APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

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APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by APOD Robot » Sun Jan 24, 2021 5:05 am

Image Massive Nearby Spiral Galaxy NGC 2841

Explanation: It is one of the more massive galaxies known. A mere 46 million light-years distant, spiral galaxy NGC 2841 can be found in the northern constellation of Ursa Major. This sharp view of the gorgeous island universe shows off a striking yellow nucleus and galactic disk. Dust lanes, small, pink star-forming regions, and young blue star clusters are embedded in the patchy, tightly wound spiral arms. In contrast, many other spirals exhibit grand, sweeping arms with large star-forming regions. NGC 2841 has a diameter of over 150,000 light-years, even larger than our own Milky Way. The featured composite image merges exposures from the orbiting 2.4-meter Hubble Space Telescope and the ground-based 8.2-meter Subaru Telescope. X-ray images suggest that resulting winds and stellar explosions create plumes of hot gas extending into a halo around NGC 2841.

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by VictorBorun » Sun Jan 24, 2021 6:09 am

trying to imagine a 3d, it's easy to see dusty outskirts as wide tyre around a thin plate wheel.

The part near the core is more puzzling. There surely is a large ghostly shine bulge through which we see the far side of the arms' ends.
But what are the dusty lanes seen very close to the core due to the backlight of the core?
They look like lying strictly in the plane.
How come the dust lanes obey the flattening viscosity of the gas disk and the stellar population of bulge does not?
Is dust more glued into near-core thinning gas then the stars?

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by VictorBorun » Sun Jan 24, 2021 6:44 am

A funny thing with this galaxy: its axis is at 68° to the line of sight from the Earth, or we may say its plane is at 22° from being edge-on.
Now that is just the slope of the dusty outskirts looming above the plane of disk.
So the part of the ridge closest to us, at 5 o'clock, has its suface just touching our line of sight. The dust becomes ink black in this view.

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Ann » Sun Jan 24, 2021 7:36 am

VictorBorun wrote:
Sun Jan 24, 2021 6:09 am
trying to imagine a 3d, it's easy to see dusty outskirts as wide tyre around a thin plate wheel.

The part near the core is more puzzling. There surely is a large ghostly shine bulge through which we see the far side of the arms' ends.
But what are the dusty lanes seen very close to the core due to the backlight of the core?
They look like lying strictly in the plane.
How come the dust lanes obey the flattening viscosity of the gas disk and the stellar population of bulge does not?
Is dust more glued into near-core thinning gas then the stars?























I'm not absolutely sure what you're asking here, Victor, mostly because I don't speak Math. But bulges are typically very different from dusty disks.

An aspect that is interesting to me is the way the dust lanes on one side of the bulge look dark and stand out sharply, while the dust lanes on the other side of the bulge are almost invisible. This is very obvious in the picture of NGC 2841, and it can be seen in the picture of M81, too.

My guess, for what it's worth, is that the dust on one side of the bulge of a galaxy that is tilted to our line of sight is blocking light from the bulge behind it, so that it becomes a "dark nebula".

The dust on the other side of the bulge (the side that is farther away from us) is actually weakly lit up by light from the bulge, which is typically brighter than the disk outside it. This makes the dust on the side that is farther away from us "disappear".


There are two more pictures of NGC 2841 that I want to show you.

























The image on the left is an SDSS picture of NGC 2841. I like SDSS images because I find them "honest". They normally use the same filters, g-r-i. The "g" filter picks out light from blue stars, the "r" filter reacts to normal red light and to H-alpha emission nebulas, and the "i" filter is sensitive to, mostly, near infrared light.

This means that in SDSS images, clusters of young stars will look blue, emission nebulas will look green, and old stars will look yellow.

Now look at the SDSS picture of NGC 2841. As you can see, it looks "overwhelmingly yellow".

Conclusion? There are young blue stars and emission nebulas in NGC 2841. Yes, there are. But the contribution they make to the overall light from NGC 2841 is vanishingly small.


