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Thread: Question about First-Gen Star Formation

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    Question about First-Gen Star Formation

    If only molecular hydrogen (H2) forms stars, and atomic hydrogen (H) doesn't bond to itself to form H2 without the presence of dust, and dust is made of elements like carbon and oxygen forged inside stars, then how did the first stars form?

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    Quote Originally Posted by CaptainToonces View Post
    If only molecular hydrogen (H2) forms stars, and atomic hydrogen (H) doesn't bond to itself to form H2 without the presence of dust, and dust is made of elements like carbon and oxygen forged inside stars, then how did the first stars form?

    Some answers here.
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    The bottom line is, either it is untrue that molecular hydrogen requires dust, or that gravitational contraction requires molecular hydrogen. Probably both are untrue, as it is just a question of timescales-- dust may help molecular hydrogen form faster, and molecular hydrogen may help the gas radiate away heat and contract faster, but all you need is contraction to get a star-- however long it takes.

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    What are the prevalent mechanisms of protostar cooling at low metallicity?

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    part 3 of http://www.mpia.de/RSF13/archive/day_5/Leroy.pdf claims that you don't need molecules to get star formation, so it appears that what happens at zero metallicity is you just go straight from atomic hydrogen to stars. I'm not sure what cooling it is using, perhaps the cooling H-minus opacity, the opacity that creates sunlight. But you still need a reasonable temperature to get the free electrons, so very cold gas probably wouldn't have much of that. It's not that easy to get cooling from very cold H atoms, but the figure in part 3 does seem to claim that's what happens. It would be better if the figure scale was included!
    Last edited by Ken G; 2019-Jan-08 at 08:39 PM.

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    A further standard response is that H2 formation proceeds (at a low rate) via a charge-exchange reaction so that even a very small fraction of ionized atomic hydrogen allows the process. Even so, most (not all) simulation of star formation in zero-metal gas generate distributions of stellar mass weighted to high values, sometimes hundreds of solar masses, where the effects of gravitation collapse with minimal cooling will still shrink to stars.

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    I've heard that also, that you get higher mass stars when there's less metallicity, presumably because the cooling is so inefficient. Note you always do need a lot of cooling though-- to make a star (by which I mean, to get core fusion), you always need about the same amount of cooling per atom. The atoms have to get to a fairly fixed temperature of 10-20 million K, so that requires a similar amount of energy lost per atom (as the kinetic energy per atom is always closely equal to the energy lost by cooling). So the time to form a star is always the time to lose that amount of energy, though in the later stages the ball of gas is not optically thin any more so the cooling rate depends on the radius and surface temperature rather than the cooling physics. Perhaps this is where the higher mass comes in-- the higher mass star can stay optically thin to higher internal temperatures (the radius is roughly proportional to the mass at given internal temperature, and the optical depth is roughly inversely proportional to mass at given internal temperature), so maybe the high mass star starts forming very slowly at first but catches up later by still being optically thin when the hydrogen gets warm enough to efficiently cool. But I'm still puzzled how they cool when dominated by cold neutral hydrogen, I know it's not 21 cm emission because that's too slow.

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    Have not found out if this paper was later proven wrong, but it may be of interest here. A first-generation star is Population III.


    https://www.sciencemag.org/news/2015...-made-big-bang

    Astronomers spot first-generation stars, made from big bang
    By Daniel CleryJun. 17, 2015 , 6:00 AM

    A team of astronomers has found the best evidence yet for the very first generation of stars, ones made only from ingredients provided directly by the big bang. Made of essentially only hydrogen and helium, these so-called population III stars are predicted to be enormous in size and to live fast and die young. Until recently, many astronomers had thought they would never be able to see such stars, because they would have all burned and died in the universe’s early history—too far for us to see. But using new instruments on the world’s top telescopes, the team found a uniquely bright galaxy that seems to bear all the hallmarks of containing population III stars.

