Normal Spiral Galaxies
Bared Spiral Galaxies
M51
M63
NGC4258
NGC4725
Summary
Comparison Chart
References
Web Sites
Image Credits
 

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Stars are grouped into “island universes” called galaxies and there are three types of galaxies in our Universe: elliptical, spiral, and irregular. Elliptical galaxies are usually small[1] and contain older stars, little dust and gas, and do not have any organized rotation. Irregular galaxies have no discernable structure or spiral arms, but they are usually small and do contain dust and gas with associated hot stars. Spiral galaxies have a bulge of older stars with prominent spiral arms with dust, gas and new star formation. They also have an organized rotation. This project will focus on the comparison of four spiral galaxies using provided images from the McDonald Observatory at Fort Davis, Texas. Using standard image reduction techniques, the data from this observatory is reduced and merged to a full color composite image. We will examine in detail the properties of a spiral galaxy. In addition, we will be using the Hubble Tuning Fork Diagram as well as reference material to provide a detailed analysis of each galaxy.

Edwin Hubble is responsible for creating the classification of galaxies using a tool called the “Tuning Fork Diagram:”

Figure 1: (Image Credit)

This diagram demonstrates the major groups (irregular galaxies are not shown on this particular diagram) and their varieties. While this chart was initially thought to be an evolutionary map of galaxies, it is now known that galaxies in one particular group will not evolve into another[2]. This project will focus on the right side of the diagram – spiral
galaxies. There are two sub-categories of spiral galaxies:

• “Normal” spiral galaxies – galaxies that have their spiral arms attached to the bulge
• Bared spiral galaxies – galaxies that have their spiral arms attached to a bar that protrudes from the bulge.

The image below shows a diagram I made of our own Milky Way galaxy – which is a spiral galaxy:

The bulge of a spiral galaxy contains older stars while the spiral arms contain dust and newer, hotter stars and their associated hydrogen dust clouds (Universe, page 582).

In order to classify spiral galaxies, it is necessary to look at two variables: the brightness of the bulge and the organization of the spiral arms.

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For normal spiral galaxies:
(http://cas.sdss.org/dr3/en/astro/galaxies/galaxies.asp) (Sparke and Gallagher, page 34 – 35)

Examples (Figure 2):

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For bared spiral galaxies:
(http://cas.sdss.org/dr3/en/astro/galaxies/galaxies.asp) (Sparke and Gallagher, page 34 – 35)

Examples (Figure 3):

Sa and SBa galaxies are almost always brighter that Sd an SBd galaxies. The organization of spiral galaxies and bared spiral galaxies are the same[3].

Some other important properties of spiral galaxies include (Sparke and Gallagher, page 172 – 214):

• Spiral galaxies produce most of the luminous light in the Universe due to new star birth in the spiral arms
• Majority of galaxies in the Universe are spiral galaxies
• Half of all spiral galaxies are in the bared spiral class
• Spiral galaxies obey the Tully-Fisher relation – brighter galaxies rotate faster
• Spiral galaxy rotation curves are dominated by Dark Matter
• Rotation of stars in the spiral arms are organized while the rotation of stars in the bulge are not (random rotation orbits about the nucleus)

While the properties of spiral galaxies are classed by the apparent visual brightness of the core, some spiral galaxies do contain active galactic nuclei, or AGN. The Seyfert galaxy is a galaxy with such an active nucleus (probably powered by a large black hole) and results in a brighter than usual bulge. The Unified AGN Model is a theory that describes the class of galaxies that result from a black hole at the center of a galaxy as well as the orientation of the galaxy toward its observer (us). Quasars, radio galaxies, and Seyfert galaxies all fall within the category of AGN, but is not covered by the scope of this project.

A Seyfert or other AGN galaxy does affect the classification system. If the bulge is brighter but the spiral arms more loose, it is required to combine classes. Galaxy NGC4258, covered below, is an example of such a galaxy. For example, a spiral galaxy “S” can have a bright core (sub-class “a” or “b”) but can have a loose spiral structure (sub-class “c” or “d”). The result will be something like Sac or Sbd.

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Galaxy M51:

In the constellation Canes Venatici, shining at a visible magnitude of 8.4 is the Whirlpool Galaxy, or M51. This galaxy also carries the designation NGC5194. Once thought to be a giant swirling nebula, this object was determined to have a spiral structure in 1845 by Lord Rosse’s 72 inch “Leviathan” telescope (O’Meara, page 160). This galaxy is about 35 million light years away and shines with a luminosity of about 10 billion Suns. Additionally, this galaxy has a mass of 160 billion Suns and is comparable in size to the Andromeda Galaxy – 100,000 light years in diameter (M31) (Burnham, page 369 – 371). M51 is a face on spiral galaxy, which allows us a detailed look at the spiral structure. Since the spiral arms are loose and bulge relatively dim, this galaxy is classed as a Sc type spiral galaxy (Burnham, page 369).  In addition, this galaxy has an interacting companion, NGC5195 (http://www.starfred.com/ngc5194a.htm). Redshift measurements of this companion galaxy show it to be the same distance as M51, so this is truly an interacting galaxy (Burnham, page 371).

The image of M51 shows the spiral structure well. It is possible to see some brighter regions of new star growth throughout the spiral structure as well as the overall blue color indicating the presence of larger, hotter new stars as well as associated dust lanes. It is also possible to see the gold color of the bulge indicating the presence of older stars. In addition, it is also possible to visualize the disruption of the smaller companion galaxy as there is no clearly defined spiral structure.

