T. Antoja

Understanding the spiral structure of the Milky Way using the local kinematic groups

We study the spiral arm influence on the solar neighbourhood stellar kinematics. As the nature of the Milky Way (MW) spiral arms is not completely determined, we study two models: the Tight-Winding Approximation (TWA) model, which represents a local approximation, and a model with self-consistent material arms named sPiral arms potEntial foRmed by obLAte Spheroids (PERLAS). This is a mass distribution with more abrupt gravitational forces. We perform test particle simulations after tuning the two models to the observational range for the MW spiral arm properties. We find that some of the currently observed MW spiral arm properties are not in obvious agreement with the TWA model. We explore the effects of the arm properties and find that a significant region of the allowed parameter space favours the appearance of kinematic groups. The velocity distribution is mostly sensitive to the relative spiral arm phase and pattern speed. In all cases the arms induce strong kinematic imprints for pattern speeds around 17 km s-1 kpc-1 (close to the 4:1 inner resonance) but no substructure is induced close to corotation. The groups change significantly if one moves only similar to 0.6 kpc in galactocentric radius, but similar to 2 kpc in azimuth. The appearance time of each group is different, ranging from 0 to more than 1 Gyr. Recent spiral arms can produce strong kinematic structures. The stellar response to the two potential models is significantly different near the Sun, both in density and in kinematics. The PERLAS model triggers more substructure for a larger range of pattern speed values. The kinematic groups can be used to reduce the current uncertainty about the MW spiral structure and to test whether this follows the TWA. However, groups such as the observed ones in the solar vicinity can be reproduced by different parameter combinations. Data from velocity distributions at larger distances are needed for a definitive constraint.

STELLAR KINEMATIC CONSTRAINTS ON GALACTIC STRUCTURE MODELS REVISITED: BAR AND SPIRAL ARM RESONANCES

We study the phase space available to the local stellar distribution using a Galactic potential consistent with several recent observational constraints. We find that the induced phase space structure has several observable consequences. The spiral arm contribution to the kinematic structure in the solar neighborhood may be as important as the one produced by the Galactic bar. We suggest that some of the stellar kinematic groups in the solar neighborhood, such as the Hercules structure and the kinematic branches, can be created by the dynamical resonances of self-gravitating spiral arms and not exclusively by the Galactic bar. A structure coincident with the Arcturus kinematic group is developed when a hot stellar disk population is considered, which introduces a new perspective on the interpretation of its extragalactic origin. A bar-related resonant mechanism can modify this kinematic structure. We show that particles in the dark matter disk-like structure predicted by recent lambda cold dark matter galaxy formation experiments, with similar kinematics to the thick disk, are affected by the same resonances, developing phase space structures or dark kinematic groups that are independent of the Galaxy assembly history and substructure abundance. We discuss the possibility of using the stellar phase space groups as constraints to nonaxisymmetric models of the Milky Way structure.

Constraints on the Galactic bar from the Hercules stream as traced with RAVE across the Galaxy

Non-axisymmetries in the Galactic potential (spiral arms and bar) induce kinematic groups such as the Hercules stream. Assuming that Hercules is caused by the effects of the outer Lindblad resonance of the Galactic bar, we model analytically its properties as a function of position in the Galaxy and its dependence on the bars pattern speed and orientation. Using data from the RAVE survey we find that the azimuthal velocity of the Hercules structure decreases as a function of Galactocentric radius, in a manner consistent with our analytical model. This allows us to obtain new estimates of the parameters of the Milky Ways bar. The combined likelihood function of the bars pattern speed and angle has its maximum for a pattern speed of Ωb = (1.89 ± 0.08) × Ω0, where Ω0 is the local circular frequency. Assuming a solar radius of 8.05 kpc and a local circular velocity of 238 km s-1, this corresponds to Ωb = 56 ± 2 km s-1 kpc-1. On the other hand, the bars orientation φb cannot be constrained with the available data. In fact, the likelihood function shows that a tight correlation exists between the pattern speed and the orientation, implying that a better description of our best fit results is given by the linear relation Ωb/Ω0 = 1.91 + 0.0044(φb(deg) - 48), with standard deviation of 0.02. For example, for an angle of φb = 30 deg the pattern speed is 54.0 ± 0.5 km s-1 kpc-1. These results are not very sensitive to the other Galactic parameters such as the circular velocity curve or the peculiar motion of the Sun, and are robust to biases in distance. Appendices are available in electronic form at http://www.aanda.org

