The ESA Gaia astrometric mission and deep photometric surveys have revolutionized our knowledge of the Milky Way. There are many ongoing efforts to search these data for substructure to find evidence of individual accretion events that built up the Milky Way and its halo. One of these newly identified features, called Nyx, was announced as an accreted stellar stream traveling in the plane of the disk.

Paper PDF: 

Nix_on_Nyx.pdf

We explore the fundamental relations governing the radial and vertical velocity dispersions of stars in the Milky Way. We determine stellar age estimates from combined studies of complementary surveys including GALAH, LAMOST, APOGEE, and the NASA Kepler and K2 missions, and obtain parallax and proper motion from {\it Gaia} DR2. We find that stellar samples from these surveys, even though they target different tracer populations and employ a variety of age estimation techniques, follow the same set of fundamental relations.

Paper PDF: 

veldisp_mw.pdf

Gaia DR2 has revealed substructures in the phase space distribution of stars in the Milky Way. In particular, ridge like structures can be seen in the (R,V_phi) plane and asymmetric arches in (V_R, V_phi) plane. We show that the ridges are also clearly present when the (R,V_phi) plane is mapped by , ,, <|z|>,[Fe/H] and [alpha/Fe]. The last three maps suggest that stars along the ridges lie preferentially close to the Galactic midplane (|z|<0.2 kpc), and, have metallicity and alpha elemental abundance similar to that of the Sun.

Paper PDF: 

Linking_ridges__arches_and_vertical_waves_in_the_kinematics_of_the_Milky_Way.pdf

We use data from the second data releases of the ESA Gaia astrometric survey and the high-resolution GALAH spectroscopic survey to analyse the structure of our Galaxy's discs. With GALAH, we can separate the \alpha-rich and \alpha-poor discs (with respect to Fe), which are superposed in both position and velocity space, and examine their distributions in action space. We examine carefully the distribution of stars in the z-V_z phase plane in which a remarkable ``phase spiral'' was recently discovered.

Paper PDF: 

galah-survey-signatures.pdf

Context: If the Galaxy is axisymmetric and in dynamical equilibrium, we expect negligible fluctuations in the residual line-of-sight velocity field. However, non-axisymmetric structures like a bar, spiral arms and merger events can generate velocity fluctuations. Recent results using the APOGEE survey claim significant fluctuations in velocity for stars in the mid plane (|z|<0.25 kpc) out to 5 kpc and suggest that the dynamical influence of the Milky Way's bar extends out to the Solar neighborhood.

Paper PDF: 

Khanna_velfluc_MW.pdf

Using GALAH survey data of nearby stars, we look at how structure in the planar (u,v) velocity distribution depends on metallicity and on viewing direction within the Galaxy. In nearby stars, with distance d < 1 kpc, the Hercules stream is most strongly seen in higher metallicity stars [Fe/H] > 0.2. The Hercules stream peak v value depends on viewed galactic longitude, which we interpret as due to the gap between the stellar stream and more circular orbits being associated with a specific angular momentum value of about 1640 km/s kpc.

The overlap between the spectroscopic Galactic Archaeology with HERMES (GALAH) survey and Gaia provides a high-dimensional chemodynamical space of unprecedented size. We present a first analysis of a subset of this overlap, of 7066 dwarf, turn-off, and sub-giant stars. These stars have spectra from the GALAH survey and high parallax precision from the Gaia DR1 Tycho-Gaia Astrometric Solution. We investigate correlations between chemical compositions, ages, and kinematics for this sample.

Paper PDF: 

Buder_AandA_2019_624_A19_GALAH_TGAS.pdf

Almost all spiral galaxies have a second disk component, the thick disk, in addition to the thin disk which defines their disk structure. Thick disks are believed to be ancient structures that predates the formation of the thin disks, but how they fit in to the overall picture of galaxy formation remains unknown. Although our Galaxy has a thick disk, the properties of this ancient component are not yet well determined.

Paper PDF: 

thickdisk_paper_GALAH.pdf

The Galactic disk retains vast amount of information about how it came to be and how it evolved over cosmic time. However, we know very little about the secular processes associated with disk evolution. One major uncertainty is the extent to which stars migrate radially through the disk, thereby washing out signatures of their past (e.g., birth sites). Recent theoretical work finds that such "blurring" of the disk can be important if spiral arms are transient phenomena. Here we describe an experiment to determine the importance of diffusion from the Solar circle with cosmic time.

How did the Galactic disk form and can the sequence of events ever be unravelled from the vast stellar inventory? This will require that some of the residual inhomogeneities from prehistory escaped the dissipative process at an early stage. Fossil hunting to date has concentrated mostly on the stellar halo, but a key source of information will be the thick disk. This is believed to be a `snap frozen' relic which formed during or shortly after the last major epoch of dissipation, or it may have formed from infalling systems early in the life of the Galaxy.