APOD: NGC 147 and NGC 185 (2021 Nov 04)

[APOD Robot]

NGC 147 and NGC 185

Explanation: Dwarf galaxies NGC 147 (left) and NGC 185 stand side by side in this sharp telescopic portrait. The two are not-often-imaged satellites of M31, the great spiral Andromeda Galaxy, some 2.5 million light-years away. Their separation on the sky, less than one degree across a pretty field of view, translates to only about 35 thousand light-years at Andromeda's distance, but Andromeda itself is found well outside this frame. Brighter and more famous satellite galaxies of Andromeda, M32 and M110, are seen closer to the great spiral. NGC 147 and NGC 185 have been identified as binary galaxies, forming a gravitationally stable binary system. But recently discovered faint dwarf galaxy Cassiopeia II also seems to be part of their system, forming a gravitationally bound group within Andromeda's intriguing population of small satellite galaxies.

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[Ann]

NGC 147 and NGC 185. Image Credit & Copyright: Dan Bartlett

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NGC 6786 and UGC 11415 Roberto Colombari.png

NGC 6786 (left) and UGC 11415. Photo: Roberto Colombari.

There are galactic pairs, and there are galactic pairs...

NGC 147 and NGC 185 have to represent one of the most common types of galaxies out there, just possibly the most common one, the spheroidal dwarf galaxy. Take a look at NGC 147 and NGC 185 and see for yourself: These guys are small, faint, featureless and yellow. Not much is happening inside them, although apparently NGC 147 has a tail of stars and NGC 185 contains a few young (~400 million years old) stars.

How different are NGC 6789 and UGC 11415! They make up the rarest of pairs, because not only are they spiral galaxies, but they are both grand design galaxies (which means that they both have two major spiral arms), and they rotate in the same direction (clockwise), and they both present the "same face" to us (face on), and they are moderately the same size. They are so unusual! I've got a challenge for you: Find another pair of spiral galaxies that show so many similarities! And find their designations, too.

Of course, there are other pairs of galaxies, too:

Large and Small Magellanic Clouds. Photo: ESO/S. Brunier.

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Deep view of the Large and Small Magellanic Clouds. Image: Y. Beletsky (LCO), D. Martinez-Delgado/ESO

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If Andromeda has a binary pair of satellite galaxies, the Milky Way certainly does, too. And the Large and Small Magellanic Clouds are far far more remarkable than NGC 147 and NGC 185. Both Magellanic Clouds are pretty hefty, particularly of course the LMC, and both of them are rich in gas and star formation, particularly of course the LMC. I am not aware of any other major spiral galaxy that has such a pair of large starforming satellites. Do any of you know of another large spiral galaxy that has its own two "Magellanic Clouds"?

The fact that the Magellanic Clouds are so unique means that they can't very well have been satellites of the Milky Way for that many billion years. More likely, they are an interacting pair that was caught up by the gravity field of the Milky Way and became bound to our galaxy fairly recently. Alternatively, the LMC may have started interacting with the SMC before they got trapped by our own big bully of a galaxy.

Note in the deep view of the Magellanic Clouds the large halos surrounding them, particularly of course the LMC. Not the "tail" that emanates from the SMC and points towards the LMC. The LMC is a well-known cannibalizer of the SMC!

Finally, let's take a look at another galactic pair that is actually a trio:

M81, M82 and NGC 3077 (bottom left). Photo: NASA/JPL-Caltech/UCLA

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Take a look at these two pictures of M81, M82 and NGC 3077. The left one was taken by NASA's Wide-Field Infrared Survey Explorer, or WISE. In the image, blue means starlight and red means dust. M81 looks fairly normal in the image (and so does unremarkable NGC 3077 at bottom left), but M82 is "exploding" in a yellow cloud of gas and dust! In the picture at right we can see the streams of neutral gas that is flowing between these three galaxies.

So galaxies come in many pairs and in some trios. Some are really unusual, while others count as the most common denizens of the Universe.