On galaxy spiral arms' nature as revealed by rotation frequencies

High-resolution N-body simulations using different codes and initial condition techniques reveal two different behaviours for the rotation frequency of transient spiral arms like structures. Whereas unbarred discs present spiral arms nearly corotating with disc particles, strong barred models (bulged or bulgeless) quickly develop a bar-spiral structure dominant in density, with a pattern speed almost constant in radius. As the bar strength decreases the arm departs from bar rigid rotation and behaves similar to the unbarred case. In strong barred models, we detect in the frequency space other subdominant and slower modes at large radii, in agreement with previous studies, however, we also detect them in the configuration space. We propose that the distinctive behaviour of the dominant spiral modes can be exploited in order to constraint the nature of Galactic spiral arms by the astrometric survey Gaia and by 2D spectroscopic surveys like Calar Alto Legacy Integral Field Area Survey (CALIFA) and Mapping Nearby Galaxies at APO (MANGA) in external galaxies.

Gaia Data Release 2: Catalogue validation

Context. The second Gaia data release (DR2) contains very precise astrometric and photometric properties for more than one billion sources, astrophysical parameters for dozens of millions, radial velocities for millions, variability information for half a million stars from selected variability classes, and orbits for thousands of solar system objects. Aims: Before the catalogue was published, these data have undergone dedicated validation processes. The goal of this paper is to describe the validation results in terms of completeness, accuracy, and precision of the various Gaia DR2 data. Methods: The validation processes include a systematic analysis of the catalogue content to detect anomalies, either individual errors or statistical properties, using statistical analysis and comparisons to external data or to models. Results: Although the astrometric, photometric, and spectroscopic data are of unprecedented quality and quantity, it is shown that the data cannot be used without dedicated attention to the limitations described here, in the catalogue documentation and in accompanying papers. We place special emphasis on the caveats for the statistical use of the data in scientific exploitation. In particular, we discuss the quality filters and the consideration of the properties, systematics, and uncertainties from astrometry to astrophysical parameters, together with the various selection functions.

Kinematic groups beyond the solar neighbourhood with RAVE

We analyse the kinematics of disc stars observed by the RAdial Velocity Experiment (RAVE) survey in and beyond the solar neighbourhood. We detect significant overdensities in the velocity distributions using a technique based on the wavelet transform. We find that the main local kinematic groups are large-scale features, surviving at least up to ˜1 kpc from the Sun in the direction of antirotation, and also at ? below the Galactic plane. We also find that for regions located at different radii than the Sun, the known groups appear shifted in the vR-vφ velocity plane. For example, the Hercules group has a larger azimuthal velocity for regions inside the solar circle and a lower value outside. We have also discovered a new group at ? in the solar neighbourhood and confirmed the significance of other previously found groups. Some of these trends detected for the first time are consistent with dynamical models of the effects of the bar and the spiral arms. More modelling is required to definitively characterize the non-axisymmetric components of our Galaxy using these groups.

Modelling the inner disc of the Milky Way with manifolds - I. A first step

We study the bar-driven dynamics in the inner part of the Milky Way by using invariant manifolds. This theory has been successfully applied to describe the morphology and kinematics of rings and spirals in external galaxies, and now, for the first time, we apply it to the Milky Way. In particular, we compute the orbits confined by the invariant manifolds of the unstable periodic orbits located at the ends of the bar. We start by discussing whether the COBE/Diffuse Infrared Background Experiment (DIRBE) bar and the Long bar compose a single bar or two independent bars and perform a number of comparisons which, taken together, argue strongly in favour of the former. More specifically, we favour the possibility that the so-called COBE/DIRBE bar is the boxy/peanut bulge of a bar whose outer thin parts are the so-called Long bar. This possibility is in good agreement both with observations of external galaxies, with orbital structure theory and with simulations. We then analyse in detail the morphology and kinematics given by five representative Galactic potentials. Two of these have a Ferrers bar, two have a quadrupole bar and the last one a composite bar. We first consider only the COBE/DIRBE bar and then extend it to include the effect of the Long bar. We find that the large-scale structure given by the manifolds describes an inner ring, whose size is similar to the near and far 3-kpc arm, and an outer ring, whose properties resemble those of the Galactic Molecular Ring. We also analyse the kinematics of these two structures, under the different Galactic potentials, and find they reproduce the relevant overdensities found in the galactic longitude-velocity CO diagram. Finally, we consider for what model parameters, the global morphology of the manifolds may reproduce the two outer spiral arms. We conclude that this would necessitate either more massive and more rapidly rotating bars, or including in the potential an extra component describing the spiral arms.

