Starship Asterisk*

Barred Spiral Galaxy NGC 6217

Explanation: Many spiral galaxies have bars across their centers. Even our own Milky Way Galaxy is thought to have a modest central bar. Prominently barred spiral galaxy NGC 6217, featured here, was captured in spectacular detail in this image taken by the Advanced Camera for Surveys on the orbiting Hubble Space Telescope in 2009. Visible are dark filamentary dust lanes, young clusters of bright blue stars, red emission nebulas of glowing hydrogen gas, a long bar of stars across the center, and a bright active nucleus that likely houses a supermassive black hole. Light takes about 60 million years to reach us from NGC 6217, which spans about 30,000 light years across and can be found toward the constellation of the Little Bear (Ursa Minor).

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Today's APOD is a nice picture of a small galaxy!


How do we know that NGC 6217 is small? Well, you can almost tell at a glance when you compare it with M61, can't you? Note that the pink emission nebulas in both galaxies are about the same size, their bright, almost white centers are about the same size, the bars (the elongated yellowish structures with dust lanes running through the centers of the galaxies are about the same size, but M61 has a much larger and more massive set of spiral arms.

Both NGC 6217 and M61 have a lot of star formation at their bar ends and in their arms. Actually it is M61 that is the oddball here, because M61 is a large galaxy, and it is quite unusual for large galaxies to have so much star formation in them. I believe that M61 is about the same size as the Milky Way, but it is much brighter due to the high proportion of highly luminous star clusters in its arms. The only large and reasonably nearby galaxy that has more star formation and bluer colors than M61 that I can think of is M101, but M101 looks "flimsier" and less massive than M61, in my opinion. It has a quite small yellow center.


So M61 is a really unusual large galaxy, but NGC 6217 is, I think, fairly typical of a richly starforming specimen of a relatively small galaxy - well, let's call them bantam weight galaxies, so they are hefty enough to be allowed to compete in the Galactic Olympic Games!

Another contender in that weight class in the Galactic Olympic Games may be M83. Actually, M83 looks larger to me than NGC 6217, because of M83's elegant and symmetrical shape and extremely well defined dust lanes:


Can we be sure that M83 really is a smallish (bantam weight) galaxy? Yes, because NASA calls it "our own Milky Way galaxys smaller cousin"! (Hey, I'm not the one that forgot to put the apostrophe in the word "galaxys", I'm just quoting NASA!)

Anyway. Some smallish galaxies experience a fantastic flurry of star formation, which is rarely seen in larger galaxies. Large galaxies have typically undergone so many at least minor mergers and so many episodes of enhanced star formation that they have severely depleted their reservoirs of prime star forming material.

Smallish galaxies, especially those that live relatively isolated (although they may have at least one small companion) may hang on to a lot more gas, particularly in relation to their overall sizes. Just look at this great picture by R. Jay GaBany of "largish" dwarf galaxy NGC 4449 and its tiny little companion (or intruder?):


Wowzers, that's what I call a starburst!

So why is it that so many richly starforming spiral galaxies appear to be barred galaxies? Well... beats me!

Ann

Ann wrote: ↑Mon Feb 21, 2022 6:41 am the bars (the elongated yellowish structures with dust lanes running through the centers of the galaxies)…
a lot of star formation at their bar ends and in their arms

Can't trust my eyes to tell a bar from an ellipsis of a disk seen at an angle. So I looked up wiki

The galaxy is inclined by an angle of 33° to the line of sight along a position angle of 162°…
…bar …along a position angle of 35.97° ± 0.35°

I took it for an instruction to mark the bar and to stretch the pic so as to compensate non-zero angle between the galaxy's rotation axis and our line of sight.
I tried to fit one arm with a spline having a clone of that spline fit the other arm

VictorBorun wrote: ↑Mon Feb 21, 2022 7:58 am

Ann wrote: ↑Mon Feb 21, 2022 6:41 am the bars (the elongated yellowish structures with dust lanes running through the centers of the galaxies)…
a lot of star formation at their bar ends and in their arms

Can't trust my eyes to tell a bar from an ellipsis of a disk seen at an angle. So I looked up wiki

The galaxy is inclined by an angle of 33° to the line of sight along a position angle of 162°…
…bar …along a position angle of 35.97° ± 0.35°

I took it for an instruction to mark the bar and to stretch the pic so as to compensate non-zero angle between the galaxy's rotation axis and our line of sight.
I tried to fit one arm with a spline having a clone of that spline fit the other arm

Click to view full size image

The white dashed line marks the bar. You can tell it's a bar because of the two dust lanes running along it.

At the ends of the bar you often find bar-end enhancement with increased star formation, because more gas - star forming material - typically gathers at the ends of the bar. In NGC 6217, the increased star formation is particularly visible at upper right. At lower left it is much weaker.

And I apologize for using a purple pen to write "Rather weak bar-end enhancement" at lower left, so that you can hardly read it.

Ann

Ann

Ignoring the bar, what's at about 11 o'clock on the rim? Looks like a ring of new stars around a void, as if from an enormous explosion!

Marked here:

How does this Galaxy compare in size to the Milky Way?
Funny how we end up talking about the dippers! Look Ma; no wings!

JohnD wrote: ↑Mon Feb 21, 2022 11:20 am Ignoring the bar, what's at about 11 o'clock on the rim? Looks like a ring of new stars around a void, as if from an enormous explosion!

