ABSTRACT
This study is based on high quality astrometric and spectroscopic data from the most recent releases by Gaia and APOGEE. We select $58\, 882$ thin and thick disc red giants in the ...Galactocentric (cylindrical) distance range 5 < R < 13 kpc and within |$z$| < 3 kpc, for which full chemo-kinematical information is available. Radial chemical gradients, $\partial \rm {M/H} / \partial \rm {R}$, and rotational velocity–metallicity correlations, $\partial V_\phi / \partial \rm {M/H}$, are re-derived firmly uncovering that the thick disc velocity–metallicity correlation maintains its positiveness over the 8 kpc range explored. This observational result is important as it sets experimental constraints on recent theoretical studies on the formation and evolution of the Milky Way disc and on cosmological models of Galaxy formation.
ABSTRACT
We analyse from an observational perspective the formation history and kinematics of a Milky Way-like galaxy from a high-resolution zoom-in cosmological simulation that we compare to those ...of our Galaxy as seen by Gaia DR2 to better understand the origin and evolution of the Galactic thin and thick discs. The cosmological simulation was carried out with the gadget-3 TreePM+SPH code using the MUlti-Phase Particle Integrator (muppi) model. We disentangle the complex overlapping of stellar generations that rises from the top-down and inside-out formation of the galactic disc. We investigate cosmological signatures in the phase-space of mono-age populations and highlight features stemming from past and recent dynamical perturbations. In the simulation, we identify a satellite with a stellar mass of $1.2 \times 10^9~\rm {M}_\odot$, i.e. stellar mass ratio Δ ∼ 5.5 per cent at the time, accreted at z ∼ 1.6, which resembles the major merger Gaia–Sausage–Enceladus that produced the Galactic thick disc, i.e. Δ ∼ 6 per cent. We found at z ∼ 0.5–0.4 two merging satellites with a stellar mass of $8.8 \times 10^8~\rm {M}_\odot$ and $5.1 \times 10^8~\rm {M}_\odot$ that are associated to a strong starburst in the star formation history, which appears fairly similar to that recently found in the solar neighbourhood. Our findings highlight that detailed studies of coeval stellar populations kinematics, which are made available by current and future Gaia data releases and in synergy with simulations, are fundamental to unravel the formation and evolution of the Milky Way discs.
Taking advantage of the recent
Gaia
Data Release 3 (DR3), we mapped chemical inhomogeneities in the Milky Way’s disc out to a distance of ∼4 kpc from the Sun, using different samples of bright giant ...stars. The samples were selected using effective temperatures and surface gravities from the GSP-Spec module, and they are expected to trace stellar populations of a different typical age. The cool (old) giants exhibit a relatively smooth radial metallicity gradient with an azimuthal dependence. Binning in Galactic azimuth
ϕ
, the slope gradually varies from dM/H/d
R
∼ −0.054 dex kpc
−1
at
ϕ
∼ −20° to ∼ − 0.036 dex kpc
−1
at
ϕ
∼ 20°. On the other hand, the relatively hotter (and younger) stars present remarkable inhomogeneities, which are apparent as three (possibly four) metal-rich elongated features in correspondence with the spiral arms’ locations in the Galactic disc. When projected onto the Galactic radius, those features manifest themselves as statistically significant bumps on top of the observed radial metallicity gradients with amplitudes up to ∼0.05–0.1 dex, making the assumption of a linear radial decrease not applicable for this sample. The strong correlation between the spiral structure of the Galaxy and the observed chemical pattern in the young sample indicates that the spiral arms might be at the origin of the detected chemical inhomogeneities. In this scenario, the spiral arms would leave a strong signature in the younger stars which progressively disappears when cooler (and older) giants are considered.
ABSTRACT We explore the vicinity of the Milky Way through the use of spectrophotometric data from the Sloan Digital Sky Survey and high-quality proper motions derived from multi-epoch positions ...extracted from the Guide Star Catalog II database. In order to identify and characterize streams as relics of the Milky Way formation, we start with classifying, selecting, and studying 2417 subdwarfs with up to 3 kpc away from the Sun as tracers of the local halo system. Then, through phase-space analysis, we find statistical evidence of five discrete kinematic overdensities among 67 of the fastest-moving stars and compare them to high-resolution N-body simulations of the interaction between a Milky Way-like galaxy and orbiting dwarf galaxies with four representative cases of merging histories. The observed overdensities can be interpreted as fossil substructures consisting of streamers torn from their progenitors; such progenitors appear to be satellites on prograde and retrograde orbits on different inclinations. In particular, of the five detected overdensities, two appear to be associated, yielding 21 additional main-sequence members, with the stream of Helmi et al. that our analysis confirms is on a high-inclination prograde orbit. The three newly identified kinematic groups could be associated with the retrograde streams detected by Dinescu and Kepley et al.; whatever their origin, the progenitor(s) would be on retrograde orbit(s) and inclination(s) within the range . Finally, we use our simulations to investigate the impact of observational errors and compare the current picture to the promising prospect of highly improved data expected from the Gaia mission.
