APOD: Shell Galaxies in Pisces (2020 Aug 27)

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APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by APOD Robot » Thu Aug 27, 2020 4:06 am

Image Shell Galaxies in Pisces

Explanation: This intergalactic skyscape features a peculiar system of galaxies cataloged as Arp 227 some 100 million light-years distant. Swimming within the boundaries of the constellation Pisces, Arp 227 consists of the two galaxies prominent right of center, the curious shell galaxy NGC 474 and its blue, spiral-armed neighbor NGC 470. The faint, wide arcs or shells of NGC 474 could have been formed by a gravitational encounter with neighbor NGC 470. Alternately the shells could be caused by a merger with a smaller galaxy producing an effect analogous to ripples across the surface of a pond. The large galaxy on the top lefthand side of the deep image, NGC 467, appears to be surrounded by faint shells too, evidence of another interacting galaxy system. Intriguing background galaxies are scattered around the field that also includes spiky foreground stars. Of course, those stars lie well within our own Milky Way Galaxy. The field of view spans 25 arc minutes or about 1/2 degree on the sky.

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by Sa Ji Tario » Thu Aug 27, 2020 4:19 am

Those that seem to have interacted are NGC 474 and NGC 467, the others do not present gravitational anomalies

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by JohnD » Thu Aug 27, 2020 9:25 am

Such symmetrical shells, all with a common centre, remind me of images of gravitational lensing.
Could this be the same?

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by Ann » Thu Aug 27, 2020 11:30 am

JohnD wrote:
Thu Aug 27, 2020 9:25 am
Such symmetrical shells, all with a common centre, remind me of images of gravitational lensing.
Could this be the same?

JOhn
No, because the arcs are not lensed background galaxies but shells of the galaxy's own stars.

I think David Malin explained it like this - take a look:

Click to play embedded YouTube video.


















Imagine that you drop a pebble in a pond. You will see ripples spread out in all directions.

Imagine, too, that you are down the seaside, by a sandy beach. You see waves lapping the shoreline. If it is a dry day, and it hasn't rained for quite some time, the sand on the beach will be mostly dry. But there will be a "wet edge" - or several wet edges - marking how far onto the shore the waves reached before receding. If you take a close look at Warwick Kent's photo, you can see two "wet edges" (or outer limits), demonstrating how far waves have reached up on the beach before receding.

David Malin said it will be the same thing with galaxies like NGC 474. "Drop a pebble in it" - or rather, make it merge with another (smallish) galaxy, and ripples of stars will progress outwards in response to the "splash". After the "outward-rippling" stars have used up all the momentum caused by the "splash" and reached their maximum "outward scatter", some of the stars will return inwards again, but some stars will remain in their new, distant position, forming luminous shells of stars.

Other stars will be flung outward more violently. A big rock thrown into a pool will make water splash and fly.

I don't know if that is what we are seeing in NGC 474, but I like to think of it that way. Note that the nice blue spiral appears to be completely unaffected. If the elliptical shell galaxy NGC 474 is more massive than spiral galaxy NGC 470 (and I think it is), its greater gravity might cause it to have more impacts than its punier neighbour.

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by orin stepanek » Thu Aug 27, 2020 1:26 pm

ngc474MP1024.jpg

She sell sea shells by the sea side! :mrgreen:
Tiny galaxies are beautiful! :D
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by JohnD » Thu Aug 27, 2020 1:36 pm

Thank you, Ann. I always suspect a theory proposed to explain an extraordinary observation. NGC 474 is unique, so far (?) If it could be explained without referring to mechanisms that could occur in any galaxy, but seemingly don't, then the alternative theory is more attractive.
If an image has been lensed by some great mass, then I expect it will show some particular characteristic. For instance the star being imaged will be some distance behind the lensing object, so would be expected to have a larger redshift than a star at the same distance as the object. Has anyone looked for such properties in the light of the shell stars?
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by Ann » Thu Aug 27, 2020 3:31 pm