Now take a look at the NASA/GALEX/Wikisky picture of NGC 2841, which shows the galaxy in ultraviolet light emitted by hot stars. As you can see, there are indeed some hot stars in NGC 2841.

What I find most interesting about the ultraviolet image (apart from the fact that it reveals outer relatively ultraviolet arms or rings of NGC 2841) is that it seems ro reveal remnants of an "inner ring" around the bulge of NGC 2841. Can you see it? There are two dark parallel lines on both sides of the bulge of NGC 2841. I think they might be remnants of a ring.

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by VictorBorun » Sun Jan 24, 2021 8:32 am

and what about visibiliy of the dust in say 1 kly from us just outside the disk?
Is it dark against the galaxy bulge?
Does it disappear the other way round, where it catch the light from the core of the Milky Way and is as bright as its background of the distant stellar haze?
Or are we too far from too small a bulge for such play?

It seems we are too far from the bulge of the Milky Way for its light to brighten the dust outside the disk and against the background of distant stellar haze.

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Ann » Sun Jan 24, 2021 10:42 am

VictorBorun wrote:
Sun Jan 24, 2021 8:32 am
and what about visibiliy of the dust in say 1 kly from us just outside the disk?
Is it dark against the galaxy bulge?
Does it disappear the other way round, where it catch the light from the core of the Milky Way and is as bright as its background of the distant stellar haze?
Or are we too far from too small a bulge for such play?

It seems we are too far from the bulge of the Milky Way for its light to brighten the dust outside the disk and against the background of distant stellar haze.


















The bulges of galaxies are often really quite bright, and the disk is typically rather faint. Therefore, if a dust lane blocks light from the bulge, the dust lane will look dark. But if the dust lane is located on the other side of the bulge, it will likely reflect some of the light that reaches it from the bulge, and therefore it will look fainter and much less dark.

In the picture of the Andromeda Galaxy by Peter Lawrence(?), we can easily see the dust lanes on the near side of the bulge, but it is much harder to see the dust lanes on the far side of the bulge.

Note that in the picture of Andromeda by Szendrői-Gábor, where the disk has been brightened by "stretching" so that we can indeed see dust lanes on the far side of the bulge, these dust lanes there still look fainter and less dark than the dust lanes on the near side of the bulge.

As for the Milky Way, I believe that it has a very thick dust lane. In any case, ours is a really big galaxy and we are located inside it. We have no chance of seeing the far side of our galaxy in optical light.

(At least not now. Maybe, in 50 million years or so, the "bobbing" motion of the Solar system in its orbit around the center of the Milky Way may have brought the Earth so high above the plane of our galaxy that we can actually see not just parts of the other side of the Milky Way but also the very center of our galaxy.)

Remind our great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great-great--etcetera-etcetera-etcetera-grandchildren to send a postcard home and tell us about the Milky Way splendors they have seen!

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by JohnD » Sun Jan 24, 2021 11:24 am

I think this may be one for Anne, but did something go wrong with the way this photo was printed?
Quote from blurb, "Dust lanes, small, pink star-forming regions, and young blue star clusters are embedded in the patchy, tightly wound spiral arms", when the whole galaxy looks pink, with precious few blue stars!

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by VictorBorun » Sun Jan 24, 2021 11:55 am

"stretching" the contrast for easier look at dust

1) I think there is an ink-dark dust lane at 11 o'clock outside the ring that represents the plane of the Milky Way.
It's dark against the bulge.
2) I think there is ghostly smitherings at 6 o'clock outside the ring that represents the plane of the Milky Way.
Maybe they not so dark.

Pixels are like this:
1) dust RGB=(48 52 59) aganst the bulge RGB=(60 65 74)
2) dust RGB=(60 64 70) aganst the stellar haze above the disk RGB=(52 52 58) — dust is somewhat brighter than the background

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Ann » Sun Jan 24, 2021 12:00 pm

JohnD wrote:
Sun Jan 24, 2021 11:24 am
I think this may be one for Anne, but did something go wrong with the way this photo was printed?
Quote from blurb, "Dust lanes, small, pink star-forming regions, and young blue star clusters are embedded in the patchy, tightly wound spiral arms", when the whole galaxy looks pink, with precious few blue stars!

