Abstracts of the Monday Programme

New phase space structures in the Gaia data: a dynamically young disk

by Teresa Antoja, Amina Helmi, Merce Romero-Gomez, et al.

We will present the new phase space substructures sampled by the six million stars dataset with 6D phase-space information from the Gaia satellite; thanks to the large amount of data and their excellent precision. The new morphologies discovered are a snail-shaped structure in the combination of vertical position and velocity; and diagonal ridges in the azimuthal velocity combined with galactic radius. The vertical structures are signatures of ongoing phase-mixing and our hypothesis is that the Milky Way disk might have been perturbed in the past. This helps us to determine properties of the possible perturbation and parameters of the Milky Way potential as well.

By considering a realistic model for the potential of the Galaxy and the associated orbital vertical frequencies; we infer that a perturbation might have occurred between 300 and 900 million years ago; which is consistent with estimates of the previous pericenter of the Sagittarius dwarf galaxy. But we will also review alternative hypothesis presented so far such as the effects of the buckling of the bar or bending waves in the disk. The in-plane diagonal ridges might be a result of phase-mixing or/and resonant effects of the bar and spiral arms. In any case; the Gaia data prove that the disk of our Galaxy strongly changes with time and is very responsive to perturbations.

Modelling vertical waves of the Galactic disc in the Solar Neighbourhood

by Morgan Bennett, Jo Bovy

Much can be learned about the structure and dynamics of the Milky Way from observations of its vertical structure. In particular; vertical oscillations in the density and velocity caused by dynamical perturbations can tell us about not only the dynamical history of the disc; but also about fundamental properties of the Galaxy itself. With Gaia Data Release 2; we have an unprecedented number of stars with 5-dimensional parameters including geometric parallax; which allows us to investigate the local vertical structure with unprecedented accuracy.

I will discuss evidence from Gaia DR2 that the Galactic Disc is not in equilibrium as commonly assumed and the importance of taking this perturbation into account when analysing our observations. I will also discuss new methods for quantifying this departure from equilibrium to help us learn about the Galactic disc's dynamical history.

Invited talk: Bringing the Milky Way into sharp focus: Gaia parallaxes, photometry and spectroscopy

by Cristina Chiappini

We combine Gaia DR2 parallaxes and optical photometry with other photometric bands from PANSTARRS, 2MASS and AllWISE and derive Bayesian distances and extinctions for stars brighter than G=18. We achieve a median precisions of 6% in distance and 0.07 mag in V-band extinction for G<14. At fainter magnitudes our precision is 20% and 0.15 mag at G=16 and 30% and 0.25 mag at G=17. We will soon make our distances and extinctions publicly available through the Gaia mirror at AIP.

As initial first applications, here we show extinction maps as a function of distance, extinction-corrected colour-magnitude diagrams, and extensive kinematic maps of the Galaxy. In the second part of the talk we show that by combining Gaia and APOGEE data will give us an unprecedented view into the MW Bulge. Finally, we discuss how our results help us devising new selection criteria of targets for the 4MOST Milky way Disk And BuLgE Low-Resolution Survey (4MIDABLE-LR), which will be the largest spectroscopic follow-up of the Gaia mission. With this survey we aim to study the kinematic and chemical structures of in the Milky Way disc and bulge region with samples of unprecedented size out to larger distances and greater precisions than conceivable with Gaia alone or by any other ongoing or planned survey.

Thanks to Gaia, 4MMIDABLE-LR will have the unprecedented opportunity of target selection criteria almost entirely based on parallax and magnitude range, hence increasing the efficiency in sampling large Milky Way volumes with well-defined and characterised selection functions.

The evolutionary history of the Milky Way disk(s) and halo from Gaia DR2 color-magnitude diagram fitting.

by Carme Gallart, Edouard Bernard, Tomas Ruiz-Lara, et al.

Gaia has provided distances and photometry; and thus color- diagrams in the absolute plane; for stars over an unprecedented vast volume in the Milky Way; encompassing not only the immediate solar neighborhood but significant fractions of the thin and thick disk and halo. This has allowed; for the first time; to derive star formation histories (SFHs) from direct modeling of these color-magnitude diagrams; using the same techniques that have been proven successful for external galaxies in the Local Group.

We will discuss SFHs derived using Gaia DR2 data for the disk and halo of the Milky Way; and compare the conclusions on the evolutionary history of these Milky Way structures with the picture offered by other techniques that have traditionally been used to study the Milky Way stellar populations.

The tilt of the velocity ellipsoid in the Milky Way with Gaia DR2 - from spherical to cylindrical alignment

by Jorrit Hagen, Amina Helmi, Lorenzo Posti, et al.