JohnD wrote:
Thu Aug 27, 2020 1:36 pm
Thank you, Ann. I always suspect a theory proposed to explain an extraordinary observation. NGC 474 is unique, so far (?) If it could be explained without referring to mechanisms that could occur in any galaxy, but seemingly don't, then the alternative theory is more attractive.
If an image has been lensed by some great mass, then I expect it will show some particular characteristic. For instance the star being imaged will be some distance behind the lensing object, so would be expected to have a larger redshift than a star at the same distance as the object. Has anyone looked for such properties in the light of the shell stars?
John
John, there are other examples of shell galaxies:























Check out this mosaic of galaxies M89 (right) and M90, too: :arrow:


Read about the shell galaxy PGC 42871 here. It is apparently a very tiny galaxy, only 20,000 light-years in diameter! For a galaxy, that's puny indeed! :shock:

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Last edited by Ann on Thu Aug 27, 2020 3:38 pm, edited 1 time in total.
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by neufer » Thu Aug 27, 2020 3:35 pm

https://en.wikipedia.org/wiki/Halton_Arp wrote:

<<Halton Christian "Chip" Arp (March 21, 1927 – December 28, 2013) was an American astronomer. He was known for his 1966 Atlas of Peculiar Galaxies, which (it was later theorized) catalogues many examples of interacting and merging galaxies, though Arp disputed the idea, claiming apparent associations were prime examples of ejections. Arp was also known as a critic of the Big Bang theory and for advocating a non-standard cosmology incorporating intrinsic redshift.

Arp was born on March 21, 1927, in New York City. He was married three times, has four daughters and five grandchildren. His bachelor's degree was awarded by Harvard (1949), and his PhD by Caltech (1953). Afterward he became a Fellow of the Carnegie Institution of Washington in 1953, performing research at the Mount Wilson Observatory and Palomar Observatory. Arp became a Research Assistant at Indiana University in 1955, and then in 1957 became a staff member at Palomar Observatory, where he worked for 29 years. In 1983 he joined the staff of the Max Planck Institute for Astrophysics in Germany. He died in Munich, Germany on December 28, 2013. He was an atheist.

Arp compiled a catalog of unusual galaxies titled Atlas of Peculiar Galaxies, which was first published in 1966. Arp realized that astronomers understood little about how galaxies change over time, which led him to work on this project. This atlas was intended to provide images that would give astronomers data from which they could study the evolution of galaxies. Arp later used the atlas as evidence in his debate on quasi-stellar objects (QSOs). Based on its citation by other astronomers, Arp's atlas is recognized now as an excellent compilation of interacting and merging galaxies. Many objects in the atlas are referred to primarily by their Arp number. Many of these objects (particularly Arp 220) are also used as spectral templates for studying high-redshift galaxies. Arp disputed the idea that the peculiar galaxies were merging, claiming, rather, that apparent associations were prime examples of ejections from a host galaxy.>>
https://en.wikipedia.org/wiki/Atlas_of_Peculiar_Galaxies wrote:
<<The Atlas of Peculiar Galaxies is a catalog of peculiar galaxies produced by Halton Arp in 1966. A total of 338 galaxies are presented in the atlas, which was originally published in 1966 by the California Institute of Technology. The primary goal of the catalog was to present photographs of examples of the different kinds of peculiar structures found among galaxies.>>
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by johnnydeep » Thu Aug 27, 2020 6:06 pm

