A disc galaxy (or disk galaxy) is a galaxy characterized by a galactic disc. This is a flattened circular volume of stars that are mainly orbiting the galactic core in the same plane. These galaxies may or may not include a central non-disc-like region (a galactic bulge). They will typically have an orbiting mass of gas and dust in the same plane as the stars. Interactions with other nearby galaxies can perturb and stretch the galactic disk.
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👉 Disc galaxy in the context of Dust lane
A dust lane consists of relatively dense, obscuring clouds of interstellar dust, observed as a dark swath against the background of brighter object(s), especially a galaxy. These dust lanes can usually be seen in spiral galaxies, such as the Milky Way, when viewed from the edge. Due to the dense and relatively thick nature of this dust, observed light from a galaxy can be reduced by dust lanes by up to several magnitudes. In the Milky Way, this attenuation of visible light makes it impossible to see the stars behind the Great Rift through the bulge around the Galactic Center from Earth. This dust, as well as the gasses also found within these lanes, mixes and combines to form stars and planets. The gas in the dust lanes is funneled toward the Central Molecular Zone. Approximately one third of the gas will combine with the CMZ. The rest will overshoot and accrete at a later time.
The presence of a dust lane is most apparent in disc galaxies that are viewed edge on. Although they are absent in many low-mass late-type galaxies. However, the absence of a dust lane does not signify a lack of dust but that it is more dispersed throughout the galaxy. Simulations have shown that in barred spiral galaxies the strength of the bar has an affect on the curvature of the dust lanes. Galaxies with weak bars result in curved dust lanes whereas strong bars result in straight dust lanes.
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Disc galaxy in the context of Lenticular galaxy
A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars (like elliptical galaxies). Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate-scale discs.
Disc galaxy in the context of Disc (galaxy)
A galactic disc (or galactic disk) is a component of disc galaxies, such as spiral galaxies like the Milky Way and lenticular galaxies. Galactic discs consist of a stellar component (composed of most of the galaxy's stars) and a gaseous component (mostly composed of cool gas and dust). The stellar population of galactic discs tend to exhibit very little random motion with most of its stars undergoing nearly circular orbits about the galactic center. Discs can be fairly thin because the disc material's motion lies predominantly on the plane of the disc (very little vertical motion). The Milky Way's disc, for example, is approximately 1 kilolight-year thick, but thickness can vary for discs in other galaxies.
Disc galaxy in the context of Velocity curve
The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. The experimental curves observed are at significant variance with gravitational theory applied to the matter observed in a galaxy. Theories involving unobservable dark matter are the main postulated explanation of this discrepancy.
Considering their mass distributions, the rotational/orbital speeds of galaxies/stars would not be expected to follow rules such as Kepler's third law applying to smaller orbital systems such as stars/planets and planets/moons with most mass at the centre. Stars revolve around their galaxy's centre at equal or increasing speed over a large range of distances. Even considering this, however, the mass estimations for galaxies based on the light they emit are far too low to explain the velocity observations.