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Register a new userEvidence of a large scale positive rotation-metallicity correlation in the Galactic thick disk
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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 disk 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 disk 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 disk and on cosmological models of Galaxy formation.
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The thick disk rotation--metallicity correlation, partial V_phi/partial[Fe/H] =40div 50 km s^{-1}dex^{-1} represents an important signature of the formation processes of the galactic disk. We use nondissipative numerical simulations to follow the evolution of a Milky Way (MW)-like disk to verify if secular dynamical processes can account for this correlation in the old thick disk stellar population. We followed the evolution of an ancient disk population represented by 10 million particles whose chemical abundances were assigned by assuming a cosmologically plausible radial metallicity gradient with lower metallicity in the inner regions, as expected for the 10-Gyr-old MW. Essentially, inner disk stars move towards the outer regions and populate layers located at higher |z|. A rotation--metallicity correlation appears, which well resembles the behaviour observed in our Galaxy at a galactocentric distance between 8 kpc and 10 kpc. In particular,we measure a correlation of partial V_phi/partial[Fe/H]simeq 60 km s^{-1}dex^{-1} for particles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6 Gyr. Our pure N-body models can account for the V_phi vs. [Fe/H] correlation observed in the thick disk of our Galaxy, suggesting that processes internal to the disk such as heating and radial migration play a role in the formation of this old stellar component. In this scenario, the positive rotation-metallicity correlation of the old thick disk population would represent the relic signature of an ancient inverse chemical (radial) gradient in the inner Galaxy, which resulted from accretion of primordial gas.
We present the measurements of Faraday rotation for 477 pulsars observed by the Parkes 64-m radio telescope and the Green Bank 100-m radio telescope. Using these results along with previous measurements for pulsars and extra-galactic sources, we analyse the structure of the large-scale magnetic field in the Galactic disk. Comparison of rotation measures of pulsars in the disk at different distances as well as with rotation measures of background radio sources beyond the disk reveals large-scale reversals of the field directions between spiral arms and interarm regions. We develop a model for the disk magnetic field, which can reproduce not only these reversals but also the distribution of observed rotation measures of background sources.
Thick disks appear to be common in external large spiral galaxies and our own Milky Way also hosts one. The existence of a thick disk is possibly directly linked to the formation history of the host galaxy and if its properties is known it can constrain models of galaxy formation and help us to better understand galaxy formation and evolution. This brief review attempts to highlight some of the characteristics of the Galactic thick disk and how it relates to other stellar populations such as the thin disk and the Galactic bulge. Focus has been put on results from high-resolution spectroscopic data obtained during the last 10 to 15 years.
Recent determinations of the radial distributions of mono-metallicity populations (MMPs, i.e., stars in narrow bins in [Fe/H] within wider [$alpha$/Fe] ranges) by the SDSS-III/APOGEE DR12 survey cast doubts on the classical thin - thick disk dichotomy. The analysis of these observations lead to the non-$[alpha$/Fe] enhanced populations splitting into MMPs with different surface densities according to their [Fe/H]. By contrast, $[alpha$/Fe] enhanced (i.e., old) populations show an homogeneous behaviour. We analyze these results in the wider context of disk formation within non-isolated halos embedded in the Cosmic Web, resulting in a two-phase mass assembly. By performing hydrodynamical simulations in the context of the $rm Lambda CDM$ model, we have found that the two phases of halo mass assembly (an early, fast phase, followed by a slow one, with low mass assembly rates) are very relevant to determine the radial structure of MMP distributions, while radial mixing has only a secondary role, depending on the coeval dynamical and/or destabilizing events. Indeed, while the frequent dynamical violent events occuring at high redshift remove metallicity gradients, and imply efficient stellar mixing, the relatively quiescent dynamics after the transition keeps [Fe/H] gaseous gradients and prevents newly formed stars to suffer from strong radial mixing. By linking the two-component disk concept with the two-phase halo mass assembly scenario, our results set halo virialization (the event marking the transition from the fast to the slow phases) as the separating event marking periods characterized by different physical conditions under which thick and thin disk stars were born.
The orbits of the least chemically enriched stars open a window on the formation of our Galaxy when it was still in its infancy. The common picture is that these low-metallicity stars are distributed as an isotropic, pressure-supported component since these stars were either accreted from the early building blocks of the assembling Milky Way, or were later brought by the accretion of faint dwarf galaxies. Combining the metallicities and radial velocities from the Pristine and LAMOST surveys and Gaia DR2 parallaxes and proper motions for an unprecedented large and unbiased sample of very metal-poor stars at $[Fe/H]leq-2.5$ we show that this picture is incomplete. This sample shows strong statistical evidence (at the $5.0sigma$ level) of asymmetry in their kinematics, favouring prograde motion. Moreover, we find that $31%$ of the stars that currently reside in the disk do not venture outside of the disk plane throughout their orbit. The discovery of this population implies that a significant fraction of stars with iron abundances $[Fe/H]leq-2.5$ formed within or concurrently with the Milky Way disk and that the history of the disk was quiet enough to allow them to retain their disk-like orbital properties.
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