vol 485 (3) ◽  
pp. 3296-3316 ◽  
Author(s):  
Christopher Wegg ◽  
Ortwin Gerhard ◽  
Marie Bieth
Keyword(s):  
Dark Matter ◽  
Dark Matter Halo ◽  
Galactic Centre ◽  
Gravitational Acceleration ◽  
Momentum Exchange ◽  
Acceleration Field ◽  
Rr Lyrae ◽  
Halo Stars ◽  
The Galaxy ◽  
Stellar Halo

Abstract From a sample of 15651 RR Lyrae with accurate proper motions in Gaia DR2, we measure the azimuthally averaged kinematics of the inner stellar halo between 1.5  and 20  kpc from the Galactic centre. We find that their kinematics are strongly radially anisotropic, and their velocity ellipsoid nearly spherically aligned over this volume. Only in the inner regions ${\lesssim } 5\, {\rm kpc}\,$ does the anisotropy significantly fall (but still with β &gt; 0.25) and the velocity ellipsoid tilt towards cylindrical alignment. In the inner regions, our sample of halo stars rotates at up to $50\, {\rm km}\, {\rm s}^{-1}\,$, which may reflect the early history of the Milky Way, although there is also a significant angular momentum exchange with the Galactic bar at these radii. We subsequently apply the Jeans equations to these kinematic measurements in order to non-parametrically infer the azimuthally averaged gravitational acceleration field over this volume, and by removing the contribution from baryonic matter, measure the contribution from dark matter. We find that the gravitational potential of the dark matter is nearly spherical with average flattening $q_\Phi ={1.01 \pm 0.06\, }$ between 5 and 20 kpc, and by fitting parametric ellipsoidal density profiles to the acceleration field, we measure the flattening of the dark matter halo over these radii to be $q_\rho ={1.00 \pm 0.09\, }\!.$

scholarly journals One Large Blob and Many Streams Frosting the nearby Stellar Halo in Gaia DR2

2018 ◽  
Vol 860 (1) ◽  
pp. L11 ◽  
Author(s):  
Helmer Koppelman ◽  
Amina Helmi ◽  
Jovan Veljanoski
Keyword(s):  
Stellar Halo ◽  
Gaia Dr2

scholarly journals Structure of the outer Galactic disc with Gaia DR2

2020 ◽  
Vol 637 ◽  
pp. A96 ◽  
Author(s):  
Ž. Chrobáková ◽  
R. Nagy ◽  
M. López-Corredoira
Keyword(s):  
Maximum Amplitude ◽  
Galactic Disc ◽  
Galactocentric Distance ◽  
The North ◽  
Density Maps ◽  
Deconvolution Technique ◽  
Best Fit ◽  
Stellar Density ◽  
Gaia Dr2 ◽  
South Asymmetry

Context. The structure of outer disc of our Galaxy is still not well described, and many features need to be better understood. The second Gaia data release (DR2) provides data in unprecedented quality that can be analysed to shed some light on the outermost parts of the Milky Way. Aims. We calculate the stellar density using star counts obtained from Gaia DR2 up to a Galactocentric distance R = 20 kpc with a deconvolution technique for the parallax errors. Then we analyse the density in order to study the structure of the outer Galactic disc, mainly the warp. Methods. In order to carry out the deconvolution, we used the Lucy inversion technique for recovering the corrected star counts. We also used the Gaia luminosity function of stars with MG <  10 to extract the stellar density from the star counts. Results. The stellar density maps can be fitted by an exponential disc in the radial direction hr = 2.07 ± 0.07 kpc, with a weak dependence on the azimuth, extended up to 20 kpc without any cut-off. The flare and warp are clearly visible. The best fit of a symmetrical S-shaped warp gives zw ≈ z⊙ + (37 ± 4.2(stat.) − 0.91(syst.))pc ⋅ (R/R⊙)2.42 ± 0.76(stat.) + 0.129(syst.)sin(ϕ + 9.3° ±7.37° (stat.) + 4.48° (syst.)) for the whole population. When we analyse the northern and southern warps separately, we obtain an asymmetry of an ∼25% larger amplitude in the north. This result may be influenced by extinction because the GaiaG band is quite prone to extinction biases. However, we tested the accuracy of the extinction map we used, which shows that the extinction is determined very well in the outer disc. Nevertheless, we recall that we do not know the full extinction error, and neither do we know the systematic error of the map, which may influence the final result. The analysis was also carried out for very luminous stars alone (MG <  −2), which on average represents a younger population. We obtain similar scale-length values, while the maximum amplitude of the warp is 20 − 30% larger than with the whole population. The north-south asymmetry is maintained.

scholarly journals Know thy star, know thy planet: chemo-kinematically characterizing TESS targets

2019 ◽  
Vol 491 (3) ◽  
pp. 4365-4381 ◽  
Author(s):  
Andreia Carrillo ◽  
Keith Hawkins ◽  
Brendan P Bowler ◽  
William Cochran ◽  
Andrew Vanderburg
Keyword(s):  
Distribution Functions ◽  
Target List ◽  
Thick Disc ◽  
Thin Disc ◽  
Candidate Target ◽  
Stellar Properties ◽  
The Individual ◽  
Host Stars ◽  
Gaia Dr2 ◽  
Kinematic Properties

ABSTRACT The Transiting Exoplanet Survey Satellite (TESS) has already begun to discover what will ultimately be thousands of exoplanets around nearby cool bright stars. These potential host stars must be well understood to accurately characterize exoplanets at the individual and population levels. We present a catalogue of the chemo-kinematic properties of 2218 434 stars in the TESS Candidate Target List using survey data from Gaia DR2, APOGEE, GALAH, RAVE, LAMOST, and photometrically derived stellar properties from SkyMapper. We compute kinematic thin disc, thick disc, and halo membership probabilities for these stars and find that though the majority of TESS targets are in the thin disc, 4 per cent of them reside in the thick disc and &lt;1 per cent of them are in the halo. The TESS Objects of Interest in our sample also display similar contributions from the thin disc, thick disc, and halo with a majority of them being in the thin disc. We also explore metallicity and [α/Fe] distributions for each Galactic component and show that each cross-matched survey exhibits metallicity and [α/Fe] distribution functions that peak from higher to lower metallicity and lower to higher [α/Fe] from the thin disc to the halo. This catalogue will be useful to explore planet occurrence rates, among other things, with respect to kinematics, component membership, metallicity, or [α/Fe].

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