Generation(s) of Stars and Galaxies

[ErnieM]

What is the source of the heavier elements that a star "consumes" during the different stages of transformation? Are the stages different between stars with planets and those with no planets?
From the debris of a exploded star. the next "generation of solar systems" is born. Do we know how many "generations" our solar system has gone through?
How about galaxies? Are new star systems formed from the ejected jets of debris as the central massive black hole ran out of solar systems to "digest"? Is there any evidence for or against this statement?
What then is the conceptual eventuality for the massive black hole? Without the tell tale ejection jets, how can we detect the presence of massive black hole wandering in space? Is it gravitational lens effect only?
Presence of dark matter is not directly observed except through the gravitational lens effect. Is there a know way to differentiate the gravitational lens effect between massive black holes and dark matter?
How do dark matter and massive black holes interact? Through gravity. Does massive black hole "consume" dark matter? If so, then it is conceivable that part of the ejected jet debris contains dark matter.

[neufer]

Why do I get the feeling that I'm being asked to do someone else's homework assignment

[ErnieM]

neufer wrote:

Why do I get the feeling that I'm being asked to do someone else's homework assignment

Huh? Can an ordinary drop out joe blow or a retired enthusiast with time on his hands or a teenager more interested in stars than twitter not have as much imagination as one who is taking or have taken a graduate degree in astronomy?

You are assuming only a university professor with a monopoly on imagination can articulate these queries.

Respectfully, if you have worth while knowledge and opinions about my questions that you are willing to share, then lets hear them and maturely use this thread. Thank you.

ErnieM

[Chris Peterson]

What is the source of the heavier elements that a star "consumes" during the different stages of transformation?

The source of all heavy elements is the same: stellar nucleosynthesis.

Are the stages different between stars with planets and those with no planets?

Not so far as anybody knows.

From the debris of a exploded star. the next "generation of solar systems" is born. Do we know how many "generations" our solar system has gone through?

There's no way to clearly define what the length of a "generation" is in this context. The lifetime of stars varies by orders of magnitude, and the Universe had evolved sufficiently by the time the Sun formed that there must have been debris from stars just a few generations removed from the Big Bang, and from stars tens or even hundreds of generations removed.

How about galaxies? Are new star systems formed from the ejected jets of debris as the central massive black hole ran out of solar systems to "digest"? Is there any evidence for or against this statement?

No star systems are observed to be forming in jets from active galaxies. But the released material might ultimately be incorporated into a star forming region in some different galaxy.

What then is the conceptual eventuality for the massive black hole?

It is believed that all black holes eventually evaporate. For a supermassive black hole, the time required is huge compared with the current age of the Universe... but is, nevertheless, finite.

Without the tell tale ejection jets, how can we detect the presence of massive black hole wandering in space? Is it gravitational lens effect only?

We can detect a black hole by the radiation emitted when matter falls into it (which isn't limited to the production of jets), or we can detect it by its gravitational effect on other bodies that we can see directly.

Presence of dark matter is not directly observed except through the gravitational lens effect.

That is not so. There are many observations that indicate the presence of dark matter, such as the nature of galaxy rotation and the movement of galaxies in clusters.

Is there a know way to differentiate the gravitational lens effect between massive black holes and dark matter?

I don't think a gravitational lens has ever been observed which can be associated with a supermassive black hole. As a rule, such objects are simply not massive enough. The black holes at the center of galaxies only represent a tiny fraction of the total mass of those galaxies- most of which is dark matter. When we observe a gravitational lens, we are seeing the effect of an entire galaxy. The only way to see lensing from a massive black hole would be to find an isolated one, which we haven't.

How do dark matter and massive black holes interact? Through gravity. Does massive black hole "consume" dark matter? If so, then it is conceivable that part of the ejected jet debris contains dark matter.

Black holes should consume dark matter in the same way they do ordinary matter. Jets are not produced from material consumed by a black hole, but are ordinary matter that is ejected before reaching the black hole, by a process that involves electromagnetic effects. Dark matter is not affected by electromagnetism, so there is no mechanism for its ejection from the vicinity of a black hole in jets. Presumably, it merely orbits black holes, and is only consumed if the orbital path crosses the event horizon.

[Ann]

How about galaxies? Are new star systems formed from the ejected jets of debris as the central massive black hole ran out of solar systems to "digest"? Is there any evidence for or against this statement?

