APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Comments and questions about the APOD on the main view screen.
Post Reply
User avatar
APOD Robot
Otto Posterman
Posts: 5345
Joined: Fri Dec 04, 2009 3:27 am
Contact:

APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by APOD Robot » Sun Apr 03, 2011 4:06 am

Image Giant Galaxy NGC 6872

Explanation: Over 400,000 light years across NGC 6872 is an enormous spiral galaxy, at least 4 times the size of our own, very large, Milky Way. About 200 million light-years distant, toward the southern constellation Pavo, the Peacock, the remarkable galaxy's stretched out shape is due to its ongoing gravitational interaction, likely leading to an eventual merger, with the nearby smaller galaxy IC 4970. IC 4970 is seen just below and right of the giant galaxy's core in this cosmic color portrait from the 8 meter Gemini South telescope in Chile. The idea to image this titanic galaxy collision comes from a winning contest essay submitted last year to the Gemini Observatory by the Sydney Girls High School Astronomy Club. In addition to inspirational aspects and aesthetics, club members argued that a color image would be more than just a pretty picture. In their winning essay they noted that "If enough colour data is obtained in the image it may reveal easily accessible information about the different populations of stars, star formation, relative rate of star formation due to the interaction, and the extent of dust and gas present in these galaxies". (Editor's note: For Australian schools, 2011 contest information is here.)

<< Previous APODDiscuss Any APOD Next APOD >>
[/b]

Guest

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Guest » Sun Apr 03, 2011 4:30 am

Wow,

I'm so used to a sort of swirlyness in "typical" galaxies--several "wraps". This one shows two very long distinct arms. And then the " bent" arm. Wow.

This site is great. I've got a lot to learn!

Who doesn't?

contactcameron@yahoo.co.uk

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by contactcameron@yahoo.co.uk » Sun Apr 03, 2011 5:51 am

Nice pic, and good reasoning. And to think I founded the astronomy club at Knox in Sydney so many years ago. Good one girls.

I note top above galaxy, far away in distance, what looks like an eliptical galaxy with mutiple cores (bright points). I wonder if this is a tight cluster of galaxies or a case of gravitational lensing. If the second, a particularly exceptional example. A serindipidous discovery perhaps! Just a thought.

Cdownunder.

User avatar
Ann
4725 Å
Posts: 13373
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Ann » Sun Apr 03, 2011 6:15 am

It's heartening that a girls' high school has an astronomy club, and even more heartening that that girls from this astronomy club would hand in a winning essay and get to image a fantastic interacting galaxy pair with the help of one of the world's foremost astronomers, Travis Rector, and one of the world's best telescopes, the Gemini Observatory.

The picture is quite delightful. The colors are subtle and beautiful, and the "texture" of the various stellar populations is fascinating. The small galaxy seems to consist of a soft smooth S-shaped center surrounded by a soft smooth mildly purple envelope. The arms of the large galaxy also seem to contain soft smooth white and purple populations, but the arms also contain finely structured brown dust features and clumpy blue young clusters. Personally I would guess that the white smooth populations are mostly old yellow stars which are either mixed with some intermediate-aged stars of spectral class F and G, or else this white population may be entirely old, but mixed with some metal-poor stars, which contain blue horizontal stars. As for the purple populations, I believe they are intermediate-aged stars dominated by the light of F and G type stars. The yellow center of NGC 6872 is probably made up of stars that are almost all old and metal-rich. One thing I wonder about, but can't explain, is the faint somewhat greenish population which seems to envelop the large galaxy like a halo. I wonder what the greenish color represents. There appears to be a small greenish dwarf galaxy in the lower left part of the picture.

The large galaxy, NGC 6872, is huge. The Principal Galaxy Catalog claims that the luminosity of this galaxy is 8.6 times that of the Milky Way! The color indexes of NGC 6872 are relatively red, which is what you can expect from a very large galaxy. That is because very big and bright galaxies always have an enormous population of small red stars. (A rare exception to this rule would be big and blue galaxy M101.) But NGC 6872 is bright enough in the far infrared that we have reason to believe that a substantial amount of star formation is still going on inside it.

NGC 6872 is quite far away. The Principal Galaxy Catalog (which is not entirely reliable when it estimates distances) says that NGC 6872 is about 190 million light years away, which is about three times farther away than the Virgo Cluster.

The bright foreground star has a color index which is almost exactly the same as that of bright and famous yellow star Arcturus.

Image

Arcturus. The star looks a bit red here.

All in all, congratulations, girls, and thanks for a lovely picture!

Ann
Last edited by Ann on Sun Apr 03, 2011 3:57 pm, edited 3 times in total.
Color Commentator

User avatar
Ann
4725 Å
Posts: 13373
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Ann » Sun Apr 03, 2011 6:20 am

contactcameron@yahoo.co.uk wrote:Nice pic, and good reasoning. And to think I founded the astronomy club at Knox in Sydney so many years ago. Good one girls.

I note top above galaxy, far away in distance, what looks like an eliptical galaxy with mutiple cores (bright points). I wonder if this is a tight cluster of galaxies or a case of gravitational lensing. If the second, a particularly exceptional example. A serindipidous discovery perhaps! Just a thought.