John
The overall "pink" (or deep apricot) color is "wrong". Or we may say that pink color is an aesthetic choice.

















The "true" overall color of NGC 2841 is yellow-white. That is to say, the overall B-V color of NGC 2841 is a bit yellower than the B-V color of the Sun.

The B-V of the Sun is 0.628 ± 0.011. The B-V of NGC 2841 is 0.870. That makes NGC 2841 a little yellower than the Sun. It makes NGC 2841 a little yellower than bright star Capella, too, because the B-V index of Capella is 0.795. But it makes NGC 2841 a little whiter than Pollux, because the B-V of Pollux is 0.991.

Anyway. The surface brightness of NGC 2841 is so low that it will always look gray through a telescope. When astronomers measure the wavelengths of the light it emits, they can conclude that we would see NGC 2841 as yellow-white, if its surface brightness was much, much, much higher.

Oh, and... you know what? I'm going to have a guess. What is the B-V color of the Milky Way?

It wouldn't surprise me if the color of our galaxy is similar to the B-V of NGC 2841. Because our galaxy, too, has huge numbers of cool stars like Capella, Pollux and Arcturus. And it really hasn't that many hot blue stars to dilute its yellow color.

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by orin stepanek » Sun Jan 24, 2021 1:07 pm

ngc2841_hstColombari_960.jpg

The galaxy looks like a perfect wheel! The central region is huge; must
have gigantic black hole! :roll:
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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Chris Peterson » Sun Jan 24, 2021 2:39 pm

VictorBorun wrote:
Sun Jan 24, 2021 6:09 am
trying to imagine a 3d, it's easy to see dusty outskirts as wide tyre around a thin plate wheel.

The part near the core is more puzzling. There surely is a large ghostly shine bulge through which we see the far side of the arms' ends.
But what are the dusty lanes seen very close to the core due to the backlight of the core?
They look like lying strictly in the plane.
How come the dust lanes obey the flattening viscosity of the gas disk and the stellar population of bulge does not?
Is dust more glued into near-core thinning gas then the stars?
I don't think there's anything unusual about the shape of this galaxy. It's your basic spherical (or oblate spherical) central bulge surrounded by a thin, flat disc of stars, gas, and dust. Nothing is backlit, nothing is sidelit, nothing is transparent or translucent. To the extent you might be seeing those things, they are optical illusions.
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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by De58te » Sun Jan 24, 2021 3:10 pm

@John D, I see a lot of blue star clusters, particularly in the arms at the top of the photo. You just got to view the enlarged image. In the small image it does look overall pink with a yellow nucleus. Blue stars are scattered all over the arms with the exception of one solid pink arm, second from the foreground spiral. This seems to be at least an area some 50,000 light years long (if the entire galaxy is 150,000 light years large ) without any blue clusters at all.

Now I have a question about observation. As already mentioned the galaxy is tilted from our viewpoint. At first I thought we are viewing it from above and the closer arms to us are the ones at the bottom of the image. Then I glanced away for awhile and looking again I had the impression that I am looking from below and the galaxy is above us like a ceiling fixture and the arms at the top are the ones closer to us. So which is it? Is there an astronical way of measuring which of the arms are closer to us?

Also at the west end of the yellow nucleus there seems to be a black spot where presumably there shouldn't be a black spot in the bright nucleus. Given the size of the galaxy I would guess the black spot is more than 10 light years wide at least. What could it be? A telescope glitch, a black spot in the Milky Way foreground, or maybe the first visible dark matter caught on camera?