    =====

    LATER ADD: Found two more papers touching on Population III stars.


    https://arxiv.org/abs/1308.4456

    One Hundred First Stars: Protostellar Evolution and the Final Masses

    Shingo Hirano, Takashi Hosokawa, Naoki Yoshida, Hideyuki Umeda, Kazuyuki Omukai, Gen Chiaki, Harold W. Yorke (Submitted on 21 Aug 2013 (v1), last revised 19 Feb 2014 (this version, v2))

    We perform a large set of radiation hydrodynamics simulations of primordial star formation in a fully cosmological context. Our statistical sample of 100 First Stars show that the first generation of stars have a wide mass distribution M_popIII = 10 ~ 1000 M_sun. We first run cosmological simulations to generate a set of primordial star-forming gas clouds. We then follow protostar formation in each gas cloud and the subsequent protostellar evolution until the gas mass accretion onto the protostar is halted by stellar radiative feedback. The accretion rates differ significantly among the primordial gas clouds which largely determine the final stellar masses. For low accretion rates the growth of a protostar is self-regulated by radiative feedback effects and the final mass is limited to several tens of solar masses. At high accretion rates the protostar's outer envelope continues to expand and the effective surface temperature remains low; such protostars do not exert strong radiative feedback and can grow in excess to one hundred solar masses. The obtained wide mass range suggests that the first stars play a variety of roles in the early universe, by triggering both core-collapse supernovae and pair-instability supernovae as well as by leaving stellar mass black holes. We find certain correlations between the final stellar mass and the physical properties of the star-forming cloud. These correlations can be used to estimate the mass of the first star from the properties of the parent cloud or of the host halo, without following the detailed protostellar evolution.

    ===

    https://arxiv.org/abs/1011.2632

    The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations

    John H. Wise (GA Tech), Matthew J. Turk (Columbia), Michael L. Norman (UCSD), Tom Abel (Stanford) (Submitted on 11 Nov 2010 (v1), last revised 30 Nov 2011 (this version, v4))

    By definition, Population III stars are metal-free, and their protostellar collapse is driven by molecular hydrogen cooling in the gas-phase, leading to large characteristic masses. Population II stars with lower characteristic masses form when the star-forming gas reaches a critical metallicity of 10^-6 - 10^-3.5 Z⊙. We present an adaptive mesh refinement radiation hydrodynamics simulation that follows the transition from Population III to II star formation. The maximum spatial resolution of 1 comoving parsec allows for individual molecular clouds to be well-resolved and their stellar associations to be studied in detail. We model stellar radiative feedback with adaptive ray tracing. A top-heavy initial mass function for the Population III stars is considered, resulting in a plausible distribution of pair-instability supernovae and associated metal enrichment. We find that the gas fraction recovers from 5 percent to nearly the cosmic fraction in halos with merger histories rich in halos above 10^7 solar masses. A single pair-instability supernova is sufficient to enrich the host halo to a metallicity floor of 10^-3 Z⊙ and to transition to Population II star formation. This provides a natural explanation for the observed floor on damped Lyman alpha (DLA) systems metallicities reported in the literature, which is of this order. We find that stellar metallicities do not necessarily trace stellar ages, as mergers of halos with established stellar populations can create superpositions of t-Z evolutionary tracks. A bimodal metallicity distribution is created after a starburst occurs when the halo can cool efficiently through atomic line cooling.
    Last edited by Roger E. Moore; 2019-Jan-09 at 04:02 PM. Reason: add 2 papers
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    Quote Originally Posted by Ken G View Post
    I've heard that also, that you get higher mass stars when there's less metallicity, presumably because the cooling is so inefficient. Note you always do need a lot of cooling though-- to make a star (by which I mean, to get core fusion), you always need about the same amount of cooling per atom. The atoms have to get to a fairly fixed temperature of 10-20 million K, so that requires a similar amount of energy lost per atom (as the kinetic energy per atom is always closely equal to the energy lost by cooling). So the time to form a star is always the time to lose that amount of energy, though in the later stages the ball of gas is not optically thin any more so the cooling rate depends on the radius and surface temperature rather than the cooling physics. Perhaps this is where the higher mass comes in-- the higher mass star can stay optically thin to higher internal temperatures (the radius is roughly proportional to the mass at given internal temperature,
    So consider a Sun-like protostar with central temperature 100 K. This is 100 000 times cooler than Sun, so 100 000 times the radius at 500 AU.
    100 K is dense and hot in old world of 2,7 K, and it is dense and hot in new world of, say, 30 K relic radiation between protostars.
    What would be prevalent routes of radiation for 100 K hydrogen?
    Quote Originally Posted by Ken G View Post
    and the optical depth is roughly inversely proportional to mass at given internal temperature),
    No. Column mass density is what is inversely proportional. A different matter.
    Quote Originally Posted by Ken G View Post
    so maybe the high mass star starts forming very slowly at first but catches up later by still being optically thin when the hydrogen gets warm enough to efficiently cool. But I'm still puzzled how they cool when dominated by cold neutral hydrogen, I know it's not 21 cm emission because that's too slow.
    As for combination, yes, there is the combination mechanism by remaining electrons (and protons) - but increased density would also promote combination of the relict ions, and thereby hinder combination of atoms. At higher densities, there are also three-particle processes to form molecules. But it is not a cooling process. It is a heating process.
    Molecular hydrogen, at low densities, is transparent. It has no dipole moment. It has stretching vibrations and rotations, but due to lack of dipole moment, these cannot radiate.
    At higher densities, infrared emission (and absorption) by collision induced lines becomes legal. But as you see, the optical depth of collisionally induced lines is not proportional to density. Therefore optical depth is not proportional to column density.

