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Barred spiral galaxy

NGC 1300, viewed nearly face-on. Hubble Space Telescope image. In the core of the larger spiral structure of NGC 1300, the nucleus shows its own extraordinary and distinct "grand-design" spiral structure that is about 3,300 light-years (1 kiloparsec) long. Only galaxies with large-scale bars appear to have these grand-design inner disks — a spiral within a spiral. Models suggest that the gas in a bar can be funneled inwards, and then spiral into the center through the grand-design disk, where it can potentially fuel a central black hole. NGC 1300 is not known to have an active nucleus, however, indicating either that there is no black hole, or that it is not accreting matter.

The image was constructed from exposures taken in September 2004 by the Advanced Camera for Surveys onboard Hubble in four filters. Starlight and dust are seen in blue, visible, and infrared light. Bright star clusters are highlighted in red by their associated emission from glowing hydrogen gas. Due to the galaxy's large size, two adjacent pointings of the telescope were necessary to cover the extent of the spiral arms. The galaxy lies roughly 69 million light-years away (21 megaparsecs) in the direction of the constellation Eridanus.

A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately half of all spiral galaxies[1]. Bars generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well[1].

Edwin Hubble classified these types of spiral galaxies as "SB" ("Spiral", "Barred") in his Hubble sequence, and arranged them into three sub-categories based on how open the arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb type galaxies lie in between. A fourth type, SBm, was subsequently created to describe somewhat irregular barred spirals, such as the Magellanic Cloud galaxies, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures.

In 2005, observations by the Spitzer Space Telescope backed up previously collected evidence that suggested the Milky Way is a barred spiral galaxy. Observations by radio telescopes had for years suggested our galaxy to be barred, but Spitzer's vision in the infrared region of the spectrum has provided a more definite calculation.

The bars

Barred spiral galaxies are relatively common, with surveys showing that up to two-thirds of all spiral galaxies contain a bar.[2] The current hypothesis is that the bar structure acts as a type of stellar nursery, fueling star birth at their centers. The bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars.[3] This process is also thought to explain why many barred spiral galaxies have active galactic nuclei, such as that seen in the Southern Pinwheel Galaxy.

The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure.[4] Another possible culprit in bar creation is tidal disruptions between galaxies.

Bars are thought to be a temporary phenomenon in the life of spiral galaxies, the bar structure decaying over time, transforming the galaxy from a barred spiral to a "regular" spiral pattern. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, stubby bars. Since so many spiral galaxies have a bar structure, it is likely that it is a recurring phenomenon in spiral galaxy development. [1]

The bulges

Studying the core of the Milky Way, scientists found out that the Milky Way's bulge was peanut-shaped. This led to the conclusion that all barred spiral galaxies have a peanut shaped bulge. When observing a distant spiral galaxy with a rotational axis perpendicular to the line of sight, or one that appears "edge-on" to the observer, the shape of the bulge can be easily observed, and therefore quickly classified as either a barred spiral or a regular spiral. Galaxy NGC 4565 has been classified as a unbarred spiral galaxy using this method.

Examples

Name Type Constellation

M58 SBc Virgo
M91 SBb Coma Berenices
M95 SBb Leo
M109 SBb Ursa Major
NGC 1300 SBbc Eridanus
NGC 1365 SBc Fornax

See also

* Galaxy morphological classification
* Galaxy formation and evolution
* Lenticular galaxy
* Spiral galaxy

References

1. ^ a b J. Binney, S. Tremaine (1987). Galactic Dynamics. Princeton, New Jersey: Princeton University Press. ISBN 0-691-08445-9.
2. ^ P. B. Eskridge, J. A. Frogel (1999). "What is the True Fraction of Barred Spiral Galaxies?". Astrophysics and Space Science 269/270: 427-430. 
3. ^ J. H. Knapen, D. Pérez-Ramírez, S. Laine (2002). "Circumnuclear regions in barred spiral galaxies - II. Relations to host galaxies". Monthly Notice of the Royal Astronomical Society 337 (3): 808-828. 
4. ^ F. Bournaud, F. Combes (2002). "Gas accretion on spiral galaxies: Bar formation and renewal". Astronomy and Astrophysics 392: 83-102. 

Links

* Britt, Robert Roy. "Milky Way’s Central Structure Seen with Fresh Clarity." SPACE.com 16 August 2005.

o An article about the Spitzer Space Telescope's Milky Way discovery

* Devitt, Terry. "Galactic survey reveals a new look for the Milky Way." 16 August 2005.

o The original press release regarding the article above, from the Univ. of Wisconsin

* SPACE.com staff writers. "'Barred' Spiral Galaxy Pic Highlights Stellar Birth." SPACE.com 2 March 2001.

* Hastings, George and Jane Hastings. Classifying Galaxies: Barred Spirals, 1995.

* Buta, Ronald, D. A. Crocker, and G. G. Byrd. "Astronomers Find Multiple Generations of Star Formation in Central Starburst Ring of a Barred Spiral Galaxy." January 15, 2000.

o A press release concerning NGC 1326

* Barred spirals come and go Sky & Telescope April 2002.

o A press release from the European Southern Observatory.

* 04/03/07: Hubble: Barred Spiral Galaxy NGC 1672

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