APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

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APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by APOD Robot » Sat Nov 28, 2020 5:05 am

Image 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
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Re: APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by Ann » Sat Nov 28, 2020 7:28 am

I'm very glad to see this APOD, and Martin Pugh and Mark Hanson are two of my favorite astrophotographers! :D

Also NGC 6822 is very interesting in itself, as a dwarf irregular galaxy. To me, however, NGC 6822 is clearly a tiny barred dwarf galaxy trying to grow into a tiny spiral galaxy!

NGC-6822_1024[1].jpg
"Barred" dwarf galaxy NGC 6822. Photo: Martin Pugh/Mark Hanson.
























In my opinion, it's clearly a bar that we see running diagonally from upper left to lower right in NGC 6822, just like the bar (the bright central elongated structure) in NGC 1073.

The main difference between NGC 6822 and NGC 1073 is, of course, that the former galaxy is so flimsy! As you can see, NGC 6822 has no central brightening. This is something it has in common with the Large Magellanic Cloud, another small (but not extremely dwarfish) barred galaxy.

heic0411d[1].jpg
The Large Magellanic Cloud. Photo: Eckhard Slawik.






















Let's look at some differences and similarities of these three galaxies:

1) All are barred.

2) But the bar of NGC 6822 is dominated by blue stars which are probably similar to Regulus and Vega, mid B-type to early A-type stars. The bar of the Large Magellanic Cloud is dominated, I would say, by stars similar to Altair, Procyon and even the Sun: late A-type, F-type and G-type stars. The bar of NGC 1073 is dominated by stars cooler than the Sun, like Pollux. But there is a large patch of bright star formation in the bar of NGC 1073, where hot bright stars are being born.

3) Note, too, that the bluest bar is the faintest and the yellowest bar is the brightest. The blue bar is faintest, because it contains the smallest number of stars, while the yellow bar is brightest, because it contains the largest number of stars. Also, galactic bars are usually yellow in color and crammed full of old stars.

4) All three galaxies display star formation outside the main bar. In NGC 1073, the brightest star formation is probably in the bar. In NGC 6822 and the LMC, most of the star formation is outside the bar.

5) NGC 1073 has a bright center (and a galactic core) in its bar. NGC 6822 and the LMC lack a bright center in their bars.

6) NGC 1073 clearly has spiral arms. The LMC has "the beginnings of" a spiral arm on both sides of its bar. Note the rounded soft arc shape at the left end of the bar, and the extended "fluff" at the right end of the bar.

7) NGC 6822 lacks spiral arms. Note, however, the blue "tail" pointing downwards from the lower right end of the bar of NGC 6822. This might just possibly be the beginning of a spiral arm, although I doubt that it will ever really grow into one.

8) The bright pink emission nebulas at top in the picture of NGC 6822 could be a "counterpart" to the blue tail at the bottom of the image. Both features could be rudiments of spiral arms.

9) In NGC 6822, most of the bright pink emission nebulas probably mark the perimeter of the galaxy's disk. If I am right about that, and if NGC 6822 has a disk in the first place, it would imply that this little galaxy displays some rotation in a preferred direction.

10) Of the three galaxies, NGC 6822 is clearly the flimsiest and least massive. The Large Magellanic Cloud is the "middle child", and NGC 1073 is the most massive of the three. But it is a lot less massive than the Milky Way.

Ann
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Re: APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by orin stepanek » Sat Nov 28, 2020 12:46 pm

Like the Ugly Duckling trying to grow into a Beautiful Swan! :mrgreen:
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Re: APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by johnnydeep » Sat Nov 28, 2020 9:55 pm

Thank you Ann for that nice write up! NGC 6822 is indeed quite a pretty little galaxy. Heretofore I had thought that the only interesting nearby galaxies were the SMC, the LMC, and of course, Andromeda. But I had forgotten my rule that with astronomical objects, there truly ARE no uninteresting things!
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Re: APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by heehaw » Sat Nov 28, 2020 10:53 pm

Pretty ratty looking galaxy! Get out of town!

sillyworm 2

Re: APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by sillyworm 2 » Sun Nov 29, 2020 12:08 am

Can anyone elaborate on the mechanics that may turn NGC 6822 into a spiral?

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Re: APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by Ann » Sun Nov 29, 2020 6:07 am

sillyworm 2 wrote:
Sun Nov 29, 2020 12:08 am
Can anyone elaborate on the mechanics that may turn NGC 6822 into a spiral?
More mass?

I'm pretty sure I once read that NGC 6822 is sitting a deep well of hydrogen gas (and presumably dark matter).

