APOD: Giant Galaxies in Pavo (2023 Jun 23)

[APOD Robot]

Giant Galaxies in Pavo

Explanation: Over 500,000 light years across, NGC 6872 (top right) is a truly enormous barred spiral galaxy, at least 5 times the size of our own very large Milky Way. The appearance of this giant galaxy's distorted and stretched out spiral arms suggests the magnificent wings of a giant bird. Of course its popular moniker is the Condor galaxy. It lies about 200 million light-years distant toward the southern constellation Pavo, the Peacock. Lined with star-forming regions, the distorted spiral arms are due to NGC 6872's gravitational interaction with the nearby smaller galaxy IC 4970, seen just above the giant galaxy's core. The Pavo galaxy group's dominant giant elliptical galaxy, NGC 6876 is below and left of the soaring Condor galaxy.

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[Ann]

Giant Galaxies in Pavo. Image Credit & Copyright: Mike Selby, Observatorio El Sauce

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The elongated spiral galaxy NGC 6872 is truly gigantic, but that is mostly (or only partly?) because its arms have been "unwound" and "stretched out", likely because of tidal interaction with its small satellite galaxy, IC 4970.

Even though NGC 6872 is huge, giant elliptical galaxy NGC 6876 is huger. You can tell at a glance that the truly humongous halo surrounding the elliptical monster almost certainly means that NGC 6876 must contain more stars and very likely much more mass that the elongated spiral.

Note the small spiral galaxy IC 4972 which is "standing up" at the bottom edge of the APOD, near 6 o'clock. You can see the galaxy here:

IC 4972. Credit: Capella Observatory.

According to my software, IC 4972 produces 40% as much light as the Milky Way. If true, that would make it comparable in size to M33, the Triangulum galaxy. But as for NGC 6872, the elongated spiral, it produces 8.6 times as much light as the Milky Way, if we are to believe my software! But giant elliptical NGC 6876 "only" produces 4.6 times as much light as our own galaxy. That would make it only half as bright as spiral galaxy NGC 6872!

That doesn't seem at all likely to me. Of course, we must remember that elliptical galaxies typically have a higher "mass to light ratio" than spirals. They are chock full of small stars which are surprisingly massive but produce very little light, whereas spiral galaxies normally contain some bright young stars that produce an enormous amount of light for their mass.

Well, whatever! It's fun to compare galaxies this way.

Ann

[VictorBorun]

My first thought looking at 2 arms, with straight long outer parts and curved-to-ring short inner parts:

there was an explosion at the centre of the core, shooting out a pair of massive jets.
The 2 jets left 2 star formation lanes.
The differential rotation bended those lanes into spirals, but what were the angular velocity differentials?

Close to the high density core the free fall acceleration was ~r^(-2), so the angular velocity was ~r^(-1.5), so the differential was ~-1.5r^(-2.5)dr
Far from the high density core the free fall acceleration was ~r, so the angular velocity was the same for every r, so the differential was 0

That would explain the arms we see.
That would also tell us that dark matter + barion matter density is pretty constant through the vast spheric cloud containing the 2 long straight parts of the arms

[orin stepanek]

Large indeed; and it looks as though it has captured a smaller Spiro!
I hope it makes a happy marriage! Down below; there is an
Elliptical that has a galaxy in its grasp & below that is another
Spiral!

[johnnydeep]

Ann said:

The elongated spiral galaxy NGC 6872 is truly gigantic, but that is mostly (or only partly?) because its arms have been "unwound" and "stretched out", likely because of tidal interaction with its small satellite galaxy, IC 4970.

Even though NGC 6872 is huge, giant elliptical galaxy NGC 6876 is huger. You can tell at a glance that the truly humongous halo surrounding the elliptical monster almost certainly means that NGC 6876 must contain more stars and very likely much more mass that the elongated spiral.

Battle of the Titans! Before reading Ann's post, I was going to ask how we know that NGC 6876 is the dominant one. Is that attribute based solely on mass? And if so, do we know for sure that NGC 6876 is more massive than NGC 6872? I couldn't find mass estimates for either.

[zendae]

Ann said:

The elongated spiral galaxy NGC 6872 is truly gigantic, but that is mostly (or only partly?) because its arms have been "unwound" and "stretched out", likely because of tidal interaction with its small satellite galaxy, IC 4970.

Even though NGC 6872 is huge, giant elliptical galaxy NGC 6876 is huger. You can tell at a glance that the truly humongous halo surrounding the elliptical monster almost certainly means that NGC 6876 must contain more stars and very likely much more mass that the elongated spiral.

Battle of the Titans! Before reading Ann's post, I was going to ask how we know that NGC 6876 is the dominant one. Is that attribute based solely on mass? And if so, do we know for sure that NGC 6876 is more massive than NGC 6872? I couldn't find mass estimates for either.

This gives a lot of info:
https://www.mdpi.com/2075-4434/5/3/30

[AVAO]

...
The elongated spiral galaxy NGC 6872 is truly gigantic, but that is mostly (or only partly?) because its arms have been "unwound" and "stretched out", likely because of tidal interaction with its small satellite galaxy, IC 4970.
...

Ann

ThanX Ann for your very exciting contribution!

...But I don't think the much smaller satellite galaxy IC 4970 has enough mass for that, even though there are two bright X-ray objects there. I would rather consider the huge NGC 6876 galaxy as the source...