The Galactic bar and the large scale velocity gradients in the Galactic disk

We investigate whether the cylindrical (galactocentric) radial velocity gradient of ∼− 3k m s −1 kpc −1 , directed radially from the Galactic center and recently observed in the stars of the solar neighborhood with the RAVE survey, can be explained by the resonant effects of the bar near the solar neighborhood. We compared the results of test particle simulations of the Milky Way with a potential that includes a rotating bar with observations from the RAVE survey. To this end we applied the RAVE selection function to the simulations and convolved these with the characteristic RAVE errors. We explored different “solar neighborhoods” in the simulations, as well as different bar models. We find that the bar induces a negative radial velocity gradient at every height from the Galactic plane, outside the outer Lindblad resonance and for angles from the long axis of the bar compatible with the current estimates. The selection function and errors do not wash away the gradient, but often make it steeper, especially near the Galactic plane, because this is where the RAVE survey is less radially extended. No gradient in the vertical velocity is present in our simulations, from which we may conclude that this cannot be induced by the bar.

Gaia Data Release 2. Mapping the Milky Way disc kinematics

Context. The second Gaia data release (Gaia DR2) contains high-precision positions, parallaxes, and proper motions for 1.3 billion sources as well as line-of-sight velocities for 7.2 million stars brighter than GRVS = 12 mag. Both samples provide a full sky coverage. Aims. To illustrate the potential of Gaia DR2, we provide a first look at the kinematics of the Milky Way disc, within a radius of several kiloparsecs around the Sun. Methods. We benefit for the first time from a sample of 6.4 million F-G-K stars with full 6D phase-space coordinates, precise parallaxes (σω/ω/≤ 20%), and precise Galactic cylindrical velocities (median uncertainties of 0.9-1.4 km s-1 and 20% of the stars with uncertainties smaller than 1 km s-1 on all three components). From this sample, we extracted a sub-sample of 3.2 million giant stars to map the velocity field of the Galactic disc from ∼5 kpc to ∼13 kpc from the Galactic centre and up to 2 kpc above and below the plane. We also study the distribution of 0.3 million solar neighbourhood stars (r < 200 pc), with median velocity uncertainties of 0.4 km s-1, in velocity space and use the full sample to examine how the over-densities evolve in more distant regions. Results. Gaia DR2 allows us to draw 3D maps of the Galactocentric median velocities and velocity dispersions with unprecedented accuracy, precision, and spatial resolution. The maps show the complexity and richness of the velocity field of the galactic disc. We observe streaming motions in all the components of the velocities as well as patterns in the velocity dispersions. For example, we confirm the previously reported negative and positive galactocentric radial velocity gradients in the inner and outer disc, respectively. Here, we see them as part of a non-axisymmetric kinematic oscillation, and we map its azimuthal and vertical behaviour. We also witness a new global arrangement of stars in the velocity plane of the solar neighbourhood and in distant regions in which stars are organised in thin substructures with the shape of circular arches that are oriented approximately along the horizontal direction in the U - V plane. Moreover, in distant regions, we see variations in the velocity substructures more clearly than ever before, in particular, variations in the velocity of the Hercules stream. Conclusions. Gaia DR2 provides the largest existing full 6D phase-space coordinates catalogue. It also vastly increases the number of available distances and transverse velocities with respect to Gaia DR1. Gaia DR2 offers a great wealth of information on the Milky Way and reveals clear non-axisymmetric kinematic signatures within the Galactic disc, for instance. It is now up to the astronomical community to explore its full potential.

The evolution of the Sun's birth cluster and the search for the solar siblings with Gaia

We use self-consistent numerical simulations of the evolution and disruption of the Suns birth cluster in the Milky Way potential to investigate the present-day phase space distribution of the Suns siblings. The simulations include the gravitational N-body forces within the cluster and the effects of stellar evolution on the cluster population. In addition the gravitational forces due to the Milky Way potential are accounted for in a self-consistent manner. Our aim is to understand how the astrometric and radial velocity data from the Gaia mission can be used to pre-select solar sibling candidates. We vary the initial conditions of the Suns birth cluster, as well as the parameters of the Galactic potential. We show that the disruption time-scales of the cluster are insensitive to the details of the non-axisymmetric components of the Milky Way model and we make predictions, averaged over the different simulated possibilities, about the number of solar siblings that should appear in surveys such as Gaia or GALAH. We find a large variety of present-day phase space distributions of solar siblings, which depend on the cluster initial conditions and the Milky Way model parameters. We show that nevertheless robust predictions can be made about the location of the solar siblings in the space of parallaxes (