APOD Robot wrote:
Thu Aug 27, 2020 4:06 am
Image Shell Galaxies in Pisces

Explanation: This intergalactic skyscape features a peculiar system of galaxies cataloged as Arp 227 some 100 million light-years distant. Swimming within the boundaries of the constellation Pisces, Arp 227 consists of the two galaxies prominent right of center, the curious shell galaxy NGC 474 and its blue, spiral-armed neighbor NGC 470. The faint, wide arcs or shells of NGC 474 could have been formed by a gravitational encounter with neighbor NGC 470. Alternately the shells could be caused by a merger with a smaller galaxy producing an effect analogous to ripples across the surface of a pond. The large galaxy on the top lefthand side of the deep image, NGC 467, appears to be surrounded by faint shells too, evidence of another interacting galaxy system. Intriguing background galaxies are scattered around the field that also includes spiky foreground stars. Of course, those stars lie well within our own Milky Way Galaxy. The field of view spans 25 arc minutes or about 1/2 degree on the sky.
Anyone know what this background galaxy is? It's oddly rectangular!
What Galaxy is This.JPG
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by Ann » Thu Aug 27, 2020 7:09 pm

johnnydeep wrote:
Thu Aug 27, 2020 6:06 pm
APOD Robot wrote:
Thu Aug 27, 2020 4:06 am
Image Shell Galaxies in Pisces

Explanation: This intergalactic skyscape features a peculiar system of galaxies cataloged as Arp 227 some 100 million light-years distant. Swimming within the boundaries of the constellation Pisces, Arp 227 consists of the two galaxies prominent right of center, the curious shell galaxy NGC 474 and its blue, spiral-armed neighbor NGC 470. The faint, wide arcs or shells of NGC 474 could have been formed by a gravitational encounter with neighbor NGC 470. Alternately the shells could be caused by a merger with a smaller galaxy producing an effect analogous to ripples across the surface of a pond. The large galaxy on the top lefthand side of the deep image, NGC 467, appears to be surrounded by faint shells too, evidence of another interacting galaxy system. Intriguing background galaxies are scattered around the field that also includes spiky foreground stars. Of course, those stars lie well within our own Milky Way Galaxy. The field of view spans 25 arc minutes or about 1/2 degree on the sky.
Anyone know what this background galaxy is? It's oddly rectangular!

What Galaxy is This.JPG

How about it's the NGC 474 Galaxy Group version of the Large Magellanic Cloud?

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by johnnydeep » Thu Aug 27, 2020 7:48 pm

Ann wrote:
Thu Aug 27, 2020 7:09 pm
johnnydeep wrote:
Thu Aug 27, 2020 6:06 pm
APOD Robot wrote:
Thu Aug 27, 2020 4:06 am
Image Shell Galaxies in Pisces

Explanation: This intergalactic skyscape features a peculiar system of galaxies cataloged as Arp 227 some 100 million light-years distant. Swimming within the boundaries of the constellation Pisces, Arp 227 consists of the two galaxies prominent right of center, the curious shell galaxy NGC 474 and its blue, spiral-armed neighbor NGC 470. The faint, wide arcs or shells of NGC 474 could have been formed by a gravitational encounter with neighbor NGC 470. Alternately the shells could be caused by a merger with a smaller galaxy producing an effect analogous to ripples across the surface of a pond. The large galaxy on the top lefthand side of the deep image, NGC 467, appears to be surrounded by faint shells too, evidence of another interacting galaxy system. Intriguing background galaxies are scattered around the field that also includes spiky foreground stars. Of course, those stars lie well within our own Milky Way Galaxy. The field of view spans 25 arc minutes or about 1/2 degree on the sky.
Anyone know what this background galaxy is? It's oddly rectangular!

What Galaxy is This.JPG

How about it's the NGC 474 Galaxy Group version of the Large Magellanic Cloud?

Ann
Though it looks similarly rectangular, this one seems more well-defined and disjoint from the others. And it's also much bluer!

Aha, I found it, and it has a name: PGC 4755 !