No star systems are observed to be forming in jets from active galaxies. But the released material might ultimately be incorporated into a star forming region in some different galaxy.

viewtopic.php?f=29&t=28114

By following the above link you can see Minkowski's Object. A jet from a black hole in a giant elliptical galaxy hits a cloud of hydrogen gas and triggers star formation there. There may well be new star systems with planets there.

Ann

[Chris Peterson]

http://asterisk.apod.com/viewtopic.php?f=29&t=28114

By following the above link you can see Minkowski's Object. A jet from a black hole in a giant elliptical galaxy hits a cloud of hydrogen gas and triggers star formation there. There may well be new star systems with planets there.

Indeed... although I wonder how much material the jet is contributing. The primary mechanism of star formation here is the shock front created in an external hydrogen cloud; without that, would we see star formation in a jet? I think the jet is a catalyst for formation, but not the source of material incorporated in new stars (outside of trace amounts).

But in any case, depending on how we interpret the original question, there seems little doubt that the massive black hole is responsible for new star formation, even if it isn't actually recycling galactic material in the process.

[ErnieM]

Chris wrote:

Ernie wrote:

1. What then is the conceptual eventuality for the massive black hole?
2. Without the tell tale ejection jets, how can we detect the presence of massive black hole wandering in space? Is it gravitational lens effect only?
3. Presence of dark matter is not directly observed except through the gravitational lens effect.
4. Is there a known way to differentiate the gravitational lens effect between massive black holes and dark matter?
5. How do dark matter and massive black holes interact? Through gravity. Does massive black hole "consume" dark matter? If so, then it is conceivable that part of the ejected jet debris contains dark matter.

In reply Chris wrote:

1. It is believed that all black holes eventually evaporate. For a supermassive black hole, the time required is huge compared with the current age of the Universe... but is, nevertheless, finite.

2. We can detect a black hole by the radiation emitted when matter falls into it (which isn't limited to the production of jets), or we can detect it by its gravitational effect on other bodies that we can see directly.

3. That is not so. There are many observations that indicate the presence of dark matter, such as the nature of galaxy rotation and the movement of galaxies in clusters.

4. I don't think a gravitational lens has ever been observed which can be associated with a supermassive black hole. As a rule, such objects are simply not massive enough. The black holes at the center of galaxies only represent a tiny fraction of the total mass of those galaxies- most of which is dark matter. When we observe a gravitational lens, we are seeing the effect of an entire galaxy. The only way to see lensing from a massive black hole would be to find an isolated one, which we haven't.

5. Black holes should consume dark matter in the same way they do ordinary matter. Jets are not produced from material consumed by a black hole, but are ordinary matter that is ejected before reaching the black hole, by a process that involves electromagnetic effects. Dark matter is not affected by electromagnetism, so there is no mechanism for its ejection from the vicinity of a black hole in jets. Presumably, it merely orbits black holes, and is only consumed if the orbital path crosses the event horizon.

As massive black hole(s) at the center of a galaxy consumes its "ordinary" matter leaving only dark matter, its electromagnetic signatures including radio, will become undetectable by current technologies. In essence the galaxy goes dark, "invisible". Even before the dark hole(s) evaporates its mass is relatively small so the mass of this dark galaxy is primarily from the "orbiting" dark matter. Therefore we only have the gravitational lens effect left and its presumed gravitational effect on neighboring galaxies to indirectly "observe" or infer presence of such "dark galaxy". Is 14 billion years long enough to evolve such "dark galaxy"? If so, is it also conceivable that the observed separated clump of dark matter in large galaxy clusters are in reality "dark galaxies".
I agree that we do not know. And we will never know unless we start searching for it. How? Very hard? Yes. Impossible? I hope not.

[Chris Peterson]

As massive black hole(s) at the center of a galaxy consumes its "ordinary" matter leaving only dark matter, its electromagnetic signatures including radio, will become undetectable by current technologies. In essence the galaxy goes dark, "invisible".

But that isn't what happens. Black holes don't easily consume matter. They are gravitational sources, and like all gravitational sources, other bodies with mass orbit them. They are not vacuum cleaners sucking everything in. The only material that gets absorbed is stuff that gets so close it interacts with other material via electromagnetic and fluid dynamic effects- things that result in a loss of energy, allowing the orbits to decay. A central black hole will never be able to absorb the vast majority of matter in a galaxy, whether ordinary or dark. The only mechanism by which galaxies will go dark is the ultimate burning out of all the stars, leaving behind cold gas and dust too thin to initiate new star formation, and the hulks of old stars: black holes, neutron stars, and white dwarfs. The Universe is nowhere near old enough for that to have happened to any galaxy.

