Spiral Arm Structure
Spiral Arm Structure
| Topics: |  Galaxy Types Galaxies |
Introduction
The structure of arms in spiral galaxies originates with their differential rotation. Spiral galaxies may have a grand design structure, with long, continuous, symmetric arms, or flocculent structure, with short spiral arm pieces, or multiple arms, which exhibit grand design structure in the inner regions but branch to many arms in the outer regions.
Grand Design Arms
Messier 081 nebulae imaged by the Palomar Observatory 1.5m telescope. Click on image for full-size view. (Source: NASA/IPAC Extragalactic Database.)
Grand design arms are caused by spiral density waves, which are sinusoidal perturbations that ripple through the disk. They may move from the outer regions inward, or the inward regions outward. Companions and internal bars both trigger waves. The wave pattern rotates as a solid body, while the arms rotate diffierentially, at a higher angular rate in the inner disk than in the outer disk. At corotation, stars and gas rotate at the same angular speed as the pattern. Waves are reinforced at corotation, and may bounce between a point outside the Inner Lindblad resonance (where the difference between the disk's angular speed and the pattern's angular speed is equal to the epicyclic period divided by the number of arms) and corotation. In this case standing waves are established, so the overall structure is governed by modes. The waves are destroyed at the Outer Lindblad resonance, which is close to R25 (where the surface brightness is 25 magnitudes per arcsecond squared). In grand design galaxies, both old and young stars are organized by the waves, so the arms are evident in both blue and infrared filters. Multiple arms may result from a superposition of 2-arm and 3-arm patterns.
Flocculent Structure
NGC 7793 imaged by the Galaxy Evolution Explorer (GALEX) satellite. (Source: NASA/IPAC Extragalactic Database.)
Flocculent structure may also result from overlapping modes. Alternatively, flocculent structure may be the result of sheared star formation sites (with stochastic self-propagating star formation) that trace out short spiral arcs in a differentially rotating disk. In this case, the spiral structure only manifests itself at blue wavelengths, tracing out the high mass (short-lived) stars.
External Links:
The NASA/Infrared Processing and Analysis Center - Extragalactic Database ("NED") is built around a master list of extragalactic objects for which cross-identifications of names have been established, accurate positions and redshifts entered to the extent possible, and some basic data collected. The images in the external links below are from the NED collection.
Bibliographic references relevant to individual objects have been compiled, and abstracts of extragalactic interest are kept on line. Detailed and referenced photometry, position, and redshift data, have been taken from large compilations and from the literature. NED also includes images from 2MASS, from the literature, and from the Digitized Sky Survey. NED's data and references are being continually updated, with revised versions being put on-line every 2-3 months.
The NASA/IPAC Extragalactic Database is operated by the Jet Propulsion Laboratory, and the California Institute of Technology, under contract with the National Aeronautics and Space Administration.
- NED's features has more information on NED's many available features.
- Guide to NED Image Results - NASA/IPAC Extragalactic Database.
- SIMBAD– Data and references for Galactic objects may be retrieved from SIMBAD (Set of Identifications, Measurements, and Bibliography for Astronomical Data), maintained by Centre de Données Astronomiques de Strasbourg, France.
- Planetary Data System– Similarly, solar system and planetary data (e.g. for Mars or for Halley's Comet) may be retrieved from NASA's Planetary Data System (PDS) at JPL.
- CDS (Centre données astronomiques de Strasbourg) – Many of the individual catalogs loaded into NED are available from CDS at the Strasbourg Astronomical Observatory, Strasbourg, France.
Grand Design Galaxy Examples:
Flocculent Spiral Galaxy Examples:
Multiple Arm Galaxy Examples:
Preview Image
The "Whirlpool Galaxy," Messier 051, also known as (NGC 5194). An example of a "Grand Design Galaxy." A multicolor optical image taken by the Hubble Space Telescope. (View full-size image.) From the NASA/IPAC Extragalactic Database collection. A close-up view of the Whirlpool Galaxy was the February 19, 2006 NASA Astronomy Photo of the Day.
Citation
Elmegreen, Debra, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Spiral Arm Structure." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 26, 2007].
<http://www.cosmosportal.org/articles/view/138204/>
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