Our home Galaxy, the Milky Way, can be used as an enormous laboratory to study the laws of physics in extreme conditions. The EMMY project is focused on the exploration of the complexity of the physical processes driving the evolution of the Galactic disc. The EMMY project aims to study the co-existence of both internal (such as the influence of the bar or spiral arms) and external (such as accretions of satellite galaxies) physical mechanisms, based on the properties of the stars that can be observed today (e.g. their spatial distributions, their motions, their chemical abundances).
The Gaia satellite is currently mapping the Milky Way by measuring positions and motions for more than one billion stars with strong accuracy and unprecedented detail. Taking advantage of the unprecedented wealth of chemo-dynamical data published in Gaia Data Release 3 (DR3), it is possible to obtain an unparalleled view of our Galaxy.
In the Milky Way disc, the stars in the inner parts are more rich in metals than those in the outer stars. This property is known as the "large-scale metallicity gradient" of the disc, and it is thought to be the result of how our Galaxy formed (inside-out formation).
Using Gaia DR3, we mapped the chemical composition of several groups of stars throughout the Galactic disc, in a radius of 13 000 light-years (4 Kpc) around the Sun. We found that the stars not only have a radial metallicity gradient, but also chemical azimuthal variations (i.e. for a given radius, the metallicity varies with Galactic azimuthal angle). We also found that the observed azimuthal variations change depending on the typical age of the consireded stars.
The young stars show interesting chemical features, apparent as chemical undulations, on top of the radial metallicity gradient. We also found an important correlation between the observed chemical maps and the position of the spiral arms in the Galaxy. Specifically, young stars located in the spiral arms of the Galaxy are typically richer in metals than those out of them.The important statistical correlations between density (i.e. spiral arm segments) and metallicity (i.e. chemical undulations) in the young stars indicate that the spiral arms of the Milky Way might be at the origin of the detected chemical inhomogeneities.
On the other hand, the old stars exhibit a relatively smooth radial metallicity gradient, whose slope (inclination) gradually changes with azimuthal angle. Possible explanations for the observed azimuthal variations in the old populations do not only include the spiral arms, but also radial migration induced by the bar, and/or the interaction with a satellite Galaxy (e.g. the Sagittarius dwarf galaxy).
Using the three-dimensional kinematics of stars in the Galactic, we detected a large-scale wave on top of the Galactic warp, with a vertical height of 150 pc, a radial half-amplitude of about 1 kpc and a total length of at least 10 kpc. The stars in the wave exhibit both radial and vertical systematic motions, consistent with a vertical wave propagating towards the outer parts of the Galactic disk. The observed wave might be a signature of the interaction with a satellite galaxy.