If NGC 6822 started accreting some of that matter (and assuming my memory serves me right and assuming the informant who claimed that there is a large reservoir of gas surrounding NGC 6822 was right in the first place), then NGC 6822 might grow up! :mrgreen:

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Re: APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by Ann » Sun Nov 29, 2020 6:53 am

johnnydeep wrote:
Sat Nov 28, 2020 9:55 pm
Thank you Ann for that nice write up! NGC 6822 is indeed quite a pretty little galaxy. Heretofore I had thought that the only interesting nearby galaxies were the SMC, the LMC, and of course, Andromeda. But I had forgotten my rule that with astronomical objects, there truly ARE no uninteresting things!
Thanks, Johnny! :D I actually think that NGC 6822 is pretty interesting as dwarf galaxies go.

Many other irregular dwarf galaxies, like for example IC 1613, are less interesting. That galaxy hasn't got a lot to show for itself, except one huge clump of star formation in one of its "corners". Perhaps I could describe IC 1613 as a tiny, skinny, emaciated, anorexic version of the Large Magellanic Cloud, where the swollen "boil" of star formation in IC 1613 "in its left upper end" would correspond to the Tarantula Nebula and R136 in a similar location in the Large Magellanic Cloud! :D

heic0411d[1].jpg
The Large Magellanic Cloud with the huge red Tarantula Nebula to the east (left).
Photo: Eckhard Slawik.























And what could I say about the Fornax dwarf, a small faint blob with no young stars, no dust clouds and no star formation at all?

I would post this picture of the Fornax dwarf with its six globular clusters and be done! :D

Ann
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Re: APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by johnnydeep » Sun Nov 29, 2020 8:05 pm

Ann wrote:
Sun Nov 29, 2020 6:53 am
johnnydeep wrote:
Sat Nov 28, 2020 9:55 pm
Thank you Ann for that nice write up! NGC 6822 is indeed quite a pretty little galaxy. Heretofore I had thought that the only interesting nearby galaxies were the SMC, the LMC, and of course, Andromeda. But I had forgotten my rule that with astronomical objects, there truly ARE no uninteresting things!
Thanks, Johnny! :D I actually think that NGC 6822 is pretty interesting as dwarf galaxies go.

And what could I say about the Fornax dwarf, a small faint blob with no young stars, no dust clouds and no star formation at all?

I would post this picture of the Fornax dwarf with its six globular clusters and be done! :D
Well, it does have six globular clusters! I'm finding the Sculptor Dwarf Elliptical to be visually even less interesting:


Though Wikipedia says it's metalicity might be noteworthy. From https://en.wikipedia.org/wiki/Sculptor_ ... etallicity :
Metallicity
The metallicity of Sculptor dwarf appears to be broken up into two distinct groups, one with [Fe/H] = -2.3 and the other with [Fe/H] = -1.5.[7] Similar to many of the other Local Group galaxies, the older metal-poor segment appears more extended than the younger metal-rich segment.[8]
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Re: APOD: NGC 6822: Barnard's Galaxy (2020 Nov 28)

Post by Ann » Sun Nov 29, 2020 9:22 pm

johnnydeep wrote:
Sun Nov 29, 2020 8:05 pm
Ann wrote:
Sun Nov 29, 2020 6:53 am
johnnydeep wrote:
Sat Nov 28, 2020 9:55 pm
Thank you Ann for that nice write up! NGC 6822 is indeed quite a pretty little galaxy. Heretofore I had thought that the only interesting nearby galaxies were the SMC, the LMC, and of course, Andromeda. But I had forgotten my rule that with astronomical objects, there truly ARE no uninteresting things!
Thanks, Johnny! :D I actually think that NGC 6822 is pretty interesting as dwarf galaxies go.

And what could I say about the Fornax dwarf, a small faint blob with no young stars, no dust clouds and no star formation at all?

I would post this picture of the Fornax dwarf with its six globular clusters and be done! :D
Well, it does have six globular clusters! I'm finding the Sculptor Dwarf Elliptical to be visually even less interesting:


Though Wikipedia says it's metalicity might be noteworthy. From https://en.wikipedia.org/wiki/Sculptor_ ... etallicity :
Metallicity
The metallicity of Sculptor dwarf appears to be broken up into two distinct groups, one with [Fe/H] = -2.3 and the other with [Fe/H] = -1.5.[7] Similar to many of the other Local Group galaxies, the older metal-poor segment appears more extended than the younger metal-rich segment.[8]
Touché, Johnny! :D

Ann
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