From https://www.irida-observatory.org/CCD/N ... otated.jpg

[rant]I'm getting tired of not being able to embed a pic more than 900 pixels wide, and having to use an inline screen-grab from my PC instead:x [/rant]
PCG 4755 And Other Galaxies Near NGC 474.JPG

I can't find much more about PGC 4755, though this link does remark on its blueness: http://cosmicneighbors.net/ngc474.htm
Galaxy NGC 474, the large galaxy on the left, has quite an unusual shape including a very faint outer "halo". Its strange morphology is probably due to interaction with the galaxy to its upper right, which is NGC 470. At the right of the image is elliptical galaxy NGC 467, which is also somewhat asymmetrical and distorted, possibly by interaction with the small edge-on spiral just below it. At the bottom right are two more edge-on galaxies: PGC 4765 on the left, and the very blue PGC 4755 on the right. At the top center of the image is the trail of asteroid (9389) Crabbegat, which passed through the field of view during one capture session. In all this is a 10-hour image with 7:1:1:1 hours of L:R:G:B.
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Last edited by johnnydeep on Thu Aug 27, 2020 9:08 pm, edited 1 time in total.
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by johnnydeep » Thu Aug 27, 2020 8:08 pm

APOD Robot wrote:
Thu Aug 27, 2020 4:06 am
Image Shell Galaxies in Pisces

Explanation: This intergalactic skyscape features a peculiar system of galaxies cataloged as Arp 227 some 100 million light-years distant. Swimming within the boundaries of the constellation Pisces, Arp 227 consists of the two galaxies prominent right of center, the curious shell galaxy NGC 474 and its blue, spiral-armed neighbor NGC 470. The faint, wide arcs or shells of NGC 474 could have been formed by a gravitational encounter with neighbor NGC 470. Alternately the shells could be caused by a merger with a smaller galaxy producing an effect analogous to ripples across the surface of a pond. The large galaxy on the top lefthand side of the deep image, NGC 467, appears to be surrounded by faint shells too, evidence of another interacting galaxy system. Intriguing background galaxies are scattered around the field that also includes spiky foreground stars. Of course, those stars lie well within our own Milky Way Galaxy. The field of view spans 25 arc minutes or about 1/2 degree on the sky.
I love this black and white image of NGC474 (from https://www.irida-observatory.org/CCD/N ... 769-th.jpg) - it really shows those galactic ripples/shells well! And in this inverted orientation, I christen it the Table Fan Galaxy!

Image
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by LouGottlieb » Fri Aug 28, 2020 6:15 am

The photo of shell galaxies today in APOD had two possible theoretical explanations accompanying it; both involved tidal interactions with
neighboring galaxies.

However, my instant impression on seeing the shells was "colossal explosion".
Do black holes ever explode?
If not, why not?

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by Ann » Fri Aug 28, 2020 7:25 am

johnnydeep wrote:
Thu Aug 27, 2020 7:48 pm
Ann wrote:
Thu Aug 27, 2020 7:09 pm
johnnydeep wrote:
Thu Aug 27, 2020 6:06 pm


Anyone know what this background galaxy is? It's oddly rectangular!

What Galaxy is This.JPG

How about it's the NGC 474 Galaxy Group version of the Large Magellanic Cloud?

Ann
Though it looks similarly rectangular, this one seems more well-defined and disjoint from the others. And it's also much bluer!

Aha, I found it, and it has a name: PGC 4755 !

From https://www.irida-observatory.org/CCD/N ... otated.jpg
Well done finding out the designation of the blue rectangular galaxy! :clap:

However, I'm not sure that PGC 4755 is bluer than the Large Magellanic Cloud. Remember that we are comparing it with the larger galaxies in this field, non of which, not even the blue-looking ring-spiral galaxy NGC 470, is very blue. The color indexes of NGC 470 is, U-B, +0.100, B-V, +0.750. The lower these values are, the bluer is the galaxy. Galaxy colors are always diluted, but NGC 470 is not very blue.

As for the fact that PGC 4755 looks more well-defined than the LMC, remember that PGC 4755 is extremely blurry in the APOD, while the picture that I posted of LMC is sharply defined, so that we can see all the irregularities. The blurriness of PGC 4755 helps smooth out everything but the main features.

My software Guide quotes the PGC catalogue to claim that while the NGC 474 group is about 100 million light-years distant (which is also what the APOD caption reports), the distance to PGC 4755 is about 240 million light-years. I have no idea if you can trust the latter distance estimate, but I guess it is at least possible that PGC 4755 is a background object.

Let me have a go at guessing what kind of a galaxy that PGC 4755 is:


















Take a look at galaxies NGC 4631 and NGC 4656 at left. Both have thick, puffed-up blue disks. NGC 4631 clearly shows off its yellow center (it looks deep orange in the picture I posted), but in NGC 4656 the center is barely yellow.

And take a look at galaxy NGC 3628 at right. This galaxy is not blue at all, but it does have a very rectangular shape.

I think PGC 4755 displays a mixture of those features. I think it has either (and most likely) a thick blue disk full of star formation, or else it might actually have a brilliant blue bar. If the blue feature is a bar, it becomes hard to explain why there is no yellow center. But if the blue feature is a bar, then it becomes much easier to explain the flaring at the edges, because that would simply be the beginnings of a faint ring or faint arms.

The lack of an obvious yellow center is easier to explain if the blue feature is a disk than if it is a bar, but the flaring at the edges becomes problematical. But as you can see in galaxy NGC 3628, flaring at the edges does occur in disk galaxies.

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by JohnD » Fri Aug 28, 2020 12:23 pm

Thank you, Ann, for the list of other shell galaxies, which demolishes my argument from uniqueness, and my previous ignorance.

You can rest your case, I am convinced about "ripples"!
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by JohnD » Fri Aug 28, 2020 12:59 pm

And this just appeared on SpaceX Daily:

https://www.facebook.com/SpaceXdaily/vi ... 8406113617

Bit less symmetrical, but convincing shells.

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by johnnydeep » Fri Aug 28, 2020 2:05 pm

Ann wrote:
Fri Aug 28, 2020 7:25 am
johnnydeep wrote:
Thu Aug 27, 2020 7:48 pm
Ann wrote:
Thu Aug 27, 2020 7:09 pm



How about it's the NGC 474 Galaxy Group version of the Large Magellanic Cloud?

Ann
Though it looks similarly rectangular, this one seems more well-defined and disjoint from the others. And it's also much bluer!

Aha, I found it, and it has a name: PGC 4755 !

From https://www.irida-observatory.org/CCD/N ... otated.jpg
Well done finding out the designation of the blue rectangular galaxy! :clap:

However, I'm not sure that PGC 4755 is bluer than the Large Magellanic Cloud. Remember that we are comparing it with the larger galaxies in this field, non of which, not even the blue-looking ring-spiral galaxy NGC 470, is very blue. The color indexes of NGC 470 is, U-B, +0.100, B-V, +0.750. The lower these values are, the bluer is the galaxy. Galaxy colors are always diluted, but NGC 470 is not very blue.

As for the fact that PGC 4755 looks more well-defined than the LMC, remember that PGC 4755 is extremely blurry in the APOD, while the picture that I posted of LMC is sharply defined, so that we can see all the irregularities. The blurriness of PGC 4755 helps smooth out everything but the main features.

My software Guide quotes the PGC catalogue to claim that while the NGC 474 group is about 100 million light-years distant (which is also what the APOD caption reports), the distance to PGC 4755 is about 240 million light-years. I have no idea if you can trust the latter distance estimate, but I guess it is at least possible that PGC 4755 is a background object.

Let me have a go at guessing what kind of a galaxy that PGC 4755 is:


















Take a look at galaxies NGC 4631 and NGC 4656 at left. Both have thick, puffed-up blue disks. NGC 4631 clearly shows off its yellow center (it looks deep orange in the picture I posted), but in NGC 4656 the center is barely yellow.

And take a look at galaxy NGC 3628 at right. This galaxy is not blue at all, but it does have a very rectangular shape.