[ErnieM]

Chris wrote:

The only mechanism by which galaxies will go dark is the ultimate burning out of all the stars, leaving behind cold gas and dust too thin to initiate new star formation, and the hulks of old stars: black holes, neutron stars, and white dwarfs. The Universe is nowhere near old enough for that to have happened to any galaxy.

From Wipedia: http://en.wikipedia.org/wiki/Dark_galax ... l_evidence

"Astronomers first suspected that there was an invisible, dark, galaxy, upon observing galaxy NGC 4254. This unusual-looking galaxy appears to be one partner in a cosmic collision. The only evidence is the following: gas is being siphoned away into a tenuous stream, and one of its spiral arms is being stretched out. The other partner in this collision is nowhere to be seen. The researchers calculated that an object with a mass of 1011 M☉ made close passage with NGC 4254 within the last 100 million years creating the gas stream and tearing at one of its arms. This was the clue that an invisible dark matter galaxy might be nearby.
Nature of dark galaxy

Origin
In 2000 astronomers found a gas cloud VIRGOHI21 and tried to find a theory of what it was and or why it could cause such a gravitational pull from NGC 4254 galaxy. After years of running out of other explanations some have concluded that VIRGOHI21 is a dark galaxy, due to the massive effect it had on NGC 4254.[1]

Size
The actual size of a dark galaxy is unknown, because they cannot be spotted with a normal telescope. There have been various estimations of the size of dark galaxies. Two potential sizes could be either double the size of the Milky Way[4] or the size of a small quasar.

Structure
Dark galaxies are composed of dark matter. Furthermore, dark galaxies are theoretically composed of hydrogen and dust.[1] Some scientists support the idea that dark galaxies may contain stars.[3] Yet the exact composition of dark galaxies is unknown because there is no conclusive way to spot them so far.

Methodology to observe dark bodies
Dark galaxies contain no visible stars, and are not visible using optical telescopes. The Arecibo Galaxy Environment Survey (AGES) is a current study using the Arecibo radio telescope to search for dark galaxies, which are predicted to contain detectable amounts of neutral hydrogen. The Arecibo radio telescope is useful where others are not because of its ability to detect the emission from this neutral hydrogen, specifically the 21 cm line.[5]

Alternative theories
Scientists do not have much explanation for some astronomic events, so some use the idea of a dark galaxy to explain these events. Little is known about dark galaxies, and some scientists believe dark galaxy is actually a newly forming galaxy. One such candidate is in the Virgo cluster. This candidate contains very few stars. Scientist classify this galaxy as a newly forming galaxy, rather than a dark galaxy.[6]

Potential dark galaxies

HE0450-2958
Main article: HE0450-2958
HE0450-2958 is an unusual quasar (a star like object that may send out radio waves and other forms of energy). This one in particular has many large red shifts.[7] HE0450-2958 has no visible host galaxy (a galaxy surrounding the quasar) detected around it. It has been suggested that this may be a dark galaxy in which a quasar has become active. However subsequent observations revealed that a normal host galaxy is probably present.[8]

HVC 127-41-330
Main article: HVC 127-41-330
HVC 127-41-330 is a cloud at high speed between the Andromeda and the Triangulum Galaxy. Astronomer Josh Simon considers this cloud to be a dark galaxy because of the speed of its rotation and its predicted mass.[9]

VIRGOHI21
Main article: VIRGOHI21
The discovery of VIRGOHI21 was announced in February 2005, and it was the first good candidate to be a true dark galaxy.[2][3][10][11] It was found when AGES was looking for the 21 cm-wavelength radio waves emitted by hydrogen (H). Its dynamics are apparently inconsistent with the predictions of the Modified Newtonian Dynamics (MOND) theory.[12] Some researchers have since discounted the possibility of VIRGOHI21 being a dark galaxy and believe it is more likely a "tidal tail"[13] of nearby galaxy NGC 4254, which is experiencing gravitational perturbations as it enters the Virgo cluster

[Chris Peterson]

Chris wrote:

The only mechanism by which galaxies will go dark is the ultimate burning out of all the stars, leaving behind cold gas and dust too thin to initiate new star formation, and the hulks of old stars: black holes, neutron stars, and white dwarfs. The Universe is nowhere near old enough for that to have happened to any galaxy.