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Lasse H » Sun Jan 24, 2021 3:17 pm

Certainly an impressive galaxy! But what is the little black dot doing there in the middle?
Looking at the original photo (that you always are able to see by clicking on the APOD), I found a "black star" to the left of the shiny core.
It helps to magnify the photo first.
Starting from the center, it can be seen about 1/4 of the way out towards the edge in the lower left of the picture. We are still within the hazy gray area, and there are some normal "white stars" near it.

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Chris Peterson » Sun Jan 24, 2021 3:20 pm

De58te wrote:
Sun Jan 24, 2021 3:10 pm
@John D, I see a lot of blue star clusters, particularly in the arms at the top of the photo. You just got to view the enlarged image. In the small image it does look overall pink with a yellow nucleus. Blue stars are scattered all over the arms with the exception of one solid pink arm, second from the foreground spiral. This seems to be at least an area some 50,000 light years long (if the entire galaxy is 150,000 light years large ) without any blue clusters at all.

Now I have a question about observation. As already mentioned the galaxy is tilted from our viewpoint. At first I thought we are viewing it from above and the closer arms to us are the ones at the bottom of the image. Then I glanced away for awhile and looking again I had the impression that I am looking from below and the galaxy is above us like a ceiling fixture and the arms at the top are the ones closer to us. So which is it? Is there an astronical way of measuring which of the arms are closer to us?

Also at the west end of the yellow nucleus there seems to be a black spot where presumably there shouldn't be a black spot in the bright nucleus. Given the size of the galaxy I would guess the black spot is more than 10 light years wide at least. What could it be? A telescope glitch, a black spot in the Milky Way foreground, or maybe the first visible dark matter caught on camera?
I think the lower edge is closer. We seem to be looking through less dust there.

Dark matter isn't dark. It is invisible. It can't be caught on a camera. (There are lots of similar black dots around the image. Almost certainly processing artifacts.)
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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by JohnD » Sun Jan 24, 2021 3:31 pm

Thank you, Anne, I wondered if like smell and taste, colour vision abberation might be an early sign of Covid!

D58te, I refer you to the previous reply from Anne the Colour Queen!
And with LasseH, do you refer to the Black Dot at 8 o'clock, on the edge of the central clearance? As you say, it must be ENORMOUS!

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by johnnydeep » Sun Jan 24, 2021 4:08 pm

APOD Robot wrote:
Sun Jan 24, 2021 5:05 am
Image Massive Nearby Spiral Galaxy NGC 2841

Explanation: It is one of the more massive galaxies known. A mere 46 million light-years distant, spiral galaxy NGC 2841 can be found in the northern constellation of Ursa Major. This sharp view of the gorgeous island universe shows off a striking yellow nucleus and galactic disk. Dust lanes, small, pink star-forming regions, and young blue star clusters are embedded in the patchy, tightly wound spiral arms. In contrast, many other spirals exhibit grand, sweeping arms with large star-forming regions. NGC 2841 has a diameter of over 150,000 light-years, even larger than our own Milky Way. The featured composite image merges exposures from the orbiting 2.4-meter Hubble Space Telescope and the ground-based 8.2-meter Subaru Telescope. X-ray images suggest that resulting winds and stellar explosions create plumes of hot gas extending into a halo around NGC 2841.
"It is one of the more massive galaxies known." Really? The Milky Way seems to be MUCH more massive! From https://en.wikipedia.org/wiki/Milky_Way:
Mass (0.8–1.5)×1012 M☉
Number of stars 100-400 billion
Size Stellar disk: 185 ± 15 kly
Dark matter halo: 1.9 ± 0.4 Mly (580 ± 120 kpc)
And from https://en.wikipedia.org/wiki/NGC_2841 (note: the article doesn't list size or number of stars):
Mass 7×1010 M☉
So, the Milky Way would seem to be at least 10 times as massive as NGC 2841.
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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Chris Peterson » Sun Jan 24, 2021 4:12 pm

johnnydeep wrote:
Sun Jan 24, 2021 4:08 pm
APOD Robot wrote:
Sun Jan 24, 2021 5:05 am
Image Massive Nearby Spiral Galaxy NGC 2841