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    Quote Originally Posted by chornedsnorkack View Post
    What would be prevalent routes of radiation for 100 K hydrogen?
    That's what is being asked, and even cooler: 10-100 K. The answer remains unclear-- the third paper just cited states that even population III environments cool by molecular hydrogen cooling (which kicks in above 100 K or so), yet the website I cited states that atomic hydrogen can dominate the cooling when there isn't sufficient dust to help molecular hydrogen form. And I saw a paper that talked about observing a cold cloud of atomic hydrogen where the expectation was that it should be dominated by molecular hydrogen (in which case you'd apparently see either CO emission or dust). So if they could see the atomic hydrogen cloud, it is probably 21 cm emission they are seeing, but that could never cool enough to cause star formation because their decay time is like 10 million years and the energy of each emission is very small. So I still don't know the source of cooling reference in part 3 of that link I gave.
    No. Column mass density is what is inversely proportional.
    True, one has to also figure out the opacity per gram, but without knowing more about what controls that, the column mass density is a good start-- especially if temperature is the key issue, not density. It first must be identified what is the emission source, and then its opacity can be analyzed. As for molecular hydrogen, it likely has very low opacity until the temperature gets up above about 100 K, since its transitions are at higher energies than that. The prevailing question is still on the table-- what is the main cooling from 10-100 K for population III gas, versus gas that has metals, dust, and more molecular hydrogen?
    At higher densities, infrared emission (and absorption) by collision induced lines becomes legal. But as you see, the optical depth of collisionally induced lines is not proportional to density. Therefore optical depth is not proportional to column density.
    It depends on the optical depth of what. Key is the various temperature regimes. To get the ball rolling, the gas has to radiate at temperatures from 10-100 K. Then it has to get from 100-1000 K, and after that atomic hydrogen will start to be a highly efficient radiator. These different regimes have different issues, but if we start with the early 10-100 K regime, molecular hydrogen is probably no help in radiating, so the issue would be what the metals and dust are doing directly, not whether or not they help molecular hydrogen form. In the window from 100-1000 K, I would think molecular hydrogen would be a much more efficient radiator than atomic hydrogen, but perhaps atomic hydrogen has the advantage of much lower opacity so the more massive pop III protostars can remain optically thin.
    Last edited by Ken G; 2019-Jan-10 at 03:23 AM.