Ann

[bls0326]

Ann : Thanks for the interesting comments and pictures. It is all info that I would not see or understand the details on my own.

[orin stepanek]

NGC147NGC185satellites.jpg

I thought NGC147 & NGC185 looked like star clusters; glad I read the
contents! On the above photo they do look like Galaxies; but on the
blowups seemed like clusters!

[Ann]

Ann : Thanks for the interesting comments and pictures. It is all info that I would not see or understand the details on my own.

Thank you very much! I'm so glad you like it!

Ann

[Ann]

Click to view full size image

I thought NGC147 & NGC185 looked like star clusters; glad I read the
contents! On the above photo they do look like Galaxies; but on the
blowups seemed like clusters!

Orin, yes, that's fascinating, isn't it? So what's the difference between a globular cluster and a galaxy?

Don't know if I'll be able to tell you, but let's have a crack at it anyway...

Dwarf spheroidal galaxy NGC 185 by Hubble. Note the big fat globular cluster at left. Also note the dust lane near the center.

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Globular cluster NGC 6388. Image: NASA, ESA, F. Ferraro (University of Bologna)

---

Note in the picture of NGC 185 that this spheroidal galaxy has at least one globular cluster. (Actually it has more; see here.) So in other words, galaxies can have globular clusters, but globular clusters can't have galaxies.

Also, galaxies are built up over billions of years. Yes indeed, a few galaxies stop forming stars very early, but most galaxides certainly keep it up for billions of years. That means their stars are of different ages, and the stars have different metallicities, because the gas that they were born from contained different levels of elements more massive than hydrogen and helium. (That, of course, is because the interstellar and intergalactic medium is constantly being enriched with heavier elements as stars die and "spit out" the heavier elements that they have forged inside them as they ran through more and more cycles of fusion in their battle against gravity.)

Globular clusters, by contrast, are formed (the way I understand it) in a single event, so that all the stars in them are the same age. The stars inside them are also very old, and many of them are 10-12 billion years old. Possibly not a single galaxy will form in a single event, never forming any new stars after that initial burst. Admittedly I can't be sure of this. However, NGC 185 certainly kept on forming stars for a long time:

Wikipedia wrote:
Martínez-Delgado, Aparicio, & Gallart (1999) looked into the star formation history of NGC 185 and found that the majority of star formation in NGC 185 happened at early times. In the last ~1 Gyr, stars have formed only near the center of this galaxy.

So NGC 185 has formed stars during the last one billion years. No globular cluster will do that. Clearly the stars that were recently born in NGC 185 were born from the cloud of gas and dust that is visible in the center of this dwarf spheroidal. No globular cluster will have an obviously visible dust cloud in its center.

All globular cluster have metal-poor stars, that is, their stars were formed from gas clouds that contained only very low levels of any other elements than hydrogen and helium. Galaxies like NGC 185 and NGC 147 are also likely to be metal-poor, but certainly not as metal-poor as a globular cluster.

According to a blurb I found summarizing a scientific paper, NGC 6388 is a very metal-rich globular cluster. And NGC 185 and NGC 147 are two metal-poor spheroidal galaxies. Nevertheless, NGC 147 and NGC 185 should still be more metal-rich than globular cluster NGC 6388. Let's take a look at two not exactly comparable color-magnitude diagrams of the stars in globular cluster NGC 6388 and dwarf spheroidal galaxy NGC 185:

Color-magnitude diagrams of NGC 147 (left) and NGC 185 (right). Diagram: Puragra Guhathakurta at ResearchGate.

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Color-magnitude diagrams for NGC 6388 and another metal-rich globular cluster, NGC 6441. Diagram: George Djorgovski at ResearchGate.

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There are differences in stellar content between the globular clusters and the dwarf spheroidal galaxies. The globulars have longer horizontal branches (which extend horizontally to the left and then curve down) than the galaxies. The galaxies, by contrast, have shorter horizontal branches, broader, thicker main sequences (at bottom) and thicker, broader blue straggler branches (which extend from the main sequence branch to the upper left towards the horizontal branch).