Explanation: It is one of the more massive galaxies known. A mere 46 million light-years distant, spiral galaxy NGC 2841 can be found in the northern constellation of Ursa Major. This sharp view of the gorgeous island universe shows off a striking yellow nucleus and galactic disk. Dust lanes, small, pink star-forming regions, and young blue star clusters are embedded in the patchy, tightly wound spiral arms. In contrast, many other spirals exhibit grand, sweeping arms with large star-forming regions. NGC 2841 has a diameter of over 150,000 light-years, even larger than our own Milky Way. The featured composite image merges exposures from the orbiting 2.4-meter Hubble Space Telescope and the ground-based 8.2-meter Subaru Telescope. X-ray images suggest that resulting winds and stellar explosions create plumes of hot gas extending into a halo around NGC 2841.
"It is one of the more massive galaxies known." Really? The Milky Way seems to be MUCH more massive! From https://en.wikipedia.org/wiki/Milky_Way:
Mass (0.8–1.5)×1012 M☉
Number of stars 100-400 billion
Size Stellar disk: 185 ± 15 kly
Dark matter halo: 1.9 ± 0.4 Mly (580 ± 120 kpc)
And from https://en.wikipedia.org/wiki/NGC_2841 (note: the article doesn't list size or number of stars):
Mass 7×1010 M☉
So, the Milky Way would seem to be at least 10 times as massive as NGC 2841.
Well, the Milky Way is very much at the high end in terms of massive galaxies.
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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Lasse H » Sun Jan 24, 2021 5:16 pm

JohnD wrote:
Sun Jan 24, 2021 3:31 pm

And with LasseH, do you refer to the Black Dot at 8 o'clock, on the edge of the central clearance? As you say, it must be ENORMOUS!
Yes, I think you mean the same object as I do. I didn't say it was enormous but it must be, if it is an object belonging to the galaxy. Near to it there are about 10 similar sized, round objects, but they are white and look like stars. So, what is the "black sheep" doing there?

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by johnnydeep » Sun Jan 24, 2021 5:38 pm

Chris Peterson wrote:
Sun Jan 24, 2021 4:12 pm
johnnydeep wrote:
Sun Jan 24, 2021 4:08 pm
APOD Robot wrote:
Sun Jan 24, 2021 5:05 am
Image Massive Nearby Spiral Galaxy NGC 2841

Explanation: It is one of the more massive galaxies known. A mere 46 million light-years distant, spiral galaxy NGC 2841 can be found in the northern constellation of Ursa Major. This sharp view of the gorgeous island universe shows off a striking yellow nucleus and galactic disk. Dust lanes, small, pink star-forming regions, and young blue star clusters are embedded in the patchy, tightly wound spiral arms. In contrast, many other spirals exhibit grand, sweeping arms with large star-forming regions. NGC 2841 has a diameter of over 150,000 light-years, even larger than our own Milky Way. The featured composite image merges exposures from the orbiting 2.4-meter Hubble Space Telescope and the ground-based 8.2-meter Subaru Telescope. X-ray images suggest that resulting winds and stellar explosions create plumes of hot gas extending into a halo around NGC 2841.
"It is one of the more massive galaxies known." Really? The Milky Way seems to be MUCH more massive! From https://en.wikipedia.org/wiki/Milky_Way:
Mass (0.8–1.5)×1012 M☉
Number of stars 100-400 billion
Size Stellar disk: 185 ± 15 kly
Dark matter halo: 1.9 ± 0.4 Mly (580 ± 120 kpc)
And from https://en.wikipedia.org/wiki/NGC_2841 (note: the article doesn't list size or number of stars):
Mass 7×1010 M☉
So, the Milky Way would seem to be at least 10 times as massive as NGC 2841.
Well, the Milky Way is very much at the high end in terms of massive galaxies.
Really? I've been trying to find a graph of galaxy mass versus frequency to no avail. Is the MW on the high end of spiral galaxy mass? What about if elliptical galaxies are also included? Is it even the most massive galaxy in the local group? WikiPedia says Andromeda is a close contender at (1.5±0.5)×1012 M☉ (from https://en.wikipedia.org/wiki/Andromeda_Galaxy)