The velocity distribution of stars close to the Sun is a sensitive probe of the gravitational potential of the Galaxy; and hence of its dark matter distribution. In particular; the shape of the dark halo (e.g. spherical; oblate; prolate) determines velocity correlations; and different halo geometries imply measurable differences. In this talk I will present the correlations (or better: tilt angles) in the velocity distribution of stars for a large fraction of the Milky Way disk; ranging from the inner (R=3 kpc) to the outer (R=13 kpc) Galaxy and up to 4 kpc away from the Galactic plane; using a high quality sample from the Gaia DR2 catalog.

We explore both the effects of random and systematic (particularly on the parallax) errors on our analysis and results. We find that the velocity ellipsoid; which measures correlations between vR and vZ; clearly changes shape moving from spherical alignment in the inner Galaxy (R<6 kpc) towards near cylindrical alignment at R~12 kpc.

We present a simple analytic function that describes the tilt angles observed. Interestingly we find that different stellar populations show similar trends in the tilt angle and that the tilt angles do not strongly vary with Galactic azimuth. The trends of the velocity ellipsoid with radius and height are then used to set additional constraints on mass models of the Milky Way.

Structure in the Galactic Disk From Clustered Star Formation

by Harshil Kamdar, Charlie Conroy

Combined with ground-based spectroscopic surveys; Gaia DR2 presents an opportunity to study the chemodynamical evolution of the Milky Way (MW) in unprecedented detail. We present a new set of simulations that evolve every star between 0.5 and 1.5 Msun born in the last 5 Gyr in a realistic MW potential that includes a bar; spiral arms; and GMCs. The simulation is the first of its kind to model the full population of star clusters comprising the Galactic disk. This allows us to resolve the small-scale structure in phase space and chemical space due to clustered star formation.

As an example of the power of combining kinematic and chemical information; we examine co-moving pairs in both the simulations and Gaia DR2. We find evidence for a high co-natal fraction for pairs with low relative velocities. These models will shed new light on long-standing questions related to the nature of clustered star formation.

The echo of the bar buckling: phase-space spirals in Gaia DR2

by Sergey Khoperskov

We present a high-resolution numerical study of the phase-space diversity in an isolated Milky Way-type galaxy. Using an N-body simulation (N~10^9) we explore the formation; evolution and spatial variation of the phase-space spirals similar to those recently discovered with Gaia DR2. For the first time in the literature; we use a self-consistent N-body simulation of an isolated Milky Way-type galaxy to show that the phase-space spirals develop naturally from vertical oscillations driven by the buckling of the stellar bar.

We claim that the physical mechanism standing behind the observed incomplete phase-space mixing process can be internal. In our model; the bending oscillations propagate outwards and produce axisymmetric variations of the mean vertical coordinate and of the vertical velocity component of about 100-200 pc and 1-2 km/s; respectively. Once the bending waves appear; they are supported for a long time via disk self-gravity. Such vertical oscillations trigger the formation of various time-dependent phase-space spirals in the entire disk.

The underlying physical mechanism implies the link between in-plane and vertical motion that leads directly to phase-space structures whose amplitude and shape are in remarkable agreement with those of the phase-space spirals observed in the Milky Way disk. In our isolated galaxy simulation; phase-space spirals are still distinguishable; at the solar neighbourhood; 3 Gyr after the buckling phase. The long-lived character of the phase-space spirals generated by the bar buckling instability cast doubts on the timing argument used so far to get back at the time of the onset of the perturbation: phase-space spirals may have been caused by perturbations originated several Gyrs ago; and not as recent as suggested so far.

Gaia DR2 reveals a star formation burst in the disc 2-3 Gyr ago

by Roger Mor, Annie C. Robin, Francesca Figueras, et al.

We developed a new theoretical framework; the Besancon Galaxy Model Fast Approximate Simulations (BGM FASt); that combined with approximate Bayesian computation techniques allows us to address fundamental questions of the Galactic structure and evolution performing multi-parameter inference. Here we use Gaia DR2 magnitudes; colours and parallaxes for stars with G<12 to explore a 15-dimensional space that includes simultaneously the initial mass function (IMF) and a non-parametric star formation history (SFH) for the Galactic disc.

We find in Gaia DR2 data an imprint of a star formation burst in the Galactic thin disc region 2-3 Gyr ago and a present SFR of 1Msun/yr. Our results show a decreasing trend of the star formation rate (SFR) from 10 Gyr to about 6 Gyr ago that is consistent with the behavior of the cosmological star formation quenching. This decreasing trend is followed by a SFR enhancement detected during a period from 5 Gyr to 1 Gyr ago.