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M63:

Also in the constellation of Canes Venatici is the 8.6 magnitude spiral galaxy M63. This galaxy also carries the designation NGC5055. Called the Sunflower galaxy, it appears as though this galaxy has lost any discernable spiral structure (O’Meara, page 188), but instead is a tight wound spiral galaxy designated with the class Sb (Burnham, page 373). This galaxy is similar to M51 in that its distance is also 35 million light years away and its luminosity is also that of 10 billion Suns. It is not as large as M51, with a diameter of 90,000 light years and has a mass of 115 billion Suns.

The core of this galaxy is very bright and does seem to overpower the brightness of the spiral arms giving the appearance of a flocculent design. This galaxy is inclined to us by 30 degrees which allows for the almost “sunflower” appearance (Burnham, page 373). While the dust lanes are not as prominent as M51, it is still possible to see the bluer color of the spiral arms versus the gold color of the bulge.

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NGC4258:

NGC4258 is in also in the Canes Venatici constellation. Also known as M106, this 8.4 magnitude galaxy is special in that it is also a Seyfert II type galaxy – that is, there is a black hole at the heart of this galaxy that results in bright radio emissions (O’Meara, page 274). The luminosity of this galaxy is much less than M51 or M63 shining at only 1.3 billion Suns. This is probably the result of this being a smaller galaxy in that it has a mass of 40 billion Suns (http://hubblesite.org/newscenter/newsdesk/archive/releases/1997/01/text/). The loose nature of the spiral arms suggests a Sc class, but the bright Seyfert nucleus is more in line with a Sb class galaxy. As such, it has been combined to be a class Sbc galaxy (Sparke and Gallagher, page 306).

The force driving the Seyfert core is suggested to be the gravity interaction of a companion galaxy, designated NGC 4217 (Mouri and Taniguchi, 2004):

The companion galaxy is seen in the upper left of NGC 4258. It is possible that the loose structure of NGC4258 is also the result of the companion galaxy.

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NGC4725:

The only bared spiral galaxy in our group of four is NGC4725. Shining at a dim magnitude of 9.2, this bared spiral is also a Seyfert galaxy as indicated by its bright nucleus. This galaxy is a distant 40 million light years and is a class SBa type galaxy – evident by the tightly wound spiral structure. What is fascinating about this galaxy is the determination of the distance to this galaxy was made possible by the use of 20 Cepheid variable stars (Gibson et al., 1999).
The diameter of this galaxy is also less than M51 or M63 at “only” 54,000 light years (Laine et al., 2002).

As mentioned, NGC4725 is a Seyfert galaxy – a Type II just like NGC4258. And just like NGC4258, the resulting energy of the Seyfert engine is also believed to be a result of a companion galaxy (Mouri and Taniguchi, 2004). The companion galaxy, NGC4747, is seen to the left of NGC4725. The size of the companion, as compared to the companion NGC4258, seems more likely to cause the gravity interactions driving the Seyfert core of NGC4725.

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Summary:

           Spiral galaxies come in a variety of flavors. Our sample included a broad spectrum of galaxies. M51, the only interacting galaxy of our group has a clearly defined spiral structure that is face on. The tight spiral structure of M63 demonstrates a bright center. A loose spiral Seyfert galaxy is also included in our roundup and demonstrates that a mixing of classes is also possible. Our final galaxy is a bared spiral and also a Seyfert galaxy. This variety allows for a broad sample of study for Astronomers. For the photographer, this offers a wide choice of subjects for display at the home or office.

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Comparison Chart:

Data gathered from Burnham’s, The NGC/IC Project, NED and Hubble.

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References:

Burnham, Robert. Burnham’s Celestial Handbook. Volume One. Dover Publications, New York. 1978.

Burnham, Robert. Burnham’s Celestial Handbook. Volume Two. Dover Publications, New York. 1978.

Freedman, Rodger and William Kaufman. Universe. Seventh Edition. W.H. Freeman and Company, New York. 2005.

Gibson, Brad et al. “The Hubble Space Telescope Key Project on the Extragalactic Distance Scale. XVII. The Cepheid Distance to NGC 4725.” The Astrophysical Journal, 512:48-64, 1999 February 10.

Laine, Seppo et al. “Nested and Single Bars in Seyfert and Non-Seyfert Galaxies.” The Astrophysical Journal, 567:97-117, 2002 March 1.

Mouri, Hideaki and Yoshiaki Tanighuchi. “Galaxy Interaction and the Starburst-Seyfert Connection.” The Astrophysical Journal, 605:144-155, 2004 April 10.

O’Meara, James. The Messier Objects. Cambridge University Press, 2000.

Sparke, Linda and John Gallagher. Galaxies in the Universe. An Introduction. Cambridge University Press, 2000.

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Websites:

Sloan Digital Sky Survey: http://cas.sdss.org/dr3/en/astro/galaxies/galaxies.asp

Galaxy Classification: http://www.astr.ua.edu/keel/galaxies/classify.html

The NGC/IC Project: http://www.ngcic.org/

NED – NASA/IPAC Extragalactic Database: http://nedwww.ipac.caltech.edu/

M51: http://www.starfred.com/ngc5194a.htm

M63: http://www.starfred.com/ngc5055.htm

NGC4258: http://hubblesite.org/newscenter/newsdesk/archive/releases/1997/01/text/

NGC4258: http://www.starfred.com/ngc4258.htm

NGC4725: http://www.starfred.com/ngc4725.htm

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Image Credits:

Figure 1: http://staff.imsa.edu/science/astro/astrometry/images/TuningFork.jpg

Figure 2 and 3: http://cas.sdss.org/dr3/en/astro/galaxies/galaxies.asp

Galaxy image provided by the McDonald Observatory and reduced by the author

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