I do know dwarf galaxies make up at least 95% (19 out of 20) or so of all galaxies. Or so the Atlas Of the Universe site implies at all size scales (e.g., http://www.atlasoftheuniverse.com/virgo.html). And https://www.quantamagazine.org/the-prob ... y-20190109 says there may be even more:
Bullock argues that around 1,000 tiny galaxies likely orbit every large galaxy like the Milky Way. In addition, dwarf galaxies also overrun the vast stretches of seemingly empty space between large galaxies. As such, there could be as many as 100,000 dwarf galaxies for every large galaxy in the universe.

Today, astronomers estimate that there are likely 100 billion galaxies across the observable universe. But these are Milky-Way-size galaxies. If you extend the count down to these mini-galaxies, then there might be more like 10 million billion galaxies in all.
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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Chris Peterson » Sun Jan 24, 2021 5:42 pm

johnnydeep wrote:
Sun Jan 24, 2021 5:38 pm
Chris Peterson wrote:
Sun Jan 24, 2021 4:12 pm
johnnydeep wrote:
Sun Jan 24, 2021 4:08 pm


"It is one of the more massive galaxies known." Really? The Milky Way seems to be MUCH more massive! From https://en.wikipedia.org/wiki/Milky_Way:



And from https://en.wikipedia.org/wiki/NGC_2841 (note: the article doesn't list size or number of stars):



So, the Milky Way would seem to be at least 10 times as massive as NGC 2841.
Well, the Milky Way is very much at the high end in terms of massive galaxies.
Really? I've been trying to find a graph of galaxy mass versus frequency to no avail. Is the MW on the high end of spiral galaxy mass? What about if elliptical galaxies are also included? Is it even the most massive galaxy in the local group? WikiPedia says Andromeda is a close contender at (1.5±0.5)×1012 M☉ (from https://en.wikipedia.org/wiki/Andromeda_Galaxy)

I do know dwarf galaxies make up at least 95% (19 out of 20) or so of all galaxies. Or so the Atlas Of the Universe site implies at all size scales (e.g., http://www.atlasoftheuniverse.com/virgo.html). And https://www.quantamagazine.org/the-prob ... y-20190109 says there may be even more:
Bullock argues that around 1,000 tiny galaxies likely orbit every large galaxy like the Milky Way. In addition, dwarf galaxies also overrun the vast stretches of seemingly empty space between large galaxies. As such, there could be as many as 100,000 dwarf galaxies for every large galaxy in the universe.

Today, astronomers estimate that there are likely 100 billion galaxies across the observable universe. But these are Milky-Way-size galaxies. If you extend the count down to these mini-galaxies, then there might be more like 10 million billion galaxies in all.
Ellipticals are the most massive. But among spirals, the MW is definitely at the high end. There are a lot more spiral galaxies that are less massive than the MW than more.
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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Ann » Sun Jan 24, 2021 5:43 pm

Lasse H wrote:
Sun Jan 24, 2021 5:16 pm
JohnD wrote:
Sun Jan 24, 2021 3:31 pm

And with LasseH, do you refer to the Black Dot at 8 o'clock, on the edge of the central clearance? As you say, it must be ENORMOUS!
Yes, I think you mean the same object as I do. I didn't say it was enormous but it must be, if it is an object belonging to the galaxy. Near to it there are about 10 similar sized, round objects, but they are white and look like stars. So, what is the "black sheep" doing there?
There are other similar round black objects in the image to the upper left of the disk. So I think that the black dot is not a physical object in the galaxy.