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    Quote Originally Posted by Ken G View Post
    In the window from 100-1000 K, I would think molecular hydrogen would be a much more efficient radiator than atomic hydrogen, but perhaps atomic hydrogen has the advantage of much lower opacity so the more massive pop III protostars can remain optically thin.
    I'm inferring from what has be said that the lower opacity is essential for massive stars because it buys more time for gravitational collapse of the cloud, otherwise it will get so hot it will push back before it gets to the mass levels of Pop III stars. Is this correct?


    Do these stars have much greater convective transports in the outer layers than normal?
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    A while back Lithium hydride dust forming in the very early universe was a popular topic related to the formation of molecular hydrogen for pop III stars but this seems to have gone out of favour and I cant find why.
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    Quote Originally Posted by George View Post
    I'm inferring from what has be said that the lower opacity is essential for massive stars because it buys more time for gravitational collapse of the cloud, otherwise it will get so hot it will push back before it gets to the mass levels of Pop III stars. Is this correct?
    In the initial stages, yes, because at fixed pressure, you get a higher initial density if you can generate a lower initial temperature. Right now there are lots of cosmic rays and other ways of heating gas that the molecular cloud has to fight against to get cool initially, so the order of events is, the gas gets very cool because of all the metals and dust, then it builds up larger and larger molecular clouds, then the gravitational instability kicks in when the mass hits the "Jeans mass". But this requires that the gas must cool itself fast enough to stay at that initial low temperature as the contraction gets going. Later, the star goes optically thick and the temperature starts to rise, and that leads to the star-- by then it doesn't really matter much what the star is made of.

    But for the first stars, there wouldn't be all those cosmic rays and interstellar turbulence, so there wouldn't be as much heating. There also would be a warmer CMB. So it's not really clear what the initial temperature would end up being-- perhaps it would be quite a bit warmer, and that might help it radiate as needed to get contraction.
    Do these stars have much greater convective transports in the outer layers than normal?
    Convection determines the internal structure after the star has begun forming, and is pretty insensitive to more or less everything. But what is at issue is how do you get stars to start contracting in the first place, long before they become convective.
    Last edited by Ken G; 2019-Jan-11 at 01:52 AM.

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    Quote Originally Posted by Ken G View Post
    But for the first stars, there wouldn't be all those cosmic rays and interstellar turbulence, so there wouldn't be as much heating. There also would be a warmer CMB. So it's not really clear what the initial temperature would end up being-- perhaps it would be quite a bit warmer, and that might help it radiate as needed to get contraction.
    Thanks.

    I did find this 2009 article helpful but it may be a bit old.

    The essence of it, if I somehow understand it, is that the trace H2 will radiate when the bump with the hydrogen atoms but temperatures will remain hotter longer allowing for the mass level to rise to several hundred solar masses, necessarily.

    Here's the georgeeze summary of that article...

    [Though the H2 characters are not sociable with light (in or out)]… H2 will nevertheless emit IR when bumping into H atoms, dropping cloud temperatures down to about 200K to 300K. This allows greater contraction. [This would be for the GNMC (Giant Non-Molecular Cloud), apparently, as] DM maintains its halo shape but the hydrogen becomes disk-like and galaxy-like. Clumps form within this disks, which become stars through a runaway collapse. But how?...

    Since the Jeans mass is proportional to the square of the gas temperature and those early clumps were almost 30X higher than that of today’s GMCs, their Jeans mass would have been almost 1000X larger. [So the lack of metals would explain inefficient cooling and allowing the greater mass.]

    The predicted masses depend primarily on the physics of the hydrogen molecule and only secondarily on the cosmological model. The H2 cannot cool the gas below 200K making this the lower limit of the first star formation.