We estimate; from our best fit model; that about the 50% of the stellar mass of the thin disc was formed during this period. The timescale and the amount of stellar mass generated during the SFR enhancement event leads us to hypothesise that its origin is not intrinsic to the disc and that an external perturbation would be needed for its explanation.

The kinematic signature of the warp in the Gaia era

by Eloisa Poggio, Ronald Drimmel

Using Gaia DR2 astrometry; we map the kinematic signature of the Galactic stellar warp out to a distance of 7 kpc from the Sun. Combining Gaia DR2 and 2MASS photometry; we identify; via a probabilistic approach; 599 494 upper main sequence stars and 12 616 068 giants without the need for individual extinction estimates. The spatial distribution of the upper main sequence stars clearly shows segments of the nearest spiral arms. The large-scale kinematics of both the upper main sequence and giant populations show a clear signature of the warp of the Milky Way; apparent as a gradient of 5-6 km/s in the vertical velocities from 8 to 14 kpc in Galactic radius (Poggio et al.; 2018MNRAS.481L..21P).

The presence of the signal in both samples; which have different typical ages; suggests that the warp is a gravitationally induced phenomenon; placing an important constraint on the possible warp formation scenario. Furthermore; while the upper main sequence stars exhibit a perturbed kinematic pattern; possibly indicating additional forces acting on the gas; the giant sample presents a smooth warp signal; as expected from a dynamically relaxed stellar population. We also present and discuss some preliminary results on the estimate of warp parameters; statistically inferred from our datasets.

Synergies between Gaia and VLBI astrometry

by Luis Henry Quiroga Nunez, Huib Jan van Langevelde, Ylva Pihlstrom, et al.

The Bar and Spiral Structure Legacy (BeSSeL) survey and the Bulge Asymmetries and Dynamical Evolution (BAaDE) project target maser stellar emission from young massive stars and evolved stars; respectively. Follow-up radio-astrometric measurements are complementary to Gaia results since the inner plane of the Galaxy is obscured at optical wavelengths. We are constructing a cross-match sample between Gaia sources and BAaDE targets.

This resulting sample provides important clues on the intrinsic properties and population distribution of evolved stars in the Galactic plane; but especially at the Galactic Bulge. For the BeSSeL targets; which are heavily obscured; we are investigating whether they can be associated with clusters of massive young stars detectable at optical wavelengths; and how such can contribute to improving the accuracy of the fundamental Galactic parameters and the Galactic spiral structure distribution.

Kinematic substructure and its variations throughout the Milky Way disk: an insight into the dynamics of our Galaxy.

by Pau Ramos, Teresa Antoja, Francesca Figueras

Using the Gaia DR2 sample with full 6D information; we found that the substructures in velocity space; mostly caused by the dynamical processes in the Milky Way disk; exhibit a continuity with galactocentric radius and azimuth never seen before. The extent can in some cases connect regions up to 3kpc apart demonstrating that the Moving Groups (MGs) are far from being local. Moreover; we are able to distinguish types of MGs based on their changes in velocity with position and relate them to different dynamical mechanisms: those preserving the energy are likely caused by phase-mixing; whereas those keeping roughly constant their angular momentum are probably linked to resonances with the bar and/or the spiral arms.

Therefore; we are most likely witnessing a non-relax Milky Way with several dynamical processes overlapping; and a tentative way to distinguish among structures of different origin. We also measure the overall gradient of the azimuthal velocity with galactocentric distance of these groups; which diverges significantly from the rotation curve; and whose value is related to the dynamics. Furthermore; we have revealed a sharper picture of the already known MGs -leading to a new estimation of the pattern speed of the bar- and new ones are identified both locally and at other Galactic neighbourhoods different from the Solar Neighbourhood.

On the cutting edge of vertical motion: Bending waves and the Galactic warp

by Merce Romero-Gomez, Cecilia Mateu, Luis A. Aguilar, et al.

We use Gaia DR2 to revisit the study of the vertical structure and motion of the Galactic disc up to large heliocentric distances. We analyse the spatial distribution and kinematics of two different populations up to R=16 kpc; a young sample mainly formed by OB stars and an older one of Red Giant Branch stars. By using two different and complementary methods (GC3 and LonKin) we confirm the age dependence of the Galactic warp; both in positions and kinematics. The warp in the Red Giant Branch stars has a larger amplitude than in the OB sample.

We find the maximum of the proper motions in Galactic latitude systematically offset from the line-of-nodes estimated from the spatial data. Based on our warp model predictions; we interpret this misalignment as the kinematic signature of the warp's lopsidedness. We also show a prominent wave-like pattern of a bending mode dominant from the Solar ring to large heliocentric distances.