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by Chris Peterson » Sun Jan 24, 2021 5:44 pm

Lasse H wrote:
Sun Jan 24, 2021 5:16 pm
JohnD wrote:
Sun Jan 24, 2021 3:31 pm

And with LasseH, do you refer to the Black Dot at 8 o'clock, on the edge of the central clearance? As you say, it must be ENORMOUS!
Yes, I think you mean the same object as I do. I didn't say it was enormous but it must be, if it is an object belonging to the galaxy. Near to it there are about 10 similar sized, round objects, but they are white and look like stars. So, what is the "black sheep" doing there?
Again, this isn't an object. It's an imaging artifact. Caused, for example, by a hot spot on a calibration frame. There are more such black spots in the image, you just need to look a little closer for them because they're in lower contrast areas.
Chris

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neufer
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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by neufer » Sun Jan 24, 2021 8:09 pm

https://en.wikipedia.org/wiki/NGC_2841 wrote:
Click to play embedded YouTube video.
Click to play embedded YouTube video.

<<The rotational behavior of NGC 2841 suggests there is a massive nuclear bulge, with a low-ionization nuclear emission-line region (LINER) at the core; a type of region that is characterized by spectral line emission from weakly ionized atoms. A prominent molecular ring is orbiting at a radius of 2–6 kpc, which is providing a star-forming region of gas and dust. The nucleus appears decoupled and there is a counter-rotating element of stars and gas in the outer parts of the nucleus, suggesting a recent interaction with a smaller galaxy.

A low-ionization nuclear emission-line region (LINER) is a type of galactic nucleus that is defined by its spectral line emission. The spectra typically include line emission from weakly ionized or neutral atoms, such as O, O+, N+, and S+. Conversely, the spectral line emission from strongly ionized atoms, such as O++, Ne++, and He+, is relatively weak. The class of galactic nuclei was first identified by Timothy Heckman in the third of a series of papers on the spectra of galactic nuclei that were published in 1980.

LINER galaxies are very common; approximately one-third of all nearby galaxies (galaxies within approximately 20-40 Mpc) may be classified as LINER galaxies. Approximately 75% of LINER galaxies are either elliptical galaxies, lenticular galaxies, or S0/a-Sab galaxies (spiral galaxies with large bulges and tightly wound spiral arms). LINERs are found less frequently in Sb-Scd galaxies (spiral galaxies with small bulges and loosely wound spiral arms), and they are very rare in nearby irregular galaxies. LINERs also may be commonly found in luminous infrared galaxies (LIRGs), a class of galaxies defined by their infrared luminosities that are frequently formed when two galaxies collide with each other. Approximately one-quarter of LIRGs may contain LINERs.

LINERs have been at the center of two major debates. First, astronomers have debated the source of energy that excites the ionized gas in the centers of these galaxies. Some astronomers have proposed that active galactic nuclei (AGN) with supermassive black holes are responsible for the LINER spectral emission. Other astronomers have asserted that the emission is powered by star formation regions. The other major issue is related to how the ions are excited. Some astronomers have suggested that shock waves propagating through the gas may ionize the gas, while others have suggested that photoionization (ionization by ultraviolet light) may be responsible.

These debates are complicated by the fact that LINERs are found in a wide variety of objects with different brightnesses and morphologies. Moreover, the debate over the energy sources for LINERs is entangled with a similar debate over whether the light from star formation regions or the light from AGN produce the high infrared luminosities seen in LIRGs.

A number of surveys have been performed to explore the connection between star formation and LINER activity. If a connection can be found between star formation activity and LINER activity, then this strengthens the possibility that LINERs are powered by the hot gas found in star formation regions. However, if star formation cannot be found in LINERs, then this definitively excludes star formation as powering LINER emission.

Recent observations with the Spitzer Space Telescope show a clear connection between LINER emission in luminous infrared galaxies (LIRGs) and star formation activity. The mid-infrared spectra of LIRGs with LINERs have been shown to look similar to the mid-infrared spectra of starburst galaxies, which suggest that infrared-bright LINERs are powered by star formation activity. However, some mid-infrared spectral line emission from AGN have also been detected in these galaxies, indicating that star formation may not be the only energy sources in these galaxies.

