Spiral galaxy
A spiral galaxy is a galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Nebulae”[1] and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.
Spiral galaxies are named for the (usually two-armed) spiral structures that extend from the center into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB stars that inhabit them. Roughly half of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. Our own Milky Way has recently (in the 1990s) been confirmed to be a barred spiral, although the bar itself is difficult to observe from our position within the Galactic disk. [2] The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, of stars in the Galactic center.[3]
Together with irregulars, spiral galaxies make up approximately 60% of galaxies in the local Universe.[4] They are mostly found in low-density regions and are rare in the centers of galaxy clusters.[5]
Structure
Spiral galaxies consist of several distinct components:
- A flat, rotating disc of (mostly newly created) stars and interstellar matter
- A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
- A near-spherical halo of stars, including many in globular clusters
- A supermassive black hole at the very center of the central bulge
The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.
Spiral arms
Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sa and SBa galaxies, for instance, have tightly wrapped arms, whereas Sc and SBc galaxies have very "loose" arms (with reference to the Hubble sequence). Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable.
Galactic bulge
A bulge is a huge, tightly packed group of stars. The term commonly refers to the central group of stars found in most spiral galaxies.
Using the Hubble classification, the bulge of Sa galaxies is usually composed of population II stars, that is old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are a great deal lesser, and are composed of young, blue, Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity), and others simply appear as higher density centers of disks, with properties similar to disk galaxies.
Many bulges are thought to host a supermassive black hole at their center. Such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole. There is a tight correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge, the M-sigma relation.
Galactic spheroid
The bulk of the stars in a spiral galaxy are located either close to a single plane (the Galactic plane) in more or less conventional circular orbits around the center of the galaxy (the galactic centre), or in a spheroidal galactic bulge around the galactic core.
However, some stars inhabit a spheroidal halo or galactic spheroid. The orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example, the Sagittarius Dwarf Elliptical Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it.
Unlike the galactic disc, the halo seems to be free of dust, and in further contrast, stars in the galactic halo are of Population II, much older and with much lower metallicity than their Population I cousins in the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters.
The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarf stars close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830. Due to their irregular movement around the centre of the galaxy—if they do so at all—these stars often display unusually high proper motion.
Origin of spiral structure
Density wave theory
Bertil Lindblad first studied the rotation of the Galaxy in 1925, but made elementary mathematical mistakes and introduced misleading notions such as epicycles, which have hampered research. In 1941, Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy’s stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms. This idea was developed into density wave theory by C. C. Lin and Frank Shu in 1964.[6] They suggested that the spiral arms were manifestations of spiral density waves, attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of the galaxy's gas and stars.
Gravitationally aligned rosettes
In a 2009 paper[7] published in Proc. Roy. Soc. A, Charles Francis and Erik Anderson pointed out errors in density wave theory. They described how mutual gravity between stars causes orbits to align on logarithmic spirals, and showed from observations of precise motions of over 20 000 local stars (within 300 parsecs), that, contrary to density wave theory, stars do move along spiral arms.
Orbital precession
In the solar system, mass is concentrated at the Sun, and planetary orbits are ellipses. In a spiral galaxy mass is distributed throughout the disc and in the halo. As a star moves towards pericentre, the gravitational mass drawing it towards the galactic centre is less than it would be if all the mass of the galaxy was concentrated at one central point. As a result the orbit of the star is less curved at pericentre than an ellipse, and the orbit precesses, forming a rosette. Imagine looking at the motion from above, from a platform rotating at the rate of precession; the orbit will be approximately elliptical with the galactic centre at the focus.
Potential in a spiral galaxy
The gravitational potential of a spiral galaxy can be described as a giant, spiral-grooved funnel. The grooves represent the gravitational field of the galaxy's spiral arms. A star near apocentre, the slowest part of its orbit, will tend to fall into a groove and then follow the groove, picking up momentum as it goes. Eventually, the star gains enough momentum to jump free of its groove. It crosses over the next-highest groove, then falls back to a higher point in its original groove (animation). As stars are drawn into an arm, the gravitational field of the arm grows stronger, drawing greater number of stars into the arm. At the same time, the funnel rotates slowly backwards due to orbital precession.
Star formation in spiral arms
Under gravity, gas clouds follow similar motion to stars. Gas in the arm is in turbulent motion, as gas clouds seek to cross in the arm and gain velocity as they approach pericentre. Whereas stars rarely collide because of their small size compared to space between them, when outgoing gas from one arm meets ingoing gas in another arm, collisions between gas clouds create regions of higher pressure, and greater turbulence. Pockets of extreme pressure due to turbulence generate the molecular clouds in which new stars form.