Both positions and kinematics also reveal a substructure (the "blob") present in the Southern part of the disc at a galactic longitude range ~ 100-120; which is not related to the Galactic warp nor to the bending mode. Gaia DR2 reveals a high degree of complexity that triggers the need for complex kinematic models; flexible enough to combine both wave-like patterns and an S-shaped lopsided warp.

Tracing the formation of the Milky Way through ultra and extremely metal-poor stars

by Federico Sestito, Nicolas F. Martin, Else Starkenburg, et al.

I will present the results of a Bayesian derivation of stellar parameters; distances; and orbits using the exquisite photometric and astrometric information from Gaia DR2 for all ultra metal-poor stars (UMPs; [Fe/H] < -4) available in the literature; as well as for the extremely metal-poor stars (EMPs; [Fe/H] < -3) observed by Pristine survey; a unique spectrophotometric survey based on a narrow-band Ca H&K filter that aims to detect and analyse EMPs stars. The Pristine survey allows me to focus on a large and homogeneous EMPs dataset; from which it is possible to better study the spatial distribution and orbits of these stars around the Milky Way; especially when cross-matched with Gaia DR2 data.

EMPs and UMPs are extremely rare objects located mainly in the Milky Way halo and because they are extremely metal poor; also relative to their neighbourhood; it is assumed that they formed in the relative pristine Galaxy short after the Big Bang. The inferred distances and orbital parameters are directly linked to the formation stages and building blocks of our Galaxy.

I will show that; even though most UMP and EMP stars have properties that link them to the inner halo or the accreted halo; a strikingly large fraction of those stars follow disc-like orbits. I will discuss how this discovery affects the different scenarios of the formation of the proto-MW.

Abstracts of the Monday Pitch Session

About proper motion of lensed QSOs in Gaia DR2

by Alex Bombrun, Maggie Lieu, Ivan Valtchanov, et al.

Even if not designed for cosmological studies; Gaia has been identified as a major source for the discovery of QSOs and lensed QSOs. Gaia not only provides new objects but also new information on these objects. Here we will focus on the proper motions of known lensed QSOs as derived in DR2. Whereas the proper motions of QSOs are statistically zero; the proper motions of lensed QSOs are significantly nonzero. However the information provided in DR2 is not sufficient to decide if these are true proper motions or artefact of the DPAC data processing.

Searching for multiple stellar populations in the GD1 stellar stream with Gaia

by Eduardo Balbinot, Ivan Cabrera-Ziri

Cold stellar streams are the remnants of globular clusters (GCs), objects which are known to have multiple stellar populations (MSP), i.e. a spread in light elements abundances and often Helium. Here, we look into the stellar stream GD1, a remnant of a GC, with an initial mass lower than any GCs known to have MSP. With the aid of Gaia, SDSS, and a variety of spectroscopic surveys we investigate the presence of MSP. We show that GD1 has the potential to be the lowest mass system at which this phenomenon is present, posing a new challenge to models of GC formation.

Shedding light on the Milky Way rotation curve with Gaia DR2

by Mariateresa Crosta, Marco Giammaria, Mario Lattanzi, et al.

We present the first attempt to apply the relativistic kinematics delivered by Gaia to trace the MW rotation curves within a general relativistic scenario. In particular; we have tested an axially symmetric; stationary and asymptotically flat Galaxy-scale metric. The mass inside a large portion of the Galaxy; far away from the central bulk; has been simplified as a pressure-less perfect fluid avoiding the bulge where resides the axis of symmetry. The study of the rotation curve profile of our Galaxy required the selection of the most suitable stellar tracers of the bulk circular velocity around the galactic center.

We selected DR2 sources according to the requirements for a proper 6-dimensional reconstruction of the phase-space location occupied by each individual star as derived by the same observer. The relativistic fit to the MW rotational data has been compared with well-studied classical models for the MW (MWC); which is comprised of a bulge; a stellar disk and a Navarro-Frenk-White (NFW) dark matter (DM) halo. To quantitatively asses this; a Monte Carlo Markov Chain (MCMC) analysis has been done and compared the results for the two models.

For the likelihood analysis the relativistic and MWC models appear almost identically consistent with the data. Nevertheless; as for the local baryonic matter density; estimated via the 00-term of then Einstein field equation; we have obtained a value that is perfectly in line with current estimates; which implies no need of extra-mass.

