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| Galactic Bar | |
|---|---|
| Name | Galactic Bar |
| Type | Stellar bar |
| Epoch | J2000 |
| Constellation | Milky Way (central region) |
| Distance | ~8 kpc (Galactic Center) |
| Major axis | ~3–5 kpc (typical) |
| Population | Disk stars, gas, dust |
Galactic Bar A galactic bar is an elongated stellar structure found in the central regions of many disk galaxies, linking the inner bulge or pseudobulge to the outer disk and often driving large-scale secular processes. Bars are observed in systems ranging from nearby spirals cataloged by Messier catalog objects to distant sources in surveys like Sloan Digital Sky Survey, and they are key features in studies by facilities such as the Hubble Space Telescope, the Spitzer Space Telescope, and the Atacama Large Millimeter/submillimeter Array. Bars are implicated in angular momentum redistribution, central star formation, and the fueling of active galactic nuclei seen in samples from the Sloan Digital Sky Survey and the Two Micron All Sky Survey.
Bars are common in morphological classifications used by astronomers such as the Hubble sequence and the de Vaucouleurs system, appearing in a large fraction of disk galaxies surveyed by projects including the Galaxy Zoo citizen science project and the Spitzer Survey of Stellar Structure in Galaxies. Observational catalogs compiled by teams using the Sloan Digital Sky Survey and the Two Micron All Sky Survey quantify bar frequency as a function of stellar mass and redshift, linking bars to populations studied in the Local Group and in more distant clusters like the Virgo Cluster and the Coma Cluster.
Bar formation is typically described by dynamical instabilities in rotating disk models such as those studied by Ostriker–Peebles instability criteria and by analyses of swing amplification in shearing disks first explored by researchers associated with the Toomre Q parameter studies. Interactions and tidal perturbations from companions—examples include those in the M81 Group and the Magellanic Clouds encountering the Milky Way—can seed bars, while internal secular evolution driven by angular momentum exchange among stars, gas, and dark matter halos described in work on the Lambda Cold Dark Matter model modifies bar strength and pattern speed. Resonances such as the corotation resonance and Lindblad resonance play central roles in orbital trapping, while exchanges with the dark matter halo (astrophysics) affect bar slowdown through dynamical friction described in studies associated with James Binney and Scott Tremaine-style theoretical frameworks.
Bars display a range of morphologies cataloged in surveys like the Ohio State University Bright Spiral Galaxy Survey and in classifications by Allan Sandage and Gerard de Vaucouleurs. Features include rectangular or boxy isophotes seen in edge-on systems such as NGC 4565, ansae or “handles” studied in galaxies like NGC 4216, and peanut- or X-shaped bulges attributed to vertical resonant heating observed in the Milky Way bulge and in extragalactic examples like NGC 128. Stellar populations within bars are probed by spectroscopic programs using instruments on the Very Large Telescope and the Keck Observatory and compared against bulge populations in works by teams from the Sloan Digital Sky Survey and the CALIFA survey.
Bars funnel gas inward—processes analyzed in papers from groups using the Atacama Large Millimeter/submillimeter Array and the Very Large Array—enhancing central star formation episodes observed in starburst systems cataloged by the Infrared Astronomical Satellite and influencing nuclear activity demographics studied in the Sloan Digital Sky Survey and ROSAT samples of active galactic nuclei. Secular evolution driven by bars can transform disks to pseudobulges characterized in studies by John Kormendy and collaborators, alter metallicity gradients traced by surveys like APOGEE and GALAH, and mediate bar-induced quenching processes considered in extragalactic contexts such as the Fornax Cluster and the Sloan Digital Sky Survey environmental analyses.
Imaging evidence for bars comes from multiwavelength campaigns by the Hubble Space Telescope, Spitzer Space Telescope, and ground-based facilities such as the Subaru Telescope and the Very Large Telescope. Kinematic signatures are obtained via integral field units used in the CALIFA survey, MaNGA program, and the SAMI Galaxy Survey, revealing noncircular motions, streaming along bar major axes, and shocks in CO emission mapped by the Atacama Large Millimeter/submillimeter Array. Photometric decompositions by groups using data from the Sloan Digital Sky Survey and the Two Micron All Sky Survey identify bar lengths and strengths through Fourier analysis methods pioneered in studies associated with the Ohio State University Bright Spiral Galaxy Survey.
N-body and hydrodynamic simulations by teams using codes such as GADGET and RAMSES explore bar formation in cosmological settings like those simulated in the Illustris and EAGLE projects, and in isolated galaxy models building on foundational work by Hohl and Athanassoula. Simulations address bar pattern speeds, secular thickening producing boxy/peanut bulges, and gas inflow leading to central concentrations, comparing outputs to observational constraints from the Milky Way bulge surveys (e.g., BRAVA and ARGOS). The role of feedback from supernovae modeled after results from the Sloan Digital Sky Survey and cosmological frameworks such as the Lambda Cold Dark Matter model is integrated to study bar survival and dissolution.
Barred galaxies appear across morphological catalogs including the Messier catalog, the New General Catalogue, and modern compilations from the Sloan Digital Sky Survey. Well-known barred examples include Messier 95, Messier 83, NGC 1097, NGC 1300, NGC 1300, NGC 1365, NGC 3351, and the barred spiral NGC 2903; nearby survey targets like NGC 1365 and NGC 4321 provide archetypes for strong and weak bars. The barred nature of the Milky Way is inferred from near-infrared mapping such as results from the COBE mission and from kinematic studies by the BRAVA and ARGOS collaborations.
Category:Galactic structures