In short: The stellar contents are different in globular clusters and dwarf spheroidals, and almost any expert would (I think) be able to tell the difference between a globular cluster and a dwarf spheroidal simply by looking at their color-magnitude diagrams.

Ann

[neufer]

<<Unlike most dwarf elliptical galaxies, NGC 185 contains young stellar clusters, and star formation proceeded at a low rate until the recent past. NGC 185 has an active galactic nucleus (AGN) and is usually classified as a type II Seyfert galaxy, though its status as a Seyfert is questioned. It is possibly the closest Seyfert galaxy to Earth, and is the only known Seyfert in the Local Group.

Type II Seyfert galaxies have the characteristic bright core, as well as appearing bright when viewed at infrared wavelengths. Their spectra contain narrow lines associated with forbidden transitions, and broader lines associated with allowed strong dipole or intercombination transitions. NGC 3147 is considered the best candidate to be a true Type II Seyfert galaxy. In some Type II Seyfert galaxies, analysis with a technique called spectro-polarimetry (spectroscopy of polarised light component) revealed obscured Type I regions. In the case of NGC 1068, nuclear light reflected off a dust cloud was measured, which led scientists to believe in the presence of an obscuring dust torus around a bright continuum and broad emission line nucleus. When the galaxy is viewed from the side, the nucleus is indirectly observed through reflection by gas and dust above and below the torus. This reflection causes the polarisation.>>

[orin stepanek]

Click to view full size image

I thought NGC147 & NGC185 looked like star clusters; glad I read the
contents! On the above photo they do look like Galaxies; but on the
blowups seemed like clusters!

Orin, yes, that's fascinating, isn't it? So what's the difference between a globular cluster and a galaxy?

Don't know if I'll be able to tell you, but let's have a crack at it anyway...

Dwarf spheroidal galaxy NGC 185 by Hubble. Note the big fat globular cluster at left. Also note the dust lane near the center.

---

Globular cluster NGC 6388. Image: NASA, ESA, F. Ferraro (University of Bologna)

---

Note in the picture of NGC 185 that this spheroidal galaxy has at least one globular cluster. (Actually it has more; see here.) So in other words, galaxies can have globular clusters, but globular clusters can't have galaxies.

Also, galaxies are built up over billions of years. Yes indeed, a few galaxies stop forming stars very early, but most galaxides certainly keep it up for billions of years. That means their stars are of different ages, and the stars have different metallicities, because the gas that they were born from contained different levels of elements more massive than hydrogen and helium. (That, of course, is because the interstellar and intergalactic medium is constantly being enriched with heavier elements as stars die and "spit out" the heavier elements that they have forged inside them as they ran through more and more cycles of fusion in their battle against gravity.)

Globular clusters, by contrast, are formed (the way I understand it) in a single event, so that all the stars in them are the same age. The stars inside them are also very old, and many of them are 10-12 billion years old. Possibly not a single galaxy will form in a single event, never forming any new stars after that initial burst. Admittedly I can't be sure of this. However, NGC 185 certainly kept on forming stars for a long time:

Wikipedia wrote:
Martínez-Delgado, Aparicio, & Gallart (1999) looked into the star formation history of NGC 185 and found that the majority of star formation in NGC 185 happened at early times. In the last ~1 Gyr, stars have formed only near the center of this galaxy.

So NGC 185 has formed stars during the last one billion years. No globular cluster will do that. Clearly the stars that were recently born in NGC 185 were born from the cloud of gas and dust that is visible in the center of this dwarf spheroidal. No globular cluster will have an obviously visible dust cloud in its center.

All globular cluster have metal-poor stars, that is, their stars were formed from gas clouds that contained only very low levels of any other elements than hydrogen and helium. Galaxies like NGC 185 and NGC 147 are also likely to be metal-poor, but certainly not as metal-poor as a globular cluster.