Bisymmetric Spirals
In a multi-arm spiral, outgoing gas meeting an arm would outweigh ingoing gas in the arm. This would tend to remove gas from the arm. In a two armed spiral, the gas in the arm has greater mass. Thus, a two-armed gaseous spiral can be stable, whereas multiarmed gaseous spirals cannot. Outgoing gas applies pressure to the trailing edge of a spiral arm with an inverse proportionality to radius. If one gaseous arm advances compared to the bisymmetric position, the pressure due to gas from the other arm will be reduced. At the same time, pressure on the retarded arm due to outgoing gas from the advanced arm will be increased. Thus gas motions preserve the symmetry of two-armed spirals.
Alignment of spin axis with cosmic voids
Recent results suggest that the orientation of the spin axis of spiral galaxies is not a chance result, but instead they are preferentially aligned along the surface of cosmic voids.[8] That is, spiral galaxies tend to be oriented at a high angle of inclination relative to the large-scale structure of the surroundings. They have been described as lining up like "beads on a string," with their axis of rotation following the filaments around the edges of the voids.[9]
Spiral nebula
“Spiral nebula” is an old term for a spiral galaxy. Until the early 20th century, most astronomers believed that objects like the Whirlpool Galaxy were just one more form of nebula that were within our own Milky Way galaxy. The idea that they might instead be other galaxies, independent of the Milky Way, was the subject of The Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mt. Wilson Observatory. In 1926, Edwin Hubble[10] observed Cepheid variables in several spiral nebulae, including the Andromeda Galaxy, proving that they are, in fact, entire galaxies outside our own. The term “spiral nebula” has since fallen into disuse.
The Milky Way
The Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1990s.[11] Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005[12] which showed the galaxy's central bar to be larger than previously suspected.
Famous examples
See also
Components
Classification
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Other
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References
- ^ Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 0300025009.
- ^ Ripples in a Galactic Pond, Scientific American, October 2005
- ^ Benjamin, R. A.; et al. (2005). "First GLIMPSE Results on the Stellar Structure of the Galaxy". The Astrophysical Journal Letters. 630 (2): L149–L152. doi:10.1086/491785. Retrieved 2007-09-21.
{{cite journal}}: Explicit use of et al. in:|first=(help); Unknown parameter|month=ignored (help) - ^ Loveday, J. (1996). "The APM Bright Galaxy Catalogue". Monthly Notices of the Royal Astronomical Society. 278 (4): 1025–1048. Retrieved 2007-09-15.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ Dressler, A. (1980). accessdate= 2007-09-15 "Galaxy morphology in rich clusters — Implications for the formation and evolution of galaxies". The Astrophysical Journal. 236: 351–365. doi:10.1086/157753.
{{cite journal}}: Check|url=value (help); Missing pipe in:|url=(help); Unknown parameter|month=ignored (help) - ^ Lin, C. C. (1964). "On the spiral structure of disk galaxies". The Astrophysical Journal. 140: 646–655. doi:10.1086/147955. Retrieved 2007-09-26.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help); Unknown parameter|month=ignored (help) - ^ Galactic spiral structure at http://rspa.royalsocietypublishing.org/content/early/2009/08/19/rspa.2009.0036.abstract
- ^ Trujillo, I. (2006). "Detection of the Effect of Cosmological Large-Scale Structure on the Orientation of Galaxies". The Astrophysical Journal. 640 (2): L111–L114. doi:10.1086/503548.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Alder, Robert (2006). "Galaxies like necklace beads". Astronomy magazine. Retrieved 2006-08-10.
- ^ Hubble, E. P. (1926). "A spiral nebula as a stellar system: Messier 33". The Astrophysical Journal. 63: 236–274. doi:10.1086/142976. Retrieved 2007-09-21.
{{cite journal}}: Unknown parameter|month=ignored (help) - ^ Chen, W.; Gehrels, N.; Diehl, R.; Hartmann, D. (1996). "On the spiral arm interpretation of COMPTEL ^26^Al map features". Space Science Reviews. 120: 315–316. Retrieved 2007-03-14.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ McKee, Maggie (August 16, 2005). "Bar at Milky Way's heart revealed". New Scientist. Retrieved 2009-06-17.
External links
- Giudice, G.F.; Mollerach, S.; Roulet, E. (1994). "Can EROS/MACHO be detecting the galactic spheroid instead of the galactic halo?". Physical Review D. 50: 2406–2413. doi:10.1103/PhysRevD.50.2406. Retrieved 2007-02-04.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - Stephens, Tim (March 6, 2007). "AEGIS survey reveals new principle governing galaxy formation and evolution". UC Santa Cruz. Retrieved 2006-05-24.
- Spiral Galaxies @ SEDS Messier pages
- SpiralZoom.com, an educational website about Spiral Galaxies and other spiral formations found in nature. For high school & general audience.
- Spiral Structure explained