Normal nearby galaxies with LINERs, however, appear to be different. A few near-infrared spectroscopic surveys have identified some LINERs in normal galaxies that may be powered by star formation. However, most LINERs in nearby galaxies have low levels of star formation activity. Moreover, the stellar populations of many LINERs appear to be very old, and the mid-infrared spectra, as observed by the Spitzer Space Telescope, do not appear similar to the spectra expected from star formation. These results demonstrate that most LINER in nearby normal galaxies may not be powered by star formation, although a few exceptions clearly exist.>>
Art Neuendorffer

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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by johnnydeep » Sun Jan 24, 2021 9:17 pm

Chris Peterson wrote:
Sun Jan 24, 2021 5:44 pm
Lasse H wrote:
Sun Jan 24, 2021 5:16 pm
JohnD wrote:
Sun Jan 24, 2021 3:31 pm

And with LasseH, do you refer to the Black Dot at 8 o'clock, on the edge of the central clearance? As you say, it must be ENORMOUS!
Yes, I think you mean the same object as I do. I didn't say it was enormous but it must be, if it is an object belonging to the galaxy. Near to it there are about 10 similar sized, round objects, but they are white and look like stars. So, what is the "black sheep" doing there?
Again, this isn't an object. It's an imaging artifact. Caused, for example, by a hot spot on a calibration frame. There are more such black spots in the image, you just need to look a little closer for them because they're in lower contrast areas.
Yup. The larger images at https://hubblesite.org/image/3845/printshop reveal the “object” as nothing more than a slightly darker patch of dust (note that the orientation of the galaxy is different than in this APOD). We should all try to refrain from letting our imaginations run wild with speculations based on single photos and scant evidence. Fortunately, this is easy for me since I have no imagination :lol2:

EDIT: actually, I was wrong. The larger images at Hubblesite.org don’t show that black artifact at all! I had the orientation wrong.
Last edited by johnnydeep on Mon Jan 25, 2021 1:58 am, edited 1 time in total.
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Re: APOD: Massive Nearby Spiral Galaxy NGC 2841 (2021 Jan 24)

Post by johnnydeep » Sun Jan 24, 2021 9:19 pm

Chris Peterson wrote:
Sun Jan 24, 2021 5:42 pm
johnnydeep wrote:
Sun Jan 24, 2021 5:38 pm
Chris Peterson wrote:
Sun Jan 24, 2021 4:12 pm

Well, the Milky Way is very much at the high end in terms of massive galaxies.
Really? I've been trying to find a graph of galaxy mass versus frequency to no avail. Is the MW on the high end of spiral galaxy mass? What about if elliptical galaxies are also included? Is it even the most massive galaxy in the local group? WikiPedia says Andromeda is a close contender at (1.5±0.5)×1012 M☉ (from https://en.wikipedia.org/wiki/Andromeda_Galaxy)

I do know dwarf galaxies make up at least 95% (19 out of 20) or so of all galaxies. Or so the Atlas Of the Universe site implies at all size scales (e.g., http://www.atlasoftheuniverse.com/virgo.html). And https://www.quantamagazine.org/the-prob ... y-20190109 says there may be even more:
Bullock argues that around 1,000 tiny galaxies likely orbit every large galaxy like the Milky Way. In addition, dwarf galaxies also overrun the vast stretches of seemingly empty space between large galaxies. As such, there could be as many as 100,000 dwarf galaxies for every large galaxy in the universe.

Today, astronomers estimate that there are likely 100 billion galaxies across the observable universe. But these are Milky-Way-size galaxies. If you extend the count down to these mini-galaxies, then there might be more like 10 million billion galaxies in all.
Ellipticals are the most massive. But among spirals, the MW is definitely at the high end. There are a lot more spiral galaxies that are less massive than the MW than more.
Ok, thanks. So it looks like we live in a somewhat special galaxy after all!
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