According to a blurb I found summarizing a scientific paper, NGC 6388 is a very metal-rich globular cluster. And NGC 185 and NGC 147 are two metal-poor spheroidal galaxies. Nevertheless, NGC 147 and NGC 185 should still be more metal-rich than globular cluster NGC 6388. Let's take a look at two not exactly comparable color-magnitude diagrams of the stars in globular cluster NGC 6388 and dwarf spheroidal galaxy NGC 185:

Color-magnitude diagrams of NGC 147 (left) and NGC 185 (right). Diagram: Puragra Guhathakurta at ResearchGate.

---

Color-magnitude diagrams for NGC 6388 and another metal-rich globular cluster, NGC 6441. Diagram: George Djorgovski at ResearchGate.

---

There are differences in stellar content between the globular clusters and the dwarf spheroidal galaxies. The globulars have longer horizontal branches (which extend horizontally to the left and then curve down) than the galaxies. The galaxies, by contrast, have shorter horizontal branches, broader, thicker main sequences (at bottom) and thicker, broader blue straggler branches (which extend from the main sequence branch to the upper left towards the horizontal branch).

In short: The stellar contents are different in globular clusters and dwarf spheroidals, and almost any expert would (I think) be able to tell the difference between a globular cluster and a dwarf spheroidal simply by looking at their color-magnitude diagrams.

Ann

Interesting analyzing! Ann! I think there be a fine line between small galaxy and globular!

[johnnydeep]

[ Jeeze, Ann, I was hoping for a nice quick asterisk reading day ...Just kidding! ]

Some of the figures in the APOD text don't seem right. It says that the two dwarfs are about 35000 ly apart and both about 2.5 Mly distant (same as Andromeda itself), but Wikipedia ways that NGC 147 is 2.58 Mly away, but that NGC 185 is only 2 Mly away. That would mean they are at least 500000 ly apart. If so, can they still be gravitationally bound to each other?

Also, it's pretty cool that both dwarfs contain at least one very discernible globular cluster (pointed out in the links), and that NGC 185 might be a Seyfert galaxy with an active black hole at its center!

[neufer]

Some of the figures in the APOD text don't seem right. It says that the two dwarfs are about 35000 ly apart and both about 2.5 Mly distant (same as Andromeda itself), but Wikipedia ways that NGC 147 is 2.58 Mly away, but that NGC 185 is only 2 Mly away. That would mean they are at least 500000 ly apart. If so, can they still be gravitationally bound to each other?

Wikipedia states that NGC 147 is 2.53 ± 0.11 Mly away,
and that NGC 185 is 2.05 ± 0.13 Mly
away for a radial separation of .48 ± 0.17 Mly (that is increasing according to the redshifts).

It also states that these distances were only determined between 2004 & 2007.

The Binary Galaxy hypothesis predates these measurements:

• The Binary Galaxies NGC 147 and NGC 185
  The Astronomical Journal, October 1998
  van den Bergh, Sidney

<<Contrary to a previously published claim, it is found that the spheroidal galaxies NGC 147 and NGC 185 probably form a stable binary system. Distance estimates place this pair on the near side of the Andromeda subgroup of the Local Group. The fact that this system has probably remained stable over a Hubble time suggests that it does not have a plunging orbit that brings it very close to M31. It is noted that the only two Local Group galaxy pairs in which the components have comparable masses also have similar morphological types. NGC 147 and NGC 185 are both spheroidal galaxies, while the LMC and SMC are both irregular galaxies. This suggests that protogalaxies of similar mass, that are spawned in similar environments, evolve into objects having similar morphologies.>>

[johnnydeep]

Some of the figures in the APOD text don't seem right. It says that the two dwarfs are about 35000 ly apart and both about 2.5 Mly distant (same as Andromeda itself), but Wikipedia ways that NGC 147 is 2.58 Mly away, but that NGC 185 is only 2 Mly away. That would mean they are at least 500000 ly apart. If so, can they still be gravitationally bound to each other?