What is that thing? In my opinion, it is most likely a region of massive star formation. The red ring would be the remnant of the molecular cloud that gave rise to the brilliant cluster that you can see in the middle of that red ring. It's a little bit like the Orion Nebula in NGC 6217!

Admittedly, the Orion Nebula is not all that similar to the red ring with the cluster inside it in NGC 6217. The difference is that the nebula i NGC 6217 is older than the Orion Nebula, and much more of the original molecular cloud in NGC 6217 has been dispersed by the strong stellar winds and photon pressure of the hot bright stars that were born from this cloud. The Orion Nebula is younger, smaller and more compact.

Yes, but we might compare the region of star formation in NGC 6217 with the very large and brilliant cluster and nebula, NGC 604, in M33, the Triangulum Galaxy:


Note that in NGC 604 too, we've got a red bubble of gas with a brilliant cluster of hot stars inside it.

Ann

JohnD wrote: ↑Mon Feb 21, 2022 11:20 am Ignoring the bar, what's at about 11 o'clock on the rim? Looks like a ring of new stars around a void, as if from an enormous explosion!
Marked here: Apod NGC 6217 marked.png

Let's estimate the size.
Coordinates
Position (RA): 16 32 39.18
Position (Dec): 78° 11' 47.82"
Field of view: 2.20 x 2.74 arcminutes
Orientation: North is 0.1° right of vertical
So the width of the posted pic is 2.2 arcminutes and at the distance of 67.2 Mly the width of the pic is 43,000 ly and the ring is some 2,000 ly in diameter A large thing but Magellanic Clouds have some stellar clusters that large, do not they

Ann wrote: ↑Mon Feb 21, 2022 2:15 pm Yes, but we might compare the region of star formation in NGC 6217 with the very large and brilliant cluster and nebula, NGC 604, in M33, the Triangulum Galaxy:

NGC 604 in M33 Hubble and Rudy Pohl.png

NGC 604 in galaxy M33. Image: Hubble and Rudy Pohl.

Note that in NGC 604 too, we've got a red bubble of gas with a brilliant cluster of hot stars inside it.
Ann

and the radius of 760 ly is fairly close to the radius almost 1,000 ly of the ring

In the upper left corned of this excellent image, I note two further galaxies. Associated with them are
two arc-like structures of red and blue star like objects. Are these artifacts of the imaging process, or could
they be produced by gravitational lensing?

Patrick70 wrote: ↑Tue Feb 22, 2022 6:31 am In the upper left corned of this excellent image, I note two further galaxies. Associated with them are
two arc-like structures of red and blue star like objects. Are these artifacts of the imaging process, or could
they be produced by gravitational lensing?

You mean these?

I don't think so. They don't look like lensed galaxies to me.


As you can see from the Hubble image, lensed background galaxies are seen to "curve around" a source of great mass, typically a galaxy cluster. Of course, more lightweight sources also curve space around them and could cause lensing. But we can see no possible source that would curve space in the direction of the funny-looking objects that you note. Also, the strange objects don't "curve in the same direction".

Gravitational lensing can cause one point source to be multiplied into several point sources. But to my knowledge, lensed point sources would not line up the way the strange point sources do in this APOD.

Ann

JohnD wrote: ↑Mon Feb 21, 2022 11:20 am Ignoring the bar, what's at about 11 o'clock on the rim? Looks like a ring of new stars around a void, as if from an enormous explosion!

There actually might have been a supernova here. Take a look at two versions of NGC 604 here:


There are actually two bubbles in NGC 604 in the Triangulum galaxy. See here what NASA wrote about the two bubbles:

NASA/Chandra 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...

Throughout the cosmic metropolis, giant bubbles in the cool dust and warm gas are filled with diffuse, multi-million degree gas that emits X-rays...

However, there is a difference between the two sides of this bifurcated stellar city...

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...

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 as you can see, supernovas probably contributed to the creation of the bubble on the left in NGC 604. Therefore, a supernova may also have contributed to the bubble in NGC 6217.

However, it seems clear to me that the obvious young massive cluster inside has contributed a lot, and in my opinion probably more, to the shaping of the bubble in NGC 6217.

Ann

Thank you, Ann! And thank you, Victor, wo estimated the bubble to be 750 Light Years across. The Local Bubble that contains the Solar System is about 1000 LY across, and is suggested to have been formed by supernovae, but is largely empty of gas, unlike those in NGC 604. https://www.newscientist.com/article/23 ... irst-time/
and https://www.nature.com/articles/s41586-021-04286-5

Like the bubble in 6217, ours is surrounded by new star formation. Is that where the gas has gone, and how?

Ann wrote: ↑Tue Feb 22, 2022 8:19 am

Patrick70 wrote: ↑Tue Feb 22, 2022 6:31 am In the upper left corned of this excellent image, I note two further galaxies. Associated with them are
two arc-like structures of red and blue star like objects. Are these artifacts of the imaging process, or could
they be produced by gravitational lensing?

Detail from APOD February 21 2022.png

...
I don't think so. They don't look like lensed galaxies to me.
...
Ann

I agree that it doesn't look at all like any GL is involved here. But this then begs the question of just what ARE those short arcs of red and blue clusters? They look exactly like parts of the larger NGC 6217 galaxy proper, and so would also seem to be "young clusters of bright blue stars, red emission nebulas of glowing hydrogen gas". But if so, what are they doing do far away?

EDIT: ok, I convinced myself that they really are just far flung clusters and nebulas of NGC 6217, and seem to be the tail end of the long spiral arc of similar clumps that extends from right to left toward the top of this image.