    Also, H2 cooling becomes inefficient at higher densities so the contraction rate is retarded further to allow at least a few hundred solar masses to accumulate in the prestellar cloud.

    Apparently, by simulations, these clumps are stable to avoid separation, typically limiting fragmentation to binary formation.

    Is this to simplistic or is it close to today's take on it?

    I also recall a few other blurbs that bring-up turbulence, especially supersonic effects, that affect formation, perhaps in both the larger cloud and in the clumps.

    Convection determines the internal structure after the star has begun forming, and is pretty insensitive to more or less everything. But what is at issue is how do you get stars to start contracting in the first place, long before they become convective.
    Yeah, I'm not sure how convection popped out of my head, though I did recently clean one ear.


    There's almost a dissonance when I try to quickly get a handle on the dance with H and the little amounts of H2, ignoring the 1 in 10 part for He.
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    I think you have the key elements there-- H2 is fine as a radiator at temperatures of a few hundred K because it has transitions at 500 K or so. So the first stars formed from clouds that were much warmer than today's GMCs, and you didn't need a lot of H2 because it radiates so well at those warmer temperatures. The key to starting star formation is that you exceed the Jeans mass (which means you simply wait long enough for enough mass to accumulate in the dark matter potential well), and you have something that can radiate well enough to maintain whatever is the temperature. It won't contract if the temperature gets too cool to radiate, so there must have been a kind of window to form the first stars as the universe was cooling. Had the process waited too long, it would have gotten too cool and missed its opportunity. But fortunately it happened in time, as you say when the temperature was still 200-300 K or so. That made all the difference.

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    Quote Originally Posted by Ken G View Post
    I think you have the key elements there-- H2 is fine as a radiator at temperatures of a few hundred K because it has transitions at 500 K or so. So the first stars formed from clouds that were much warmer than today's GMCs, and you didn't need a lot of H2 because it radiates so well at those warmer temperatures.
    That's so interesting partly because it's oxymoronic as well. [To shrink primordial gas, heat it. ]

    The key to starting star formation is that you exceed the Jeans mass (which means you simply wait long enough for enough mass to accumulate in the dark matter potential well), and you have something that can radiate well enough to maintain whatever is the temperature. It won't contract if the temperature gets too cool to radiate, so there must have been a kind of window to form the first stars as the universe was cooling. Had the process waited too long, it would have gotten too cool and missed its opportunity. But fortunately it happened in time, as you say when the temperature was still 200-300 K or so. That made all the difference.
    It really is interesting that massive stars are a must due to the necessary higher temperatures; the gas had to be hot enough to cool, and apparently, the clouds were more massive in that smaller and hotter universe. It's the H2 nuance to this that separates it from the idea of making ice faster by using warm water. [This is another subject worthy of a Cool Astronomy facts repository.]
    Last edited by George; 2019-Jan-13 at 07:23 PM.
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    Quote Originally Posted by George View Post
    That's so interesting partly because it's oxymoronic as well. [To shrink primordial gas, heat it. ]
    Basically, yes. But keep in mind it works because you are also adding mass, so it's ultimately the gravity, not the heating, that causes it to shrink. But yes, if you don't keep the temperature up, and there are no metals, it will resist contracting even if you add as much mass as you like, because it will tend to "bounce back" rather than contract.
    It really is interesting that massive stars are a must due to the necessary higher temperatures; the gas had to be hot enough to cool, and apparently, the clouds were more massive in that smaller and hotter universe.
    In fact there are two issues there-- the higher T means larger Jeans mass, but the Jeans mass is for the whole (non-)molecular cloud. The star mass is something else again, but starting with more massive clouds helps get more massive stars, once you also include some additional details about how stars fragment from the cloud, and when they cease to fragment any further.

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    Quote Originally Posted by Ken G View Post
    In fact there are two issues there-- the higher T means larger Jeans mass, but the Jeans mass is for the whole (non-)molecular cloud.
    Thanks for clarifying that. Is it too soon to have confidence in any IMF for Pop III stars?
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    Yes, there's just not enough data. There isn't even a good theoretical explanation for the IMF of the normal stars that we do see!

