Barred spiral galaxy

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 galaxiescite book
author=J. Binney, S. Tremaine
year=1987
title=Galactic Dynamics
publisher=Princeton University Press
location=Princeton, New Jersey
id=ISBN 0-691-08445-9] . Bars generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well.

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. [cite journal | author=P. B. Eskridge, J. A. Frogel | title=What is the True Fraction of Barred Spiral Galaxies? | journal=Astrophysics and Space Science | year=1999 | volume=269/270 | pages=427–430 | url=http://adsabs.harvard.edu/abs/1999Ap&SS.269..427E | doi=10.1023/A:1017025820201] 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. [cite journal | author=J. H. Knapen, D. Pérez-Ramírez, S. Laine | title=Circumnuclear regions in barred spiral galaxies - II. Relations to host galaxies | journal=Monthly Notice of the Royal Astronomical Society | year=2002 | volume=337 | issue=3 | pages=808–828 | url=http://adsabs.harvard.edu/abs/2002MNRAS.337..808K | doi=10.1046/j.1365-8711.2002.05840.x ] 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. [cite journal | author=F. Bournaud, F. Combes | title=Gas accretion on spiral galaxies: Bar formation and renewal | journal=Astronomy and Astrophysics | year=2002 | volume=392 | pages=83–102 | url=http://adsabs.harvard.edu/abs/2002A&A...392...83B | doi=10.1051/0004-6361:20020920] Another possible cause of bar creation is tidal disruptions between galaxies. Fact|date=April 2008

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.Fact|date=July 2007 Since so many spiral galaxies have a bar structure, it is likely that it is a recurring phenomenon in spiral galaxy development. [ [http://www.sciamdigital.com/index.cfm?fa=Products.ViewIssuePreview&ARTICLEID_CHAR=3BC08F0C-2B35-221B-67A9F2AE04AFC79A Ripples in a Galactic Pond] , Scientific American, October 2005]

Recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadena discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts. [ [http://newswise.com/articles/view/542997/ Barred Spiral Galaxies are Latecomers to the Universe] Newswise, Retrieved on July 29, 2008.]

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.Fact|date=October 2007

Examples

See also

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

References

External links

*Britt, Robert Roy. [http://www.space.com/scienceastronomy/050816_milky_way.html "Milky Way’s Central Structure Seen with Fresh Clarity."] [http://www.space.com/ SPACE.com] 16 August, 2005.
**An article about the Spitzer Space Telescope's Milky Way discovery
*Devitt, Terry. [http://www.news.wisc.edu/11405.html "Galactic survey reveals a new look for the Milky Way."] 16 August, 2005.
**The original press release regarding the article above, from the Univ. of Wisconsin
*SPACE.com staff writers. [http://www.space.com/scienceastronomy/astronomy/hubble_bar_010302.html "'Barred' Spiral Galaxy Pic Highlights Stellar Birth."] [http://www.space.com/ SPACE.com] 2 March, 2001.
*Hastings, George and Jane Hastings. [http://www.smv.org/hastings/bsmain.htm Classifying Galaxies: Barred Spirals] , 1995.
*Buta, Ronald, D. A. Crocker, and G. G. Byrd. [http://bama.ua.edu/~rbuta/press-release.html "Astronomers Find Multiple Generations of Star Formation in Central Starburst Ring of a Barred Spiral Galaxy."] January 15, 2000.
**A press release concerning NGC 1326
* [http://www.govertschilling.nl/artikelen/archief/2002/0204/020401_st.htm Barred spirals come and go] "Sky & Telescope" April 2002.
* [http://www.spaceref.com/news/viewpr.html?pid=6736 "ESO Provides An Infrared Portrait of the Barred Spiral Galaxy Messier 83."] November 29, 2001.
**A press release from the European Southern Observatory.
* [http://www.exploration-space.com/03-apr-2007-esa-2.html 04/03/07: Hubble: Barred Spiral Galaxy NGC 1672]