APOD: NGC 6822: Barnard's Galaxy (2021 Dec 02)

[APOD Robot]

NGC 6822: Barnard's Galaxy

Explanation: Grand spiral galaxies often seem to get all the glory, flaunting their young, bright, blue star clusters in beautiful, symmetric spiral arms. But small galaxies form stars too, like nearby NGC 6822, also known as Barnard's Galaxy. Beyond the rich starfields in the constellation Sagittarius, NGC 6822 is a mere 1.5 million light-years away, a member of our Local Group of galaxies. A dwarf irregular galaxy similar to the Small Magellanic Cloud, NGC 6822 is about 7,000 light-years across. Brighter foreground stars in our Milky Way have a spiky appearance. Behind them, Barnard's Galaxy is seen to be filled with young blue stars and mottled with the telltale pinkish hydrogen glow of star forming regions in this deep color composite image.

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[Ann]

First of all, this is a lovely APOD!

Let's try to describe NGC 6822 and, well, "classify" it! Right away we must note that NGC 6822 is a small, light-weight dwarf galaxy, but it does have some interesting structures. Let's compare it with the Large Magellanic Cloud, which is a "small" galaxy but much bigger and heftier than NGC 6822.

Infrared image of the Large Magellanic Cloud. Photo: ESO/VMC Survey.

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NGC6822LRGB1024[1].jpg

NGC 6822. Photo: Dietmar Hager, Eric Benson.

Two things are immediately obvious when you compare the (infrared) image of the Large Magellanic Cloud and Dietmar Hager and Eric Benson's picture of NGC 6822. Both NGC 6822 and the LMC are barred galaxies, with an elongated straight segment running through their centers.

And both the LMC and NGC 6822 have what we might call bar-end enhancements. Things "flare out" at the ends of the bars.

Compare the structures of NGC 6822 and the LMC with the old, (probably) completely non-starforming galaxy NGC 936:

NGC 936. Photo: ESO.

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Candy wrapped in paper.

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Wait, you say, why does NGC 936 look like a round piece of candy wrapped in paper? Where is the bar? Well, the bar grew old and merged with the surrounding disk and there used to be a ring in the center of the bar which also grew old and merged with the surrounding disk. See here:

Spiral galaxy NGC 1512. NASA, ESA, Hubble.

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NGC 4394. ESA/Hubble.

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Note that NGC 1512 has a very bright inner ring (certainly surrounding a supermassive black hole ) at the center of its bar. NGC 4394 also has a ring and a black hole, but there is not a lot of star formation in the ring. Note the enhanced bar ends of both galaxies.

Note, too, how both NGC 1512 and NGC 4394 have outer rings surrounding their bars. It is as if the enhanced star formation at the ends of the bars produced "arms" that curved inwards until they met each other and formed a ring. (Or perhaps they overlap to form the appearance of a ring). Note that the LMC displays large bar end enhancements that curve inwards, as if they they were reaching for each other to try to form a ring.

And NGC 6822? There are things going on at the ends of the bar in NGC 6822, too.

NGC 6822 APOD December 2 2021 annotated.png

1= Young blue bar without any mass concentration in its center

2= Bar-end enhancement

3= Bar-end enhancement

4= The beginnings of a spiral arm (maybe?)

5= Clump of blue stars some distance from the main body of NGC 6822

6= The beginnings of a yellow disk

7= Enhanced star formation at the bar end

So what will happen when NGC 6822 runs out of gas and stops forming stars? (Although I read somewhere that NGC 6822 sits in a rich gas cloud and has a lot of star forming material.) Well, I guess it might turn into one of those innumerable shapeless yellow dwarf spheroidal galaxies, the galactic "plankton" of the Universe!

Dwarf spheroidal galaxy NGC 147. https://upload.wikimedia.org/wikipedia/commons/9/90/NGC_0147_2MASS.jpg.

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That's it for me! Johnny, I tried to insert text into my annotated image, but I wasn't able to produce any text in there!

Ann

[VictorBorun]

7= Enhanced star formation at the bar end

A Benjamin Walliser asks whether the pinkish spheres are SuperNova remnants.
FaceBook commenters answer No, these are all just H II regions, where stars are forming

I wonder if spheres are no tell-tales of SN remnants.

Would not it be logical to suggest that a wild star forming region probably forms a supergiant star that burns through in 1 million years rather than in 10-100 million years of an ordinary blue giant's life and so is quick to make a large SNR sphere to dominate the star forming region's look?

[Ann]

7= Enhanced star formation at the bar end

A Benjamin Walliser asks whether the pinkish spheres are SuperNova remnants.
FaceBook commenters answer No, these are all just H II regions, where stars are forming

I wonder if spheres are no tell-tales of SN remnants.

Would not it be logical to suggest that a wild star forming region probably forms a supergiant star that burns through in 1 million years rather than in 10-100 million years of an ordinary blue giant's life and so is quick to make a large SNR sphere to dominate the star forming region's look?