With the launch of the Gaia mission, general relativity (GR) is now at the very core of astrometry. Given the high level of accuracy of the measurements, the development of a suitable relativistic ...model for carrying out the correct data processing and analysis has become a critical necessity; its primary goal is to have a consistent set of stellar astrometric parameters by which to map a relativistic kinematic of a large portion of the Milky Way and, therefore, taking the first step of the cosmic distance ladder to higher accuracy. To trace light trajectories back to the emitting stars requires an appropriate treatment of local gravity and a relativistic definition of the observable, according to the measurement protocol of GR, so that astrometry cannot be set apart from fundamental physics. Consequently, the final Gaia outputs, following completion of its operational life, will have important new implications and an overwhelming potential for astrophysical phenomena requiring the highest precision. In this regard, the present work establishes the background GR procedure to treat such relativistic measurements from within the weak gravitational field of the Solar System. In particular, we make the method explicit in the framework of the RAMOD relativistic models, consistent with the IAU (standard) resolutions and, therefore, suitable for validating the GREM approach baselined for Gaia.
Modern astrometry is based on angular measurements at the microarcsecond level. At this accuracy, a fully general relativistic treatment of the data reduction is required. This paper concludes a ...series of articles dedicated to the problem of relativistic light propagation, presenting the final, microarcsecond version of a relativistic astrometric model that enables us to trace back a light path to its source through the nonstationary gravitational field of the moving bodies in the solar system. The previous model is used as test bed for numerical comparisons with the present one. We also test different versions of the computer code implementing the model at different levels of complexity to start exploring the best tradeoff between numerical efficiency and the microarcsecond accuracy that needs to be reached.
We develop here a new approach for the relativistic modeling of the photons moving into a quasi-Minkowskian spacetime, where the metric is generated by an arbitrary N-body distribution within an ...isolated solar system. Our model is built on the prescriptions of the theory of general relativity and leaves the choices of the metric, as well as that of the motion of the observer, arbitrary. Adopting a quasi-Minkowskian expression of the metric accurate to order (v/c) super(2), we conduct a thorough numerical test campaign to verify correctness and reliability of the model equations. The test results show that the model behaves according to predictions. Specifically, comparisons to true (simulated) data demonstrate that stellar distances are reconstructed up to the specified level of accuracy. Although the (v/c) super(2) approximation is not always sufficient for its application to future astrometric experiments, which require modeling to (v/c) super(3), this work serves also as a natural test ground for the higher order model, whose formulation is now close to completion and will be presented in a forthcoming paper.
Aims. We demonstrate the feasibility of determining parallaxes for nearby objects with the Wide Field Camera on the United Kingdom Infrared Telescope (UKIRT) using the UKIRT Infrared Deep Sky Survey ...as a first epoch. We determine physical parameters for ULAS J003402.77-005206.7, one of the coolest brown dwarfs currently known, using atmospheric and evolutionary models with the distance found here. Methods. Observations over the period 10/2005 to 07/2009 were pipeline processed at the Cambridge Astronomical Survey Unit and combined to produce a parallax and proper motion using standard procedures. Results. We determined $\pi = 79.6 \pm 3.8$ mas, $\mu_{\alpha} = -20.0 \pm 3.7$ mas/yr and $\mu_{\delta} = -363.8 \pm 4.3$ mas/yr for ULAS J003402.77-005206.7. Conclusions. We have made a direct parallax determination for one of the coolest objects outside of the solar system. The distance is consistent with a relatively young, 1-2 Gyr, low mass, 13-20 MJ, cool, 550-600 K, brown dwarf. We present a measurement of the radial velocity that is consistent with an age between 0.5 and 4.0 Gyr.
ABSTRACT
This work presents the results of a kinematic analysis of the Galaxy that uses a new model as applied to the newest available Gaia data. We carry out the Taylor decomposition of the velocity ...field up to second order for 18 million high luminosity stars (i.e. OBAF-type stars, giants, and subgiants) from the Gaia DR3 data. We determine the components of mean stellar velocities and their first and second partial derivatives (relative to cylindrical coordinates) for more than 28 thousand points in the plane of our Galaxy. We estimate Oort’s constants A, B, C, and K and other kinematics parameters and map them as a function of Galactocentric coordinates. The values found confirm the results of our previous works and are in excellent agreement with those obtained by other authors in the solar neighbourhood. In addition, the introduction of second order partial derivatives of the stellar velocity field allows us to determine the values of the vertical gradient of the Galaxy azimuthal, radial, and vertical velocities. Also, we determine the mean of the Galaxy rotation curve for Galactocentric distances from 4 to 18 kpc by averaging Galactic azimuths in the range −30° < θ < + 30° about the direction Galactic Centre – Sun – Galactic anticentre. Maps of the velocity components and of their partial derivatives with respect to coordinates within 10 kpc of the Sun reveal complex substructures, which provide clear evidence of non-axisymmetric features of the Galaxy. Finally, we show evidence of differences in the Northern and Southern hemispheres stellar velocity fields.
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