From Wipedia: http://en.wikipedia.org/wiki/Dark_galax ... l_evidence

"Astronomers first suspected that there was an invisible, dark, galaxy, upon observing galaxy NGC 4254. This unusual-looking galaxy appears to be one partner in a cosmic collision. The only evidence is the following: gas is being siphoned away into a tenuous stream, and one of its spiral arms is being stretched out. The other partner in this collision is nowhere to be seen. The researchers calculated that an object with a mass of 1011 M☉ made close passage with NGC 4254 within the last 100 million years creating the gas stream and tearing at one of its arms. This was the clue that an invisible dark matter galaxy might be nearby.

This is an entirely different thing. If dark matter galaxies exist, they are not galaxies that went dark. There hasn't been enough time for galaxies to go dark. This Wikipedia article is about hypothetical galaxies made from dark matter only. While interesting, I don't see the connection between them and the normal galaxies and supermassive black holes under discussion here.

[Ann]

I checked out http://en.wikipedia.org/wiki/Dark_galax ... l_evidence and was surprised at some things it said. For example, I found this confusing:

In 2000 astronomers found a gas cloud VIRGOHI21 and tried to find a theory of what it was and or why it could cause such a gravitational pull from NGC 4254 galaxy. After years of running out of other explanations some have concluded that VIRGOHI21 is a dark galaxy, due to the massive effect it had on NGC 4254.

Surely a cloud of gas isn't a dark matter galaxy? Surely gas isn't dark matter?

Dark galaxies are composed of dark matter. Furthermore, dark galaxies are theoretically composed of hydrogen and dust.[1] Some scientists support the idea that dark galaxies may contain stars.[3] Yet the exact composition of dark galaxies is unknown because there is no conclusive way to spot them so far.

I thought all galaxies contained at least some gas, at least some dust, at least some stars, and at least some dark matter. So what's the difference between dark matter galaxies and ordinary galaxies, if both contain the same "ingredients"?

And if a dark galaxy contains hydrogen, dust and stars as well as dark matter, why would it be so very hard to spot it?

This discussion reminds me of an extraordinary galaxy found by the great astrophotographer David Malin. Here you can read about that galaxy. According to this link, galaxy Malin 1 is the largest spiral ever found, with a disk that is 650,000 light-years across. The disk of the Milky Way is usually said to be 100,000 light-years across. Malin 1 contains a normal barred spiral of Hubble type SB0 or SBa (meaning it lacks or mostly lacks star formation) in its center. The bright bar of the normal-sized disk is 30,000 light-years long. Even though this central part of Malin 1 is "normally bright" and the extended outer disk is almost invisible, Aaron Barth of the University of California estimates that 80% of Malin 1's light comes from the extended disk. And Malin 1 emits eight times more light than the entire Milky Way, according to this link.

Malin 1 is isolated and undisturbed in space, which almost certainly explains, or at least contributes to, its large size and its low surface brightness. It seems certain that the disk contains some dark matter. But would Malin 1 be described as a dark matter galaxy? I wouldn't describe it like that, in view of the fact that it does contain a lot of stars and a lot of hydrogen gas - more than ten times more gas than the Milky Way.

Ann

[Chris Peterson]

Surely a cloud of gas isn't a dark matter galaxy? Surely gas isn't dark matter?

No, but a dark matter galaxy could surely contain some normal matter as well- in this case gas.

I thought all galaxies contained at least some gas, at least some dust, at least some stars, and at least some dark matter. So what's the difference between dark matter galaxies and ordinary galaxies, if both contain the same "ingredients"?

Who knows? It's highly uncertain if dark matter galaxies even exist. And if they do, they may not be "galaxies" at all, as we understand the term. But I think the answer to your question is that they contain the same ingredients, but in very different proportions.

And if a dark galaxy contains hydrogen, dust and stars as well as dark matter, why would it be so very hard to spot it?

Because they don't contain much hydrogen and dust, and what is there is cold, making it hard to detect. And they contain few if any stars. If they exist at all, of course.

[ErnieM]

This is an entirely different thing. If dark matter galaxies exist, they are not galaxies that went dark. There hasn't been enough time for galaxies to go dark. This Wikipedia article is about hypothetical galaxies made from dark matter only. While interesting, I don't see the connection between them and the normal galaxies and supermassive black holes under discussion here.