Cdownunder.
Well, thanks for founding the club! Congratulations.

As for that galaxy which seems to contain multiple cores, well, those things may be lensed galaxies or something, but they may also be regions of star formation in the large galaxy. But I agree with you that someone should take a better look!

Ann
Color Commentator

User avatar
bystander
Apathetic Retiree
Posts: 21571
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by bystander » Sun Apr 03, 2011 8:19 am

Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

patcon

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by patcon » Sun Apr 03, 2011 10:23 am

Are all spiral galaxies rotating in a clockwise direction and why do they rotate?

User avatar
Ann
4725 Å
Posts: 13373
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Ann » Sun Apr 03, 2011 10:38 am

There is a pretty nice ultraviolet image of NGC 6872 here: http://www.wikisky.org/?object=NGC+6872 ... urce=GALEX It is hard to find the dwarf galaxy here, isn't it?

As for galacitc rotation, it is completely random. Some galaxies rotate clockwise and others anticlockwise. You shold ask Chris more about that.

Ann
Color Commentator

User avatar
owlice
Guardian of the Codes
Posts: 8406
Joined: Wed Aug 04, 2004 4:18 pm
Location: Washington, DC

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by owlice » Sun Apr 03, 2011 10:54 am

patcon wrote:Are all spiral galaxies rotating in a clockwise direction and why do they rotate?
A good discussion of this, complete with Yogi Berra quotes, appears here.
A closed mouth gathers no foot.

User avatar
NoelC
Creepy Spock
Posts: 876
Joined: Sun Nov 20, 2005 2:30 am
Location: South Florida, USA; I just work in (cyber)space
Contact:

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by NoelC » Sun Apr 03, 2011 1:25 pm

A fantastic image and a heart-warming story to go with. It's good to know young folks aren't all wrapped up in hype and fashion, but are thinking about science.

I have a question about the telescope itself... Is the instrument not equatorially mounted? I ask because the spikes emanating from the bright star are rotated, apparently, in the time between red-filtered and green-filtered exposures. Normally in an equatorially mounted telescope one would expect the spikes - side effects of the structure in the telescope itself - to remain oriented the same at all times as compared to the objects in the sky. If this is not the case, and the scope is mounted, say, in an alt-az orientation, then do they rotate the camera during exposures?

-Noel

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18805
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by neufer » Sun Apr 03, 2011 1:53 pm

NoelC wrote:
Is the instrument not equatorially mounted? I ask because the spikes emanating from the bright star are rotated, apparently, in the time between red-filtered and green-filtered exposures. Normally in an equatorially mounted telescope one would expect the spikes - side effects of the structure in the telescope itself - to remain oriented the same at all times as compared to the objects in the sky. If this is not the case, and the scope is mounted, say, in an alt-az orientation, then do they rotate the camera during exposures?
I thought that equatorial mounts were now obsolete with computer control.
Art Neuendorffer

User avatar
Chris Peterson
Abominable Snowman
Posts: 18112
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA
Contact:

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Chris Peterson » Sun Apr 03, 2011 3:07 pm

patcon wrote:Are all spiral galaxies rotating in a clockwise direction and why do they rotate?
"Clockwise" and "counterclockwise" are not physical concepts in the case of galaxies. They have no intrinsic direction of rotation (what would that even mean?) but only an apparent direction, based on the side we see them from. And we see them in equal numbers from either side, so the distribution of apparent rotation direction is equal as well.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
https://www.cloudbait.com

Richard Melton

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Richard Melton » Sun Apr 03, 2011 9:03 pm

This giant galaxy (and many others seen at this site) has an enormous, bright white area in the middle, spanning several thousand lightyears. What is the composition of this area?

User avatar
Chris Peterson
Abominable Snowman
Posts: 18112
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA
Contact:

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Chris Peterson » Sun Apr 03, 2011 9:24 pm

Richard Melton wrote:This giant galaxy (and many others seen at this site) has an enormous, bright white area in the middle, spanning several thousand lightyears. What is the composition of this area?
Stars.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
https://www.cloudbait.com

User avatar
rstevenson
Quis custodiet ipsos custodes?
Posts: 2704
Joined: Fri Mar 28, 2008 1:24 pm
Location: Halifax, NS, Canada

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by rstevenson » Sun Apr 03, 2011 9:43 pm

Richard Melton wrote:This giant galaxy (and many others seen at this site) has an enormous, bright white area in the middle, spanning several thousand lightyears. What is the composition of this area?
The Stars, Like Dust, by Isaac Asimov.

Rob

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18805
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by neufer » Sun Apr 03, 2011 11:43 pm

Image Giant Galaxy NGC 6872

Sydney Girls High School Astronomy Club
Image
Art Neuendorffer

User avatar
Ann
4725 Å
Posts: 13373
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Ann » Mon Apr 04, 2011 1:59 am

Chris Peterson wrote:
Richard Melton wrote:This giant galaxy (and many others seen at this site) has an enormous, bright white area in the middle, spanning several thousand lightyears. What is the composition of this area?
Stars.
The answer is correct, of course, but a bit short.

