The thick disk is one of the structural components of about 2/3 of all disk galaxies, including the Milky Way. It was discovered first in external edge-on galaxies. Soon after, it was proposed as a distinct galactic structure in the Milky Way, different from the thin disk and the halo in the 1983 article by Gilmore & Reid. It is supposed to dominate the stellar number density between above the galactic plane and, in the solar neighborhood, is composed almost exclusively of older stars. Its stellar chemistry and stellar kinematics (composition and motion of it stars) are also said to set it apart from the thin disk. Compared to the thin disk, thick disk stars typically have significantly lower levels of metals—that is, the abundance of elements other than hydrogen and helium.
The thick disk is a source of early kinematic and chemical evidence for a galaxy's composition and thus is regarded as a very significant component for understanding galaxy formation.
With the availability of observations at larger distances away from the Sun, more recently it has become apparent that the Milky Way thick disk does not have the same chemical and age composition at all galactic radii. It was found instead that it is metal poor inside the solar radius, but becomes more metal rich outside it. Additionally, recent observations have revealed that the average stellar age of thick disk stars quickly decreases as one moves from the inner to the outer disk.
It was shown that there is a diversity of thick disc formation scenarios. In general, various scenarios for the formation of this structure have been proposed, including:
Thick disks come from the heating of the thin disk
It is a result of a merger event between the Milky Way and a massive dwarf galaxy
More energetic stars migrate outwards from the inner galaxy to form a thick disk at larger radii
The disk forms thick at high redshift with the thin disk forming later
Disk flaring combined with inside-out disk formation
Scattering by massive clumps: stars born in massive gas clumps tend to be scattered to a thick disc and to be enriched in alpha-elements, while those formed out of these clumps form a thin disc and are alpha-poor
Although the thick disk is mentioned as a bona fide galactic structure in numerous scientific studies and it's even thought to be a common component of disk galaxies in general, its nature is still under dispute.
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Le disque épais est une des structures composant la plupart des galaxies à disque, dont la Voie lactée. Son existence pour notre Galaxie, comme une structure distincte du disque mince et du halo stellaire galactique a été suggérée pour la première fois par Gilmore et Read en 1983. Il se situe entre 1-5 kpc au-dessus du plan galactique et est composé presque entièrement d'étoiles âgées. La composition chimique et la cinématique des étoiles le composant semblent également différents de ceux du disque mince.
alt= Carte stellaire de l'environnement proche du Soleil dans notre Voie Lactée, comprenant les 2000 étoiles les plus brillantes situées à moins de 500 parsecs, d'après les relevés du satellite Hipparcos. Le centre galactique serait situé 8000 parsecs vers la droite. Le Soleil est figuré en bleu au centre. Le disque galactique est mis en évidence par la densité d'étoiles brillantes au centre de l'image en vue de côté.
vignette|La galaxie elliptique NGC 3923 et son halo. Un halo galactique est une composante, grossièrement sphérique, d'une galaxie qui s'étend au-delà de la partie principale visible de la galaxie. Le halo galactique est constitué : d'un halo stellaire ; d'une couronne galactique (un gaz chaud, c'est-à-dire un plasma) ; d'un halo de matière noire. La distinction entre le halo et le corps principal de la galaxie est la plus nette dans les galaxies spirales, où la forme sphérique du halo contraste avec le disque galactique.
The aim of this course is to acquire the basic knowledge on specific dynamical phenomena related to the origin, equilibrium, and evolution of star
clusters, galaxies, and galaxy clusters.
Cosmology is the study of the structure and evolution of the universe as a whole. This course describes the principal themes of cosmology, as seen
from the point of view of observations.
Explore les relations de dispersion, les réponses des systèmes aux perturbations et la stabilité des systèmes en rotation, en se concentrant sur l'origine des structures spirales dans les galaxies.