The Wikipedia article did not elaborate on how the dark galaxy was formed. Your are adding your own editorial.

Let us take a broader look at the evolution process of galaxies and let me present another way a galaxy can go dark. In the merging process of two or more galaxies, the dominant galaxy or galaxies eats up all the ordinary matter from the weakest one leaving only the dark galaxy with the central black hole and dark matter. From here, the dance between the galaxies continues for million if not billion of years until the weakest dark galaxy is either gobbled up by the dominant one or instantaneously thrown away in the jet stream ejection process or by the sling shot effect of gravity.

Our own Milky way is now accepted to have several small satellite dark galaxies partly responsible for the warping of the disk.

WMAP Measurements
The Wilkinson Microwave Anisotropy Probe (WMAP) was designed to achieve more precise measurements of the anisotropy in CMB. Within the framework of the "Lambda-CDM Model" of the universe, the WMAP data indicate that the age of the universe is 12.73 ± 0.12 billion years old.

WMAP data also indicate that the current universe consists of 4.6% ordinary baryonic matter; 23% unknown dark matter; 72% dark energy; and less than 1% neutrinos. The ratio of the energy density to to the critical density was determined to be 1.0052 ± 0.0064 . These findings created big excitement in the scientific community. The academic papers [1][2] of the WMAP collaboration were among the most cited papers in history.

Following link shows how much the composition of the universe have changed in 13.7 billion year.
http://upload.wikimedia.org/wikipedia/c ... ontent.png

Correct me if I am wrong in presuming the the 4.6% atoms represent the ordinary matter today and the combined 17% photons and atoms were the ordinary matter 13.7 years ago. Dark matter is more dramatic, down to 23% from 63 %. Losing matter, dark and ordinary, at this rate must have happen not at solar system but at galactic scale.

Stars die at different rate. Some die faster and younger than others and galaxy compositions are also different specially at the early universe. So how can one be so conclusive that there is not enough time for galaxies to go dark.

[Chris Peterson]

Let us take a broader look at the evolution process of galaxies and let me present another way a galaxy can go dark. In the merging process of two or more galaxies, the dominant galaxy or galaxies eats up all the ordinary matter from the weakest one leaving only the dark galaxy with the central black hole and dark matter. From here, the dance between the galaxies continues for million if not billion of years until the weakest dark galaxy is either gobbled up by the dominant one or instantaneously thrown away in the jet stream ejection process or by the sling shot effect of gravity.

I would argue that this should not be described as a galaxy going dark. It's just a galactic collision (a common thing) where the gravitational dynamics happen to operate like a filter, separating dark matter and ordinary matter unevenly. Indeed, an object was recently observed where the dark matter halo appears to have been substantially separated from the ordinary matter core. Nothing is going dark, however. You have the same total luminosity before and after. Nor is there any reason to think the central black hole wouldn't remain with the ordinary matter. Why assume it would end up with the dark matter? That seems dynamically unlikely.

Our own Milky way is now accepted to have several small satellite dark galaxies partly responsible for the warping of the disk.

Reference? I don't think anything like this is widely accepted... I don't even know that it has been proposed. It is suggested with good arguments that the warping of the disc is associated with dark matter, but not dark matter galaxies.

Correct me if I am wrong in presuming the the 4.6% atoms represent the ordinary matter today and the combined 17% photons and atoms were the ordinary matter 13.7 years ago. Dark matter is more dramatic, down to 23% from 63 %. Losing matter, dark and ordinary, at this rate must have happen not at solar system but at galactic scale.

The Universe has a (presumably) fixed amount of energy, and during its early evolution, that energy changed form. When most of this was occurring, there were no galaxies.

Stars die at different rate. Some die faster and younger than others and galaxy compositions are also different specially at the early universe. So how can one be so conclusive that there is not enough time for galaxies to go dark.

We don't observe any galaxies consisting only of massive stars. Quite the opposite, every galaxy is dominated by low mass stars with lifetimes longer than the current age of the Universe. No theory that I'm familiar with predicts galaxies which could have gone dark. The lack of theory, combined with the lack of observation, makes a good case for no galaxies having "gone dark".