Wikipedia states that NGC 147 is 2.53 ± 0.11 Mly away,
and that NGC 185 is 2.05 ± 0.13 Mly
away for a radial separation of .48 ± 0.17 Mly (that is increasing according to the redshifts).

It also states that these distances were only determined between 2004 & 2007.

The Binary Galaxy hypothesis predates these measurements:

• The Binary Galaxies NGC 147 and NGC 185
  The Astronomical Journal, October 1998
  van den Bergh, Sidney

<<Contrary to a previously published claim, it is found that the spheroidal galaxies NGC 147 and NGC 185 probably form a stable binary system. Distance estimates place this pair on the near side of the Andromeda subgroup of the Local Group. The fact that this system has probably remained stable over a Hubble time suggests that it does not have a plunging orbit that brings it very close to M31. It is noted that the only two Local Group galaxy pairs in which the components have comparable masses also have similar morphological types. NGC 147 and NGC 185 are both spheroidal galaxies, while the LMC and SMC are both irregular galaxies. This suggests that protogalaxies of similar mass, that are spawned in similar environments, evolve into objects having similar morphologies.>>

So the implication is...what, exactly? Is our best guess now that the galaxies are about 500000 ly apart (not 37000), and they may (or may not) be a "pair"? A different "source" in Wikipedia with a table of all the satellites of Andromeda, has NGC 147 at 2.2 Mly, and NGC 185 at 2.01 Mly.

[ I really lament how difficult it is to accurately determine distances to anything more than a few tens of thousands of lightyears away. ]

[Ann]

Some of the figures in the APOD text don't seem right. It says that the two dwarfs are about 35000 ly apart and both about 2.5 Mly distant (same as Andromeda itself), but Wikipedia ways that NGC 147 is 2.58 Mly away, but that NGC 185 is only 2 Mly away. That would mean they are at least 500000 ly apart. If so, can they still be gravitationally bound to each other?

Wikipedia states that NGC 147 is 2.53 ± 0.11 Mly away,
and that NGC 185 is 2.05 ± 0.13 Mly
away for a radial separation of .48 ± 0.17 Mly (that is increasing according to the redshifts).

It also states that these distances were only determined between 2004 & 2007.

The Binary Galaxy hypothesis predates these measurements:

• The Binary Galaxies NGC 147 and NGC 185
  The Astronomical Journal, October 1998
  van den Bergh, Sidney

<<Contrary to a previously published claim, it is found that the spheroidal galaxies NGC 147 and NGC 185 probably form a stable binary system. Distance estimates place this pair on the near side of the Andromeda subgroup of the Local Group. The fact that this system has probably remained stable over a Hubble time suggests that it does not have a plunging orbit that brings it very close to M31. It is noted that the only two Local Group galaxy pairs in which the components have comparable masses also have similar morphological types. NGC 147 and NGC 185 are both spheroidal galaxies, while the LMC and SMC are both irregular galaxies. This suggests that protogalaxies of similar mass, that are spawned in similar environments, evolve into objects having similar morphologies.>>

So the implication is...what, exactly? Is our best guess now that the galaxies are about 500000 ly apart (not 37000), and they may (or may not) be a "pair"? A different "source" in Wikipedia with a table of all the satellites of Andromeda, has NGC 147 at 2.2 Mly, and NGC 185 at 2.01 Mly.

[ I really lament how difficult it is to accurately determine distances to anything more than a few tens of thousands of lightyears away. ]

The implication is that you should view astronomical distance estimates as given by Wikipedia with a healthy dose of skepticism and keep an open mind.

Instead of believing every word of Wikipedia, take a look at the "visible evidence":

NGC 147 /left) and NGC 185 (right)

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Color-magnitude diagrams of NGC 147 (left) and NGC 185 (right). Diagram: Puragra Guhathakurta at ResearchGate.