If you look at this NOAO image of the Andromeda Galaxy and check out the highest resolution, you'll get a good idea of how the stellar composition of a galaxy typically changes from the edges to the center: http://www.noao.edu/image_gallery/html/im0638.html

You can see that the outermost parts are dominated by bright but widely scattered blue stars. As you work your way towards the center of the galaxy, the blue stars generally get a little fainter and less blue, and they get mixed with noticably yellow stars. Then the blue stars seem to disappear, and they are replaced with stars that are yellow, fainter and more and more numerous. Close to the center they are basically all yellow (except for, in some cases, nuclear rings of star formation), and the stellar density goes up a lot. Because the stars are packed so close together (relatively speaking: they are still far apart in absolute terms), most color photos that aim at portraying both the inner and the outer parts of a galaxy will overexpose the inner part, because it is so much brighter. Therefore this overexposed part will look white. In reality, though, it is yellow from the light of old yellow stars.
The yellow center of galaxy M81 is so bright that the center looks white. Even so, I'd say that the arms have been articificially brightened here to make them easier to see. In reality, the yellow bulge of M81 dominates over the arms even more than it appears to do here.
A Hubble image of M81, not at full resolution.

A remarkably different sort of galaxy, NGC 4395:
NGC 4395 all but lacks a yellow population. Almost all the light from it seems to come from blue stars.

The thing to remember is that bright blue stars are young and short-lived. They have formed recently and will disappear soon enough, unless they are replaced with new bright blue stars. Faint yellow stars, certainly when they are packed close together at the center of a galaxy, are old and very long-lived. The reason for this is mass. Massive stars burn bright and blue, but they also burn out quickly and die. Low-mass stars smoulder yellow and red and can hoard their meager energy supplies for billions and billions of years.

Ann
Color Commentator

Jim Leff
Science Officer
Posts: 192
Joined: Thu Dec 09, 2010 2:00 pm

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Jim Leff » Mon Apr 04, 2011 2:59 am

Are there any representations anywhere of what the nightime sky would look like close to a dense galactic center?

Richard Melton

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by Richard Melton » Mon Apr 04, 2011 5:03 am

Mahalo Ann for the detailed explanation. I am incredibly fascinated by this website, but it is new to me so I appreciate the patience with my lack of knowledge in this science.

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18805
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by neufer » Fri Jan 11, 2013 12:57 am

http://www.universetoday.com/99378/ wrote: Behold: The Largest Known Spiral Galaxy
by Nancy Atkinson,Universe Today, January 10, 2013

<<Astronomers have long known that a spectacular barred spiral galaxy named NGC 6872 is a behemoth, but by compiling data from several space- and ground-based observatories and running a few computer simulations, they have now determined this is the largest spiral galaxy we know of.

Measuring tip-to-tip across its two outsized spiral arms, NGC 6872 spans more than 522,000 light-years, making it more than five times the size of our Milky Way galaxy.

“Without GALEX’s ability to detect the ultraviolet light of the youngest, hottest stars, we would never have recognized the full extent of this intriguing system,” said lead scientist Rafael Eufrasio, from the Goddard Space Flight Center the Catholic University of America in Washington. He presented the findings Thursday at the American Astronomical Society meeting in Long Beach, California.

But this galaxy didn’t get so gargantuan all on its own. Astronomers think large galaxies, including our own, grew through mergers and acquisitions — assembling over billions of years by absorbing numerous smaller systems.

The galaxy’s unusual size and appearance stem from its interaction with a much smaller disk galaxy named IC 4970, which has only about one-fifth the mass of NGC 6872. The odd couple is located 212 million light-years from Earth in the southern constellation Pavo. Intriguingly, the gravitational interaction of NGC 6872 and IC 4970 may have done the opposite, spawning what may develop into a new small galaxy.

“The northeastern arm of NGC 6872 is the most disturbed and is rippling with star formation, but at its far end, visible only in the ultraviolet, is an object that appears to be a tidal dwarf galaxy similar to those seen in other interacting systems,” said team member Duilia de Mello, a professor of astronomy at Catholic University.

The researchers used archived data from the Galaxy Evolution Explorer (GALEX) mission, and studied the galaxy across the spectrum using data from the European Southern Observatory’s Very Large Telescope, the Two Micron All Sky Survey, and NASA’s Spitzer Space Telescope.>>
Art Neuendorffer

User avatar
bystander
Apathetic Retiree
Posts: 21571
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Re: APOD: Giant Galaxy NGC 6872 (2011 Apr 03)

Post by bystander » Sat Jan 12, 2013 9:49 pm

neufer wrote:
Behold: The Largest Known Spiral Galaxy
by Nancy Atkinson, Universe Today, January 10, 2013

GALEX Reveals the Largest-Known Spiral Galaxy
NASA | JPL-Caltech | GSFC | GALEX | 2013 Jan 10

Largest Spiral Galaxy Is a Stretch
Science Shot | Govert Schilling | 2013 Jan 11
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

Post Reply