Galaxies in Pegasus (2009 August 6)

[bystander]

http://apod.nasa.gov/apod/ap090806.html

APOD: 2007 November 24 - Galaxies in Pegasus
Galaxies in Pegasus (APOD 24 Nov 2007)

[bystander]

APOD: 2008 October 22 - Beautiful Spiral NGC 7331
NGC 7331 Galaxy looks bent (APOD 22 Oct 2008)

APOD: 2008 July 12 - NGC 7331 and Beyond
NGC 7331 Wheels within wheels (APOD 12 Jul 2008)

APOD: 2006 July 27 - NGC 7331 and Beyond
Our galaxy, like NGC 7331? (APOD 27 Jul 2006)

APOD: 2004 July 1 - NGC 7331: A Galaxy So Inclined
Spitzer Press Release: SSC2004-12

[bystander]

APOD: 2003 August 12 - X-rays from Stephan s Quintet
APOD: 2000 November 13 - Disorder in Stephan's Quintet
APOD: 1998 January 10 - Disorder in Stephan's Quintet

Hubble News Release: STScI-2001-22

[neufer]

APOD: 2003 August 12 - X-rays from Stephan s Quintet
APOD: 2000 November 13 - Disorder in Stephan's Quintet
APOD: 1998 January 10 - Disorder in Stephan's Quintet

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July 19, 2001: In the beginning of the 1946 holiday film classic "It's a Wonderful Life," angelic figures take on the form of a famous group of compact galaxies known as Stephan's Quintet. In reality, these galaxies aren't so heavenly. Pictures from the Hubble telescope show that Stephan's Quintet has been doing some devilish things. At least two of the galaxies have been involved in high-speed, hit-and-run accidents, which have ripped stars and gas from neighboring galaxies and tossed them into space. But the galactic carnage also has spawned new life. Arising from the wreckage are more than 100 star clusters and several dwarf galaxies. The young clusters, each harboring up to millions of stars, are shown clearly for the first time in pictures taken by Hubble's Wide Field and Planetary Camera 2.
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• A lot of people asking for help for a man named George Bailey.
  
  George Bailey. Yes. Tonight's his crucial night. You're right.
  
  We'll have to send someone down immediately. Whose turn is it?
  
  That's why I came to see you, sir. It's that clock-maker's turn again.
  
  Oh, Clarence. Hasn't got his wings yet, has he?
  
  We've passed him up right along. Because, you know, sir, he's got the I.Q. of a rabbit.

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<<Stephan's Quintet, a compact group of galaxies discovered about 130 years ago and located about 280 million light years from Earth, provides a rare opportunity to observe a galaxy group in the process of evolving from an X-ray faint system dominated by spiral galaxies to a more developed system dominated by elliptical galaxies and bright X-ray emission. Being able to witness the dramatic effect of collisions in causing this evolution is important for increasing our understanding of the origins of the hot, X-ray bright halos of gas in groups of galaxies.>>

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<<Stephan's Quintet is a visual grouping of five galaxies of which four form the first compact galaxy group ever discovered. The group was discovered by Édouard Stephan in 1877 at Marseilles Observatory. The group is the most studied of all the compact galaxy groups. The brightest member of the visual grouping is NGC 7320 that is shown to have extensive H II regions, identified as red blobs, where active star formation is occurring.

More recently known, these galaxies are of interest because of their violent collisions. Four of the five galaxies in Stephan's Quintet are on collision courses with one another. The NASA Spitzer Space Telescope revealed the presence of a huge intergalactic shock wave, shown by the magnificent green arc produced by one galaxy falling into another at millions of miles per hour. As NGC 7318B collides with NGC 7318A, gas spreads throughout the cluster, atoms of hydrogen are heated in the shock wave, producing the green glow. The molecular hydrogen seen here is one of the most turbulent formations of molecular hydrogen ever seen. This phenomenon was discovered by an international team of scientists of the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg. Most notable is the fact that this collision can help provide a view into what happened in the postulated beginning of the universe some 14 billion years ago.

Also of interest, NGC 7320 indicates a small redshift (790 km/s) while the other four exhibit large redshifts (near 6600 km/s). Since galactic redshift is proportional to distance, NGC 7320 is only a foreground projection and is ~39 million ly from Earth versus the 210-340 million ly of the other five.

NGC 7319 has a type 2 Seyfert nucleus.>>

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<<Great meaning and mystery dwell in a patch of sky no larger than a thumbtack held at arm's length. Nothing about this piece of sky has been easily understood ever since its discovery. Inhabited here is the great far-off malestrom of Stephan's Quintet. The five galaxies in the center of this image (there is another to the left) seem to dance with graceful promenades. This would imply that all five galaxies are located near to each other in space. However, long ago astronomers noted that the bluish spiral galaxy (NGC 7320, bottom galaxy) had a recessional velocity, due to the expansion of the universe, that is considerably smaller than the other four galaxies. Since these velocities imply distances, it would seem that NGC 7320 could not be party to this galactic gala.

For years, many argued that chains of stars between NGC 7320 and the other galaxies positively placed it at the same distance. Only recently was the issue finally put to rest, quite literally resolved, by the sharp vision of the Hubble Space Telescope. Individual stars, clusters, and nebulae are quite clearly seen in NGC 7320 and not in any of the other galaxies owing to its foreground front seating. And so from what once seemed an irreparable rend in the theory of cosmological redshift has now been satisfyingly sewn back together as a good description of our expanding universe.

Just as quickly as one nagging question was answered by the Hubble Space Telescope, did another equally mysterious question arise. Near to the nucleus of NGC 7319 (top left galaxy) a quasar shines brightly. As labeled in the inset below (and detected in the data shown here) this quasar once again seems to play the game of next galaxy on the dance card. Quasars are generally described as being super luminous galaxies formed during the early universe. But if this quasar is associated with NGC 7319, the understanding of quasars and the scale of the universe is once again in jeopardy. The reason that this quasar is so puzzling is that there is very little absorption of its light due to the effect of the gas and dust of NGC 7319. Perhaps, as some astronomers suggest, some quasars are actually the stripped cores of devoured galaxies that have been subsequently spit out by the surviving galaxy such as NGC 7319. Many other galaxies seem to have a high number of detected quasars near to them. This could be an observational bias or perhaps in this case the light of the quasar just happens to shine through a fortuitous window of NGC 7319. Only the future will tell the fate of these far-off mysteries. More information can be gleaned from a paper found here.>>

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[BMAONE23]

http://www.nasa.gov/multimedia/imagegal ... _1408.html
Nice find Art
The "Cosmic Smiley"

[RachlinResearch]

In the August 6, 2009 APOD "Galaxies in Pegasus", I noticed a faint "hole" centered about half-way between NGC 7331 and Stephan's Quintet. Enhancing the image by equalizing the histogram (e.g., using gThumb) highlights it nicely. What is it? An imaging artifact caused by internal reflections maybe? A gap in an interstellar cloud or a dark cloud? Just curious. Regards. John Rachlin.

[Chris Peterson]

In the August 6, 2009 APOD "Galaxies in Pegasus", I noticed a faint "hole" centered about half-way between NGC 7331 and Stephan's Quintet. Enhancing the image by equalizing the histogram (e.g., using gThumb) highlights it nicely. What is it? An imaging artifact caused by internal reflections maybe? A gap in an interstellar cloud or a dark cloud? Just curious. Regards. John Rachlin.

It looks like a dust shadow not completely removed by flat field correction. When you push the contrast that far, you can see other flat field defects as well- several smaller dust shadows (the dust was closer to the sensor) and some vignetting.