Victor, there are ways to distinguish between supernova remnants and emission nebulas caused by star formation.

NGC 604: Wall Divides East and West Sides of Cosmic Metropolis. X-ray: NASA/CXC/CfA/R. Tuellmann et al.; Optical: NASA/AURA/STScI

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Chandra Space Telescope wrote:

A new study unveils NGC 604, the largest region of star formation in the nearby galaxy M33, in its first deep, high-resolution view in X-rays. This composite image from Chandra X-ray Observatory data (colored blue), combined with optical light data from the Hubble Space Telescope (red and green), shows a divided neighborhood where some 200 hot, young, massive stars reside.
...
However, there is a difference between the two sides of this bifurcated stellar city. (Rollover the image above or view this separate annotated image for the location of the "wall".) On the western (right) side, the amount of hot gas found in the bubbles corresponds to about 4300 times the mass of the sun. This value and the brightness of the gas in X-rays imply that the western part of NGC 604 is entirely powered by winds from the 200 hot massive stars.
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The situation is different on the eastern (left) side of NGC 604. On this side, the X-ray gas contains 1750 times the mass of the sun and winds from young stars cannot explain the brightness of the X-ray emission. The bubbles on this side appear to be much older and were likely created and powered by young stars and supernovas in the past.

So, in short: There are 200 massive hot young stars on the western (right) side of NGC 604, and the collisions of the fierce winds of these massive stars can explain all of the X-rays in this part of the nebula. But on the eastern (left) side there are few young stars, and their winds are not enough to explain all the X-rays in this part of the nebula. Therefore, a lot of the X-rays here must emanate from supernova remnants inside this part of NGC 604.

Young emission nebulas and hot stars in NGC 6822. Photo: ESO.

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Supernova remnant Cassiopeia A. Image credit: NASA/CXC/SAO.

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The nebulas of NGC 6822 are full of hot bright stars, and look like "typical" emission nebulas. Admittedly they are strangely round, and in the Milky Way we find very few obviously bubble-shaped nebulas. That probably has to do with all the much stronger forces acting on the nebulas that are present in a giant galaxy like the Milky Way compared with the mild forces present in a dwarf galaxy. (I apologize if "forces" is not the correct word here.)

As for supernova remnants, I think they tend to look more "tattered" than emission nebulas, although that might have to do with what wavelengths and filters you use to photograph them.

Ann

[VictorBorun]

so one of two lobes, the more spherical one, is probably an SN remnant in a still active star forming region?

[Ann]

so one of two lobes, the more spherical one, is probably an SN remnant in a still active star forming region?

No, there have been one or more supernovas in the lobe on the left, not in the lobe on the right.

NGC 604 Chandra annotated.png

Ann

[orin stepanek]

NGC6822LRGB.jpg

Have small galaxies like these emerged on their own or maybe were
ripped off from galaxies? Anyway pretty little galaxy!
Of the SMC and LMC; does that stream mean that they are merging
with the Milky Way?

[Ann]

Click to view full size image

Have small galaxies like these emerged on their own or maybe were
ripped off from galaxies? Anyway pretty little galaxy!
Of the SMC and LMC; does that stream mean that they are merging
with the Milky Way?

I have read somewhere that NGC 6822 "is sitting at the bottom of a deep gas well", which is to say that there is plenty of star- and planetforming material in its vicinity. So my guess is that NGC 6822 has formed "in situ", without much (or any) "help" from other galaxies.

I have very recently read that according to Gaia measurements, the LMC is going to collide with the Milky Way in 2 billion years.

Ann

[orin stepanek]

[quote=Ann post

I have read somewhere that NGC 6822 "is sitting at the bottom of a deep gas well", which is to say that there is plenty of star- and planetforming material in its vicinity. So my guess is that NGC 6822 has formed "in situ", without much (or any) "help" from other galaxies.

I have very recently read that according to Gaia measurements, the LMC is going to collide with the Milky Way in 2 billion years.

Ann
[/quote]

Thanks!

[johnnydeep]

Ann wrote: "That's it for me! Johnny, I tried to insert text into my annotated image, but I wasn't able to produce any text in there!"

You seem to have managed ok with a different image in a subsequent post, but here you go:

ngc 6822 annotated.png

[VictorBorun]

NGC 6822 has formed "in situ", without much (or any) "help" from other galaxies.

I have very recently read that according to Gaia measurements, the LMC is going to collide with the Milky Way in 2 billion years.

Let's compare Barnard's Galaxy to globular cluster NGC 2808, a possible core remnant of GAIA-Enceladus-Sausage Galaxy after it had been devoured by Milky Way some 8–11 billion years ago.

Their angle size is the same: 14 minutes.
But Barnard's Galaxy is 50 times farther away: 1.6 Mly / 30 kly .

So one can guess that Barnard's Galaxy will left a core-gone-GC 50 times smaller in diameter than the galaxy.