[ErnieM]

Chris wrote:

ErnieM wrote:
Let us take a broader look at the evolution process of galaxies and let me present another way a galaxy can go dark. In the merging process of two or more galaxies, the dominant galaxy or galaxies eats up all the ordinary matter from the weakest one leaving only the dark galaxy with the central black hole and dark matter. From here, the dance between the galaxies continues for million if not billion of years until the weakest dark galaxy is either gobbled up by the dominant one or instantaneously thrown away in the jet stream ejection process or by the sling shot effect of gravity.

I would argue that this should not be described as a galaxy going dark. It's just a galactic collision (a common thing) where the gravitational dynamics happen to operate like a filter, separating dark matter and ordinary matter unevenly. Indeed, an object was recently observed where the dark matter halo appears to have been substantially separated from the ordinary matter core. Nothing is going dark, however. You have the same total luminosity before and after. Nor is there any reason to think the central black hole wouldn't remain with the ordinary matter. Why assume it would end up with the dark matter? That seems dynamically unlikely.

From article Star-Crossed: Milky Way's......http://www.scientificamerican.com/artic ... ite-spiral

"The Sagittarius Dwarf Galaxy, first discovered in 1994, is a satellite galaxy that is slowly being torn apart and ingested into the larger Milky Way. In the process, however, Sagittarius seems to have been making its presence felt. A group of astrophysicists at the University of Pittsburgh, the University of California, Irvine, and Florida Atlantic University simulated the gravitational infall of Sagittarius over the past few billion years to uncover what effects the dwarf galaxy may have had on the Milky Way."

I would argue that over time, Sagittarious Dwarf can become another dark satellite galaxy.

Chris wrote:

ErnieM wrote:
Correct me if I am wrong in presuming the the 4.6% atoms represent the ordinary matter today and the combined 17% photons and atoms were the ordinary matter 13.7 years ago. Dark matter is more dramatic, down to 23% from 63 %. Losing matter, dark and ordinary, at this rate must have happen not at solar system but at galactic scale.

The Universe has a (presumably) fixed amount of energy, and during its early evolution, that energy changed form. When most of this was occurring, there were no galaxies.

No argument. And massive black holes, alone or in dark galaxies, is another form of energy but not visible.

Chris wrote:

ErnieM wrote:
Our own Milky way is now accepted to have several small satellite dark galaxies partly responsible for the warping of the disk.

Reference? I don't think anything like this is widely accepted... I don't even know that it has been proposed. It is suggested with good arguments that the warping of the disc is associated with dark matter, but not dark matter galaxies.

From article "How Dark Matter Messes....http://www.scientificamerican.com/artic ... our-galaxy

"The Magellanic Clouds are not the only satellites of the Milky Way. Astronomers have counted some two dozen. This video shows their three- dimensional position relative to the plane of the galaxy, where the sun and most other stars lie. Here, the mystery is why there are not more satellites: by rights, our galaxy should have hundreds. The Sloan Digital Sky Survey, which scanned the region shown by the pinkish cone, partially filled in the gap by finding another dozen satellites. They are extremely dim and composed mostly of dark matter. The rest of the satellites out there may be completely invisible."

From article "The Dark Side of the Milky Way" http://www.scientificamerican.com/artic ... -milky-way

"Dark matter is one of the great scientific mysteries of our time, but once astronomers accepted its existence, the answers to many other cosmic mysteries fell into place.
Whatever this unknown material may be, it seems to explain why the disk of our Milky Way galaxy has such a pronounced warp at its outer rim. Orbiting satellite galaxies naturally tend to distort the galaxy, but their gravitational effect would be too weak without the amplification that dark matter provides.

Another question dark matter answers is why the Milky Way appears to have fewer such satellite galaxies than models predict it should. It turns out that the satellites are probably out there, though composed almost entirely of dark matter, making them hard to detect."

Chris wrote:

ErnieM wrote:
Stars die at different rate. Some die faster and younger than others and galaxy compositions are also different specially at the early universe. So how can one be so conclusive that there is not enough time for galaxies to go dark.