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    Quote Originally Posted by Ken G View Post
    It won't contract if the temperature gets too cool to radiate, so there must have been a kind of window to form the first stars as the universe was cooling. Had the process waited too long, it would have gotten too cool and missed its opportunity. But fortunately it happened in time, as you say when the temperature was still 200-300 K or so. That made all the difference.
    Does it mean that there might have been warm, dense hydrogen clouds formed that at 100...150 K were too cool to radiate in the 28,2 \mum line, and were in hydrostatic equilibrium, unable to contract any further?

    What became of such clouds?

    How fast did first metal atoms travel intergalactic distances? A star once formed needs over a million years (for Eddington limit reasons) to exhaust protium and form and release metals - and then the metals needed to get across voids between galaxy clusters to reach galaxy clusters in which no stars had formed.

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    Might be of interest here.

    https://arxiv.org/abs/1901.03344

    Constraining First Star Formation with 21cm-Cosmology

    Anna T. P. Schauer, Boyuan Liu, Volker Bromm (Submitted on 10 Jan 2019)

    Within standard ΛCDM cosmology, Population III (Pop III) star formation in minihalos of mass M_\mathrm{halo}\gtrsim 5\times10^5 M_\odot provides the first stellar sources of Lymanα (Lyα) photons. The Experiment to Detect the Global Epoch of Reionization Signature (EDGES) has measured a strong absorption signal of the redshifted 21 cm radiation from neutral hydrogen at z\approx 17, requiring efficient formation of massive stars before then. In this paper, we investigate whether star formation in minihalos plays a significant role in establishing the early Lyα background required to produce the EDGES absorption feature. We find that Pop III stars are important in providing the necessary Lyα-flux at high redshifts, and derive a best-fitting average Pop III stellar mass of \sim 120 M_\odot per minihalo. Further, it is important to include baryon-dark matter streaming velocities in the calculation, to limit the efficiency of Pop~III star formation in minihalos. Without this effect, the cosmic dawn coupling between 21 cm spin temperature and that of the gas would occur at redshifts higher than what is implied by EDGES.
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    Quote Originally Posted by chornedsnorkack View Post
    Does it mean that there might have been warm, dense hydrogen clouds formed that at 100...150 K were too cool to radiate in the 28,2 \mum line, and were in hydrostatic equilibrium, unable to contract any further?
    Maybe so, I don't know under what circumstances you can get enough molecular hydrogen to allow cooling at those temperatures. Also, the site I linked to earlier talked about atomic hydrogen cooling, but didn't quantify the axes so I'm still unclear on how efficient that is, or even what the process is. Cold atomic hydrogen can radiate in the 21 cm line, but that seems way too slow.
    What became of such clouds?
    Good question, I've certainly never heard it speculated that pristine gas left over from the Big Bang might still persist. Perhaps it all found ways to radiate and form stars, or metals worked their way in (galaxies emit winds that would contain metals).
    How fast did first metal atoms travel intergalactic distances? A star once formed needs over a million years (for Eddington limit reasons) to exhaust protium and form and release metals - and then the metals needed to get across voids between galaxy clusters to reach galaxy clusters in which no stars had formed.
    Yes, but a million years is a blink of the eye, and even the distances between galaxies can be crossed in a billion years or so by galactic wind speeds. So cross-fertilization of metals seems possible on the Hubble time. It sounds like the population III situation is still an area of active research, especially now that there is some data.

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    How did high velocity clouds and dark galaxies form, and how do they persist?
    Are there any dark galaxy clusters/superclusters?

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    High velocity clouds are thought to be baryonic gas stripped out of a galaxy by tidal effects, presumably galactic collisions. Dark galaxies have sketchy observational evidence, but were theorized before they were found, so there must be some reason to expect