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| Thick Disk | |
|---|---|
| Name | Thick Disk |
| Type | Galactic component |
| Location | Milky Way and external galaxies |
| Known for | Old stellar populations, enhanced scale height |
Thick Disk
The Thick Disk is a major stellar component of disk galaxies characterized by a larger vertical scale height, older stellar ages, and distinct kinematic and chemical signatures compared to thin disk and halo components. Identified in studies of the Milky Way and resolved in external systems such as Andromeda Galaxy and edge-on spirals in the Hubble Space Telescope surveys, it provides constraints on galaxy assembly, mergers, and secular evolution. Research on the Thick Disk connects observations from missions like Gaia and Sloan Digital Sky Survey with simulations from groups using codes such as Illustris and EAGLE.
The Thick Disk denotes the population of stars, gas remnants, and dust distributed with an exponential vertical profile exceeding that of the thin disk in galaxies like the Milky Way. First distinguished in star count studies toward the Galactic Pole and infrared surveys by teams at institutions including the Harvard-Smithsonian Center for Astrophysics and the Max Planck Institute for Astronomy, it was formalized following analyses from the European Southern Observatory and observations from the Hubble Deep Field. The component contrasts with the thin disk, the central bulge, and the extended stellar halo, and has been linked observationally to events such as past accretion episodes studied by groups at the Carnegie Institution for Science.
Structurally, the Thick Disk displays a larger scale height (typically ~0.7–1.5 kpc in the Milky Way) and a longer scale length in some systems than the thin disk, with surface density measurements from the Two Micron All Sky Survey and spectroscopy from the Apache Point Observatory contributing to parameter estimates. Its vertical density profile is generally well fit by a single or double exponential described in models by researchers at the Instituto de Astrofísica de Canarias. Photometric decompositions using data from the Spitzer Space Telescope and Sloan Digital Sky Survey separate thin and thick components in edge-on galaxies observed by teams affiliated with the NASA Goddard Space Flight Center.
Proposed formation scenarios include heating of a primordial thin disk by minor mergers associated with systems like the Sagittarius Dwarf Spheroidal Galaxy, direct accretion of stars from disrupted satellites studied by groups at the Max Planck Institute for Astronomy, in situ star formation under high turbulence in early disks modeled by the Institute for Computational Cosmology, and radial migration driven by transient spirals and bars investigated by teams at Princeton University. Cosmological simulations from projects such as Illustris and EAGLE demonstrate multiple pathways: merger-driven thickening linked to events similar to the Gaia-Enceladus interaction, dissipative collapse during early gas-rich phases examined by researchers at Columbia University, and secular processes emphasized in models from the University of California, Santa Cruz.
Detection methods include star counts toward the Galactic Pole using databases like the 2MASS catalog, kinematic separation with proper motions from Gaia and radial velocities from surveys such as RAVE, and chemical tagging via high-resolution spectroscopy from instruments at the Keck Observatory and European Southern Observatory (ESO). Photometric decomposition of edge-on galaxies observed with the Hubble Space Telescope and the Subaru Telescope isolates thick disk light profiles, while integral field units on the Very Large Telescope enable resolved stellar population studies similar to those performed by the Anglo-Australian Observatory.
Thick Disk stars are predominantly old (ages ≳8–12 Gyr) and show enhanced alpha-element abundances ([alpha/Fe]) relative to thin disk counterparts, based on analyses from the APOGEE survey and high-resolution studies at the European Southern Observatory (ESO). Elements such as magnesium and oxygen are elevated in thick disk spectra examined by teams at Carnegie Observatories, indicating rapid star formation histories linked to early enrichment from core-collapse supernovae studied in contexts like the Supernova Legacy Survey. Metallicity distributions span a broad range with mean [Fe/H] lower than the thin disk, as revealed by the GALAH survey and chemical evolution models developed at the Max Planck Institute for Astrophysics.
Kinematically, Thick Disk stars exhibit larger velocity dispersions in the radial, azimuthal, and vertical components measured by Gaia and ground-based spectroscopic efforts from the Anglo-Australian Telescope. The mean rotational lag relative to the thin disk and asymmetric drift have been quantified in studies affiliated with Princeton University and the University of Cambridge, consistent with heating or accretion origins. Dynamical modeling using N-body and hydrodynamic simulations from groups behind Illustris and EAGLE explores the influence of spiral structure, bars (as in Milky Way models), and satellite interactions like the encounter with the Sagittarius Dwarf Spheroidal Galaxy on thick disk kinematics.
Observations of edge-on galaxies in surveys by the Hubble Space Telescope and the Spitzer Space Telescope reveal thick disks as ubiquitous features in late-type systems; notable cases include NGC 891 and NGC 4565, with photometric decompositions performed by researchers at the Max Planck Institute for Astronomy. Integral field spectroscopy from the Very Large Telescope and long-slit data from the Keck Observatory detect similar chemical and kinematic signatures, while deep imaging in the Sloan Digital Sky Survey and surveys by the Pan-STARRS team enable statistical studies of thick disk prevalence across morphological types cataloged by the NASA/IPAC Extragalactic Database.
Category:Galactic structure