---

Are NGC 147 and NGC 185 close together in the sky? Are they about equally bright? Are they the same morphological type? Are they about the same apparent size? Are they more or less the same color? Do their color-magnitude diagrams look pretty much the same?

Answer: Yes. Yes. Yes. Yes. Yes. And yes.

I'd say that because NGC 147 and NGC 185 have so many similarities, and are so close together in the sky, they are indeed at a very similar distance from us. I can't tell you what the distance between them is, but it would surprise me very much if it is as much as 500,000 light-years.

Ann

[neufer]

Wikipedia states that NGC 147 is 2.53 ± 0.11 Mly away,
and that NGC 185 is 2.05 ± 0.13 Mly
away for a radial separation of .48 ± 0.17 Mly (that is increasing according to the redshifts).

It also states that these distances were only determined between 2004 & 2007.

The Binary Galaxy hypothesis predates these measurements:

• The Binary Galaxies NGC 147 and NGC 185
  The Astronomical Journal, October 1998
  van den Bergh, Sidney

<<Contrary to a previously published claim, it is found that the spheroidal galaxies NGC 147 and NGC 185 probably form a stable binary system. Distance estimates place this pair on the near side of the Andromeda subgroup of the Local Group. The fact that this system has probably remained stable over a Hubble time suggests that it does not have a plunging orbit that brings it very close to M31. It is noted that the only two Local Group galaxy pairs in which the components have comparable masses also have similar morphological types. NGC 147 and NGC 185 are both spheroidal galaxies, while the LMC and SMC are both irregular galaxies. This suggests that protogalaxies of similar mass, that are spawned in similar environments, evolve into objects having similar morphologies.>>

So the implication is...what, exactly? Is our best guess now that the galaxies are about 500000 ly apart (not 37000), and they may (or may not) be a "pair"? A different "source" in Wikipedia with a table of all the satellites of Andromeda, has NGC 147 at 2.2 Mly, and NGC 185 at 2.01 Mly.

[ I really lament how difficult it is to accurately determine distances to anything more than a few tens of thousands of lightyears away. ]

Our best guess now that the galaxies are about 500000 ly apart (not 37000)
; hence, they are highly unlikely to be a "pair".

[johnnydeep]

Wikipedia states that NGC 147 is 2.53 ± 0.11 Mly away,
and that NGC 185 is 2.05 ± 0.13 Mly
away for a radial separation of .48 ± 0.17 Mly (that is increasing according to the redshifts).

It also states that these distances were only determined between 2004 & 2007.

The Binary Galaxy hypothesis predates these measurements:

So the implication is...what, exactly? Is our best guess now that the galaxies are about 500000 ly apart (not 37000), and they may (or may not) be a "pair"? A different "source" in Wikipedia with a table of all the satellites of Andromeda, has NGC 147 at 2.2 Mly, and NGC 185 at 2.01 Mly.

[ I really lament how difficult it is to accurately determine distances to anything more than a few tens of thousands of lightyears away. ]

The implication is that you should view astronomical distance estimates as given by Wikipedia with a healthy dose of skepticism and keep an open mind.

Instead of believing every word of Wikipedia, take a look at the "visible evidence":

NGC 147 /left) and NGC 185 (right)

---

Color-magnitude diagrams of NGC 147 (left) and NGC 185 (right). Diagram: Puragra Guhathakurta at ResearchGate.

---

Are NGC 147 and NGC 185 close together in the sky? Are they about equally bright? Are they the same morphological type? Are they about the same apparent size? Are they more or less the same color? Do their color-magnitude diagrams look pretty much the same?

Answer: Yes. Yes. Yes. Yes. Yes. And yes.

I'd say that because NGC 147 and NGC 185 have so many similarities, and are so close together in the sky, they are indeed at a very similar distance from us. I can't tell you what the distance between them is, but it would surprise me very much if it is as much as 500,000 light-years.