We don't observe any galaxies consisting only of massive stars. Quite the opposite, every galaxy is dominated by low mass stars with lifetimes longer than the current age of the Universe. No theory that I'm familiar with predicts galaxies which could have gone dark. The lack of theory, combined with the lack of observation, makes a good case for no galaxies having "gone dark"

From article Dark-Matter Dwarf Galaxies..http://www.scientificamerican.com/artic ... rf-galaxie

"New evidence suggests that hundreds of unseen dwarf galaxies made of dark matter encircle our Milky Way and other large, visible galaxies.
.
.
The scientists remain puzzled as to why these dark-matter galaxies contain few or no stars, however, given that 10 to 20 percent of their mass should exist as normal matter. "It's difficult to hide that much material," Dalal observes. The findings will appear in the June 10 issue of the Astrophysical Journal."

[Chris Peterson]

I would argue that this should not be described as a galaxy going dark. It's just a galactic collision (a common thing) where the gravitational dynamics happen to operate like a filter, separating dark matter and ordinary matter unevenly. Indeed, an object was recently observed where the dark matter halo appears to have been substantially separated from the ordinary matter core. Nothing is going dark, however. You have the same total luminosity before and after. Nor is there any reason to think the central black hole wouldn't remain with the ordinary matter. Why assume it would end up with the dark matter? That seems dynamically unlikely.

From article Star-Crossed: Milky Way's......http://www.scientificamerican.com/artic ... ite-spiral
I would argue that over time, Sagittarious Dwarf can become another dark satellite galaxy.

I don't think so. What happens is that the Sagittarius Dwarf Galaxy is gradually absorbed into the Milky Way- a type of merger that occurs all over the Universe. Nothing is going dark, and there's nothing to suggest that the dark matter of the SDG will remain distinct from the dark matter of the Milky Way.

No argument. And massive black holes, alone or in dark galaxies, is another form of energy but not visible.

They are energy, but not a different form. Black holes are in the "atoms" category: ordinary baryonic matter.

Our own Milky way is now accepted to have several small satellite dark galaxies partly responsible for the warping of the disk.

Reference? I don't think anything like this is widely accepted... I don't even know that it has been proposed. It is suggested with good arguments that the warping of the disc is associated with dark matter, but not dark matter galaxies.

From article "How Dark Matter Messes....http://www.scientificamerican.com/artic ... our-galaxy

The article does not suggest that dark galaxies around the Milky Way are widely accepted. It says that there are satellite galaxies made up of various mixes of ordinary and dark matter (which was already known), and speculates about galaxies with even higher dark-to-ordinary matter ratios (which have not been observed, and are not required to explain the shape of the Milky Way- as the first article you reference makes clear)

From article "The Dark Side of the Milky Way" http://www.scientificamerican.com/artic ... -milky-way

"Dark matter is one of the great scientific mysteries of our time, but once astronomers accepted its existence, the answers to many other cosmic mysteries fell into place.
Whatever this unknown material may be, it seems to explain why the disk of our Milky Way galaxy has such a pronounced warp at its outer rim. Orbiting satellite galaxies naturally tend to distort the galaxy, but their gravitational effect would be too weak without the amplification that dark matter provides.

Sure. But this is about dark matter, not dark matter galaxies.

From article Dark-Matter Dwarf Galaxies..http://www.scientificamerican.com/artic ... rf-galaxie

"New evidence suggests that hundreds of unseen dwarf galaxies made of dark matter encircle our Milky Way and other large, visible galaxies.
.
.
The scientists remain puzzled as to why these dark-matter galaxies contain few or no stars, however, given that 10 to 20 percent of their mass should exist as normal matter. "It's difficult to hide that much material," Dalal observes. The findings will appear in the June 10 issue of the Astrophysical Journal."

This is a ten-year old article. It would be interesting to check up on what these researchers have published recently. In any case, however, this is all made confusing by conflicting terminology. In general, a "dark matter galaxy" these days refers to a hypothetical isolated galaxy. What this article is discussing is clumps of dark matter orbiting inside or just outside the dark matter halo of large galaxies. Calling a non-homogeneous mix of dark matter around a large galaxy a halo with dark satellite galaxies confuses two different concepts. And in the context of the earlier discussion, these clumps of dark matter (if they exist) don't seem to be associated with any ordinary matter- including supermassive black holes.

[neufer]

From article Dark-Matter Dwarf Galaxies..http://www.scientificamerican.com/artic ... rf-galaxie

"New evidence suggests that hundreds of unseen dwarf galaxies
made of dark matter encircle our Milky Way and other large, visible galaxies.
..............................................................
The scientists remain puzzled as to why these dark-matter galaxies contain few or no stars, however, given that 10 to 20 percent of their mass should exist as normal matter. "It's difficult to hide that much material," Dalal observes. The findings will appear in the June 10 issue of the Astrophysical Journal."