Click to view full size image 1 or image 2

big: https://live.staticflickr.com/65535/529 ... 185d_k.jpg
Credit: X-ray: NASA/CXC/SAO/M.Machacek
Background: composit image - jac berne (flickr)

[Ann]

...
The elongated spiral galaxy NGC 6872 is truly gigantic, but that is mostly (or only partly?) because its arms have been "unwound" and "stretched out", likely because of tidal interaction with its small satellite galaxy, IC 4970.
...

Ann

ThanX Ann for your very exciting contribution!

...But I don't think the much smaller satellite galaxy IC 4970 has enough mass for that, even though there are two bright X-ray objects there. I would rather consider the huge NGC 6876 galaxy as the source...

Click to view full size image 1 or image 2

biggg: https://live.staticflickr.com/65535/529 ... d258_o.jpg
composit image - jac berne (flickr)

That certainly sounds plausible, AVAO!

And thank you , as always, for your images!

Ann

[VictorBorun]

...But I don't think the much smaller satellite galaxy IC 4970 has enough mass for that, even though there are two bright X-ray objects there. I would rather consider the huge NGC 6876 galaxy as the source...

Click to view full size image 1 or image 2

biggg: https://live.staticflickr.com/65535/529 ... d258_o.jpg
composit image - jac berne (flickr)

There are two X-sources in the core! Wow

[johnnydeep]

Ann said:

The elongated spiral galaxy NGC 6872 is truly gigantic, but that is mostly (or only partly?) because its arms have been "unwound" and "stretched out", likely because of tidal interaction with its small satellite galaxy, IC 4970.

Even though NGC 6872 is huge, giant elliptical galaxy NGC 6876 is huger. You can tell at a glance that the truly humongous halo surrounding the elliptical monster almost certainly means that NGC 6876 must contain more stars and very likely much more mass that the elongated spiral.

Battle of the Titans! Before reading Ann's post, I was going to ask how we know that NGC 6876 is the dominant one. Is that attribute based solely on mass? And if so, do we know for sure that NGC 6876 is more massive than NGC 6872? I couldn't find mass estimates for either.

This gives a lot of info:
https://www.mdpi.com/2075-4434/5/3/30

That article is about "The Globular Cluster System of the Galaxy NGC 6876". I didn't see anything specific in it about mass.

[Ann]

Ann said:



Battle of the Titans! Before reading Ann's post, I was going to ask how we know that NGC 6876 is the dominant one. Is that attribute based solely on mass? And if so, do we know for sure that NGC 6876 is more massive than NGC 6872? I couldn't find mass estimates for either.

This gives a lot of info:
https://www.mdpi.com/2075-4434/5/3/30

That article is about "The Globular Cluster System of the Galaxy NGC 6876". I didn't see anything specific in it about mass.

Oh groan, I had just written a long post and I just lost it!!!

Okay, here's the gist of it. NGC 6876 is a giant elliptical galaxy, and such galaxies are typically the brightest galaxies in the Universe. The reason why bright yellow galaxies are so massive is that their light comes almost exclusively from old yellow-orange stars, the overwhelming majority of which are small yellow-orange embers, which are extremely faint for their mass. For example, small red Proxima Centauri, the nearest star from the Earth (apart from the Sun) contains some 12% as much mass as the Sun, but radiates only some 2 parts in a thousand of the Sun's energy, and only 5 parts in a hundred thousand of the Sun's visible light!

By contrast, bright blue R136a1, the brightest star in the Tarantula Nebula in the Large Magellanic Cloud, contains almost 200 times the Sun's mass, but radiates almost 4.7 million times the Sun's luminosity!

Alpha, Beta and Proxima Centauri (circled). Credit: Skatebiker of Wikimedia Commons.

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Brilliant blue R136a1 in the R136 cluster in the Tarantula nebula. Credit: ESO/P. Crowther/C.J. Evans

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Therefore, bright yellow galaxies must be extremely massive, because their light comes from humongous numbers of mostly very faint yellow-orange stars. Wikipedia wrote that M87 in the Virgo cluster may be 200 times as massive as the Milky Way. But large blue sprawling spiral galaxy M101 may be less massive than the Milky Way!

Note that the outstretched arms of NGC 6872 are very blue in this composite image where blue represents ultraviolet light from relatively small numbers of very hot bright stars:

NGC 6872 and NGC 6876. Image credit: Mike Selby.

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NGC 6872 in ultraviolet, visible and 3.6 micron infrared light. Credit: NASA's Goddard Space Flight Center/ESO/JPL-Caltech/DSS

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One thing that makes elliptical galaxy NGC 6876 look really impressive to me is its huge yellow halo. The very three-dimensional nature of the (quite bright) halo means that it contains huge numbers of small stars.

Of course, in the end it is the amount of dark matter that determines which of these two massive galaxies, NGC 6872 or NGC 6876, is the most massive!

Ann

[VictorBorun]

Of course, in the end it is the amount of dark matter that determines which of these two massive galaxies, NGC 6872 or NGC 6876, is the most massive!
Ann

We have to use gravitation lensing to map the dark matter. JWST would see a lot of dim background galaxies…
This little comma-thing in a pentagon of dots in the centre might prove to be one

[johnnydeep]

Thanks, Ann. The diffuse halos around ellipticals are indeed impressive. The more so because they represent hundreds of billions of STARS, and are NOT the mere dust that is prominently visible in spirals and much less massive.