Ann

I certainly don't believe every word of Wikipedia particularly when different pages contain different info, as is the case here. But do two things close in the sky, that look similar, and have very similar populations of stars at similar evolutionary states necessarily imply that they are physically close as well? I guess it could imply that they formed together at about the same time? And even if makes it more probable than not, does anything imply that both are close to Andromeda rather than 500000 ly away? Maybe just that dwarf galaxies are only found in the near environs of larger galaxies, and having them be so far away is very unlikely.

[Ann]

I certainly don't believe every word of Wikipedia particularly when different pages contain different info, as is the case here. But do two things close in the sky, that look similar, and have very similar populations of stars at similar evolutionary states necessarily imply that they are physically close as well? I guess it could imply that they formed together at about the same time? And even if makes it more probable than not, does anything imply that both are close to Andromeda rather than 500000 ly away? Maybe just that dwarf galaxies are only found in the near environs of larger galaxies, and having them be so far away is very unlikely.

I'd say yes, they are indeed physically close, if they have so many similarities and are seen close to one another in the sky.

Let me show you what I mean:

Tadpole background galaxies annotated.png

Take a look at the four groups of galaxies that I have outlined among the background galaxies of the Tadpole galaxy.

1) This is a group of small-looking and very red-looking galaxies. I'd say they are very far away, very redshift-reddened, and, indeed, physically close.

2) This is two galaxies that look yellow-white, and they can even be seen interacting. They are much closer than the red-looking galaxies, much less redshift-reddened, and they are indeed very close to one another.

3) This is a group of tiny blue galaxies (unless they are some "fluff" from the tail of the Tadpole). I'm going to say that they are galaxies. If they are galaxies they are clearly very, very far away, because they are so tiny, but they may well be intrinsically small as well. They are clearly very redshift-reddened and also close together. So why do they look blue? It is because they are forming massive stars at a furious rate, which emit copious amounts of far ultraviolet light. This light has been redshift-reddened to a shade of blue.

4) This is a chance alignment of galaxies that in all probability have nothing to do with one another. Look how wildly different their colors are. These galaxies are not close to one another in space.

Ann

[neufer]

So the implication is...what, exactly? Is our best guess now that the galaxies are about 500000 ly apart (not 37000), and they may (or may not) be a "pair"? A different "source" in Wikipedia with a table of all the satellites of Andromeda, has NGC 147 at 2.2 Mly, and NGC 185 at 2.01 Mly.

Our best guess now that the galaxies are about 500000 ly apart (not 37000)
; hence, they are highly unlikely to be a "pair".

NGC 147 & NGC 185 are two of dozens of "Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy"
which often appear close together but are, in fact, hundreds of thousands of light years apart.

Andromeda.png

A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy
Authors: Rodrigo A. Ibata, Geraint F. Lewis, Anthony R. Conn, et al.
--------------------------------------------------------------------
The distribution of dwarf galaxies around Andromeda.
The filled circles represent the dwarf galaxies used in this analysis.
The PAndAS survey area is shaded in blue.
The red circles are galaxies that lie in a plane centered on Andromeda.
--------------------------------------------------------------------
The Curious Case of Andromeda’s Satellites
by Nick Hand | Jan 26, 2013 | Daily Paper Summaries
.....................................................................
<<Using the Canada-France-Hawaii Telescope, the authors (Ibata et al.) have constructed a large-scale panoramic image of Andromeda and its several dwarf satellites, as part of the Pan-Andromeda Archaeological Survey (PAndAS). Distance measurements for 27 of the dwarf galaxies were obtained using the brightnesses of red giant stars in the individual satellites.