This is a ten-year old article. It would be interesting to check up on what these researchers have published recently. In any case, however, this is all made confusing by conflicting terminology. In general, a "dark matter galaxy" these days refers to a hypothetical isolated galaxy. What this article is discussing is clumps of dark matter orbiting inside or just outside the dark matter halo of large galaxies. Calling a non-homogeneous mix of dark matter around a large galaxy a halo with dark satellite galaxies confuses two different concepts. And in the context of the earlier discussion, these clumps of dark matter (if they exist) don't seem to be associated with any ordinary matter- including supermassive black holes.

[ErnieM]

Chris wrote:

I don't think so. What happens is that the Sagittarius Dwarf Galaxy is gradually absorbed into the Milky Way- a type of merger that occurs all over the Universe. Nothing is going dark, and there's nothing to suggest that the dark matter of the SDG will remain distinct from the dark matter of the Milky Way.

Who is suggesting otherwise? Eventually. Long after it has gone dark and most or all of its ordinary matter is separated from the dark matter and absorbed into the Milky Way. There are observed evidences that such separation does occur.

Check the "In Brief" of the article http://www.scientificamerican.com/artic ... -milky-way
Partly it says and I quote "Another question dark matter answers is why the Milky Way appears to have fewer such satellite galaxies than models predict it should. It turns out that the satellites are probably out there, though composed almost entirely of dark matter, making them hard to detect."

Chris wrote:

This is a ten-year old article. It would be interesting to check up on what these researchers have published recently. In any case, however, this is all made confusing by conflicting terminology. In general, a "dark matter galaxy" these days refers to a hypothetical isolated galaxy. What this article is discussing is clumps of dark matter orbiting inside or just outside the dark matter halo of large galaxies. Calling a non-homogeneous mix of dark matter around a large galaxy a halo with dark satellite galaxies confuses two different concepts. And in the context of the earlier discussion, these clumps of dark matter (if they exist) don't seem to be associated with any ordinary matter- including supermassive black holes.

No argument. The other article makes the case for the isolated dark matter galaxy orbiting the Milky Way.

[ErnieM]

Chris wrote:

This is a ten-year old article. It would be interesting to check up on what these researchers have published recently. In any case, however, this is all made confusing by conflicting terminology. In general, a "dark matter galaxy" these days refers to a hypothetical isolated galaxy. What this article is discussing is clumps of dark matter orbiting inside or just outside the dark matter halo of large galaxies. Calling a non-homogeneous mix of dark matter around a large galaxy a halo with dark satellite galaxies confuses two different concepts.

The following articles prove that "dark matter galaxies" are not hypothetical isolated one.
Astronomers find a Dark Matter Galaxy, far, far away http://web.mit.edu/newsoffice/2012/dark ... -0119.html
Ghost Galaxies of early universe found by Hubble ... http://www.foxnews.com/scitech/2012/07/ ... 071513122/

Furthermore, dark matter filaments are even found separate and outside of the galaxy clusters. http://www.hindustantimes.com/HTNext/Li ... 86339.aspx

This brings to mind the "Under the Street Light" story that goes

"One dark evening a man was on his hands and knees under a street light looking through the grass.
A pedestrian asked what he was looking for.
"The keys to my car." replied the man.
Having some time and feeling helpful, the pedestrian joined the man in his search.
After a while, with no success, the pedestrian asked: "Where were you when you lost your keys?"
"Over there by my car." the man gestured.
The pedestrian was puzzled. "Why are you looking for them here?"
The man without keys explained: "The light's better!"

Astronomers are now looking for dark matter beyond the street lights.

[ErnieM]

This article "Herschel Measures Dark Matter for Star Forming Galaxies" answers all the questions I raised on this discussion.
http://www.nasa.gov/mission_pages/hersc ... 10216.html
This gives me a new and more logical understanding of stars and galaxy formation and the important role of dark matter. The process begun sometime after the BB, when gravity from coalescing dark matter creating a "wells" of various size in space for gas within the vicinity to "fall and collect". What happens next, stars and galaxy forming or not, depends on the resulting combined mass in the "well".
Theoretically, there could still be many dark matter only "wells" from the BB, with little or no star forming gas floating in space. In summary, "dark galaxies or wells" of dark matter are the beginning not the end.