Discovery of a new nearby moving group

by Verena Fürnkranz, Joao Alves, Stefan Meingast

Today; we know that young groups of stars with similar age and kinematics can appear not only as clusters but also as extended groups; spanning several degrees across the sky. The wealth of data provided by the second data release of the Gaia mission delivers many new insights into such groups; particularly for their kinematics. Previous work suggested that the Pleiades cluster together with the AB Doradus moving group have a common origin.

In this talk; I will present further evidence of the common origin of the Pleiades cluster and the AB Dor moving group. Moreover; based on kinematic data available in Gaia DR2; I will also present proof for the existence of a previously unknown co-moving stellar group. In particular; I will speculate on the connection between this new group and already known clusters and associations; investigating a common origin scenario; similar to the Pleiades and the AB Doradus moving group.

Classical pulsators in the Magellanic Bridge as seen by OGLE and Gaia

by Anna Jacyszyn-Dobrzeniecka

The Magellanic Bridge; a direct evidence of LMC and SMC interactions; constitutes one of our closest tidally stripped stellar populations. I will present results of a three-dimensional study of classical and anomalous Cepheids as well as RR Lyrae stars in the area between the Magellanic Clouds. Classical Cepheids form a bridge-like connection that closely follows the neutral hydrogen and young population on-sky distributions. Anomalous Cepheids are more spread in every dimension; whereas RR Lyrae stars seem to resemble two overlapping extended structures rather than a bridge-like connection.

The analysis is mainly based on data from the OGLE survey that monitors the Bridge since 2010. I will also show the Gaia DR2 classical pulsators distribution that is consistent with our main results; especially in terms of older population. Moreover; I will also present a reanalysis of Belokurov et al. (2017) RR Lyrae candidates in the Bridge using Gaia DR1. Following their technique; we tried to reproduce their results of a bridge-like connection formed by old pulsators. I will show that we are not able to obtain similar results without many spurious sources located in the area of interest.

Stellar Stream map of the Milky Way Halo : Application of STREAMFINDER onto ESA/Gaia DR2

by Khyati Malhan

A panoramic map of the stellar streams of the Milky Way would be presented that was obtained based upon astrometric and photometric measurements from the Gaia DR2 catalogue. This was achieved using the STREAMFINDER algorithm to detect structures along plausible orbits that are consistent with the Gaia proper motion measurements. A rich network of criss-crossing streams in the halo was found. Some of these structures were previously-known; but several others were new discoveries. With these initial discoveries; we are starting to unravel the complex formation of the halo of our Galaxy.

The bulge and bar region seen by APOGEE and Gaia DR2

by Anna Queiroz, Cristina Chiappini, Friedrich Anders, et al.

We present a first analysis of the inner region of our Galaxy, using distances and extinctions derived by StarHorse (Queiroz et al 2018) Bayesian tool. Well constrained distances and extinctions were estimated using the latest releases of Gaia and APOGEE surveys, which provide astrometry, photometry, spectral atmospheric parameters and chemical abundances with high-resolution. Using a Bayesian approach is the only solution to get reliable distances in the Bulge region.

Indeed, with our method we can extend the volume for which reliable distances and extinctions can be derived with respect to those using only the Gaia parallaxes. With StarHorse we obtain typical distance uncertainties of 20% in this region. The method has been extensively tested with simulations and external samples, and it has been used by a wide range of publications studying the Milky Way.

We made a geometric selection of the bar and bulge region using StarHorse resulting distances from Gaia, 2MASS, WISE and PanSTARRS photometry for around 300 million stars without spectroscopy (Anders et al. in prep). We then focus our analysis on a sub-sample for which APOGEE information is also available, making an analysis of the kinematics and chemical properties of selected stars in the Bulge that trace the bar structure.

Rotating globular clusters in the Gaia era

by Alice Zocchi, Anna Lisa Varri

Globular clusters have long been considered to be spherically symmetric; isotropic; non-rotating stellar systems. However; this simple picture has been revolutionized by a series of recent discoveries about their kinematic and structural properties; which challenge our theoretical understanding of their formation and evolution. As a first step to address and describe the dynamical complexity of these systems; the family of LIMEPY models has been introduced (Gieles & Zocchi 2015). LIMEPY models are self-consistent; spherical; and non-rotating; with parameterised prescriptions for the energy truncation and for the amount of radially biased pressure anisotropy. However; for many GCs; rotation is important and cannot be neglected when describing their dynamics.

By including an angular momentum dependent term in the distribution function of LIMEPY models; we have developed a family of rotating models which will contribute to form a more realistic dynamical paradigm for this class of stellar systems (Zocchi & Varri; in prep.). I will present this new family of rotating models and their properties; and I will show an example of their application to describe the dynamics of globular clusters as probed by Gaia data.