I think PGC 4755 displays a mixture of those features. I think it has either (and most likely) a thick blue disk full of star formation, or else it might actually have a brilliant blue bar. If the blue feature is a bar, it becomes hard to explain why there is no yellow center. But if the blue feature is a bar, then it becomes much easier to explain the flaring at the edges, because that would simply be the beginnings of a faint ring or faint arms.

The lack of an obvious yellow center is easier to explain if the blue feature is a disk than if it is a bar, but the flaring at the edges becomes problematical. But as you can see in galaxy NGC 3628, flaring at the edges does occur in disk galaxies.

Ann
Ok, thanks for the further explanations. Perhaps PGC 4755 isn't so particularly blue after all. But, it's still pretty interesting: how do we ask the people who control Hubble to take a closer look at it? :ssmile:
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by sillyworm 2 » Fri Aug 28, 2020 3:35 pm

In the reverse B/W image of NGC 3923 the center is oblong.Would this represent the 2 Black Holes on their way to merging?

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by neufer » Fri Aug 28, 2020 4:10 pm

sillyworm 2 wrote:
Fri Aug 28, 2020 3:35 pm

In the reverse B/W image of NGC 3923 the center is oblong.Would this represent the 2 Black Holes on their way to merging?
  • If so then then the globular clusters of NGC 3923 are unaware of it.
https://en.wikipedia.org/wiki/NGC_3923 wrote: <<NGC 3923 is an elliptical galaxy located in the constellation Hydra. It is located at a distance of circa 90 million light years from Earth, which, given its apparent dimensions, means that NGC 3923 is about 155,000 light years across. NGC 3923 is an example of a shell galaxy where the stars in its halo are arranged in layers. NGC 3923 has up to 42 shells, the highest number among all shell galaxies, and its shells are much more subtle than those of other shell galaxies. The shells of this galaxy are also symmetrical, while other shell galaxies are more skewed. Concentric shells of stars enclosing a galaxy are quite common and are observed in many elliptical galaxies. In fact, every tenth elliptical galaxy exhibits this onion-like structure, which has never been observed in spiral galaxies. The shell-like structures are thought to develop as a consequence of galactic cannibalism, when a larger galaxy ingests a smaller companion. As the two centers approach, they initially oscillate about a common center, and this oscillation ripples outwards forming the shells of stars just as ripples on a pond spread when the surface is disturbed. Based on the velocity dispersion of the globular clusters of NGC 3923 the mass of the supermassive black hole of the galaxy was estimated to be (5.3±2.5)×108 M.

With deep imaging were also detected a stream extending from the core of NGC 3923 and a small elliptical galaxy on its axis, which is a probable progenitor of some the shells. Another stream lies south of the core of NGC 3923, and a hook like structure lies at the northwest.

One supernova has been detected in NGC 3923, SN 2018aoz, a type Ia supernova with peak magnitude 12.7. It was discovered on April 2, 2018.

NGC 3923 is the brightest galaxy in a galaxy group known as the NGC 3923 galaxy group. Within 25 arcminutes from NGC 3923 have been detected seven dwarf elliptical galaxies. NGC 3904 is located 37 arcminutes away. Other galaxies in the group include NGC 3885, ESO 440-27, and ESO440-011. Other nearby galaxies include NGC 3617, NGC 3673, NGC 3717, NGC 3936, and NGC 4105.>>
https://en.wikipedia.org/wiki/Elliptical_galaxy#Evolution wrote: <<It is widely accepted that the merging of smaller galaxies due to gravitational attraction plays a major role in shaping the growth and evolution of elliptical galaxies. Such major galactic mergers are thought to have been more common at early times. Minor galactic mergers involve two galaxies of very different masses, and are not limited to giant ellipticals. The Milky Way galaxy, depending upon an unknown tangential component, is on a four- to five-billion-year collision course with the Andromeda Galaxy. It has been theorized that an elliptical galaxy will result from a merger of the two spirals.