Rather than a random distribution of satellites, they found that 15 of the satellite galaxies are aligned in a planar structure, which is nearly a million light years in diameter but only 30,000 light years thick. The authors also measured the velocities of these satellite galaxies and found that 13 of the 15 galaxies are coherently rotating around Andromeda. They conclude that these results indicate the presence of a planar structure of satellite galaxies around Andromeda with a confidence level of 99.998%. It is important to note that a planar distribution of satellite galaxies has also been observed around the Milky Way, the only other galaxy where such a mapping is possible.>>

<<In the cold dark matter theory, structure grows hierarchically, with small objects collapsing under their self-gravity first and merging in a continuous hierarchy to form larger and more massive objects. Predictions of the cold dark matter paradigm are in general agreement with observations of cosmological large-scale structure. However, several discrepancies between the predictions of the particle cold dark matter paradigm and observations of galaxies and their clustering have arisen including: The disk of satellites problem: Dwarf galaxies around the Milky Way and Andromeda galaxies are observed to be orbiting in thin, planar structures whereas the simulations predict that they should be distributed randomly about their parent galaxies.>>

[johnnydeep]

Ok, so there is a "galaxy of galaxies" orbiting Andromeda galaxy, and the majority of those orbit in a thin disk about 30 Kly thick. But due to the different distance values cited, I'm still not sure how close NGC 147 and NGC 185 really are.

[johnnydeep]

And to Ann's observations about several groups of background galaxies seen in the visual proximity of the Tadpole galaxy, I'm not convinced that their similarity and visual closeness are indicative of actual physical closeness. Could a similar argument be made of these two galaxies in the same FOV?

similar, near each other background galaxies in the FOV of the tadpole galaxy.JPG

[neufer]

Ok, so there is a "galaxy of galaxies" orbiting Andromeda galaxy, and the majority of those orbit in a thin disk about 30 Kly thick. But due to the different distance values cited, I'm still not sure how close NGC 147 and NGC 185 really are.

A disk "nearly a million light years in diameter but only 30,000 light years thick"
will contain galaxies ~500,000 light years apart, on average.

(Note: Distances were obtained using the brightnesses of red giant stars and Ann is biased against anything red.)

[johnnydeep]

Ok, so there is a "galaxy of galaxies" orbiting Andromeda galaxy, and the majority of those orbit in a thin disk about 30 Kly thick. But due to the different distance values cited, I'm still not sure how close NGC 147 and NGC 185 really are.

A disk "nearly a million light years in diameter but only 30,000 light years thick"
will contain galaxies ~500,000 light years apart, on average.

Of course that would depend on how many galaxies there are in that volume.

(Note: Distances were obtained using the brightnesses of red giant stars and Ann is biased against anything red.)

[neufer]

A disk "nearly a million light years in diameter but only 30,000 light years thick"
will contain galaxies ~500,000 light years apart, on average.

The average distance between two points in a circular disk:

• 128r/(45π) [= 452.7 kly for r = 500 kly]

[johnnydeep]

A disk "nearly a million light years in diameter but only 30,000 light years thick"
will contain galaxies ~500,000 light years apart, on average.

The average distance between two points in a circular disk:

• 128r/(45π) [= 452.7 kly for r = 500 kly]

Oh, I see what you were saying! Pick any two random points (or centers of dwarf galaxies), and they will be 500 kly apart on average. It doesn't matter how many points (or galaxies) are in that disk! I was thinking "distance from one galaxy to the next nearest one".

[Guest]

"The disk of satellites problem: Dwarf galaxies around the Milky Way and Andromeda galaxies are observed to be orbiting in thin, planar structures whereas the simulations predict that they should be distributed randomly about their parent galaxies."

Does this imply that the simulations are flawed?
And does it imply that the orbiting dwarf galaxies and the main galaxies may have been formed from very large rotating gas (and dust?) clouds, (similar to the planar solar system)?

[neufer]

"The disk of satellites problem: Dwarf galaxies around the Milky Way and Andromeda galaxies are observed to be orbiting in thin, planar structures whereas the simulations predict that they should be distributed randomly about their parent galaxies."

Does this imply that the simulations are flawed?
And does it imply that the orbiting dwarf galaxies and the main galaxies may have been formed from very large rotating gas (and dust?) clouds, (similar to the planar solar system)?

That's the only conclusion I can come up with (...especially considering that gas densities would have been much higher in the early universe).