It is believed that black holes may play an important role in limiting the growth of elliptical galaxies in the early universe by inhibiting star formation.

The traditional portrait of elliptical galaxies paints them as galaxies where star formation finished after an initial burst at high-redshift, leaving them to shine with only their aging stars. Elliptical galaxies typically appear yellow-red, which is in contrast to the distinct blue tinge of most spiral galaxies. In spirals, this blue color emanates largely from the young, hot stars in their spiral arms. Very little star formation is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies. However, in recent years, evidence has shown that a reasonable proportion (~25%) of early-type (E, ES and S0) galaxies have residual gas reservoirs and low level star-formation.>>
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by Chris Peterson » Fri Aug 28, 2020 4:34 pm

sillyworm 2 wrote:
Fri Aug 28, 2020 3:35 pm
In the reverse B/W image of NGC 3923 the center is oblong.Would this represent the 2 Black Holes on their way to merging?
If we could actually resolve a structure as non-spherical due to a pair of black holes, they would probably be billions of years away from merging.
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by sillyworm 2 » Fri Aug 28, 2020 6:38 pm

THANKS! I was not implying that the event would happen anytime soon.I was wondering if the oblong center represented there being 2 Black Holes.

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by neufer » Fri Aug 28, 2020 9:19 pm

sillyworm 2 wrote:
Fri Aug 28, 2020 6:38 pm

THANKS! I was not implying that the event would happen anytime soon.
I was wondering if the oblong center represented there being 2 Black Holes.
I don't think that anyone has determined which came first:
  • 1) super massive black holes or
    2) the galaxies themselves
If galaxies came first then these early mergers no doubt resulted in just one super massive black hole.

However, if black holes came first then it is at least possible that multiple super massive black holes still exist.
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by JohnD » Fri Aug 28, 2020 10:21 pm

I proposed the same cause (two BHs) for the bar on some galaxies, several APODs ago: viewtopic.php?f=9&t=40902&p=305207&hilit=JohnD#p305207 Post 5 but no one responded. I don't think that is necessary for an 'oblong' galaxy when on seen from the side will look like that, but I still wonder about the bars. Thanks, mine's a double.

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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by neufer » Sat Aug 29, 2020 2:10 pm

JohnD wrote:
Fri Aug 28, 2020 10:21 pm

I proposed the same cause (two BHs) for the bar on some galaxies, several APODs ago: viewtopic.php?f=9&t=40902&p=305207&hilit=JohnD#p305207 Post 5 but no one responded. I don't think that is necessary for an 'oblong' galaxy when on seen from the side will look like that, but I still wonder about the bars. Thanks, mine's a double.
:arrow: Do you attribute the bar of our own Milky Way to two BHs :?:
Art Neuendorffer

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Chris Peterson
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Re: APOD: Shell Galaxies in Pisces (2020 Aug 27)

Post by Chris Peterson » Sat Aug 29, 2020 2:20 pm

neufer wrote:
Sat Aug 29, 2020 2:10 pm
JohnD wrote:
Fri Aug 28, 2020 10:21 pm

I proposed the same cause (two BHs) for the bar on some galaxies, several APODs ago: viewtopic.php?f=9&t=40902&p=305207&hilit=JohnD#p305207 Post 5 but no one responded. I don't think that is necessary for an 'oblong' galaxy when on seen from the side will look like that, but I still wonder about the bars. Thanks, mine's a double.
:arrow: Do you attribute the bar of our own Milky Way to two BHs :?:
It's not even clear why a pair of black holes in the center of a galaxy would produce an asymmetric structure of stars. The mass of the stars in a galaxy's core hugely outmasses one or two black holes. And the black holes would be orbiting each other, so they wouldn't be maintaining a fixed orientation that you'd probably need to form any structure in the first place. I'd expect the core of a galaxy that contained two black holes to have the same structure that the core of any unbarred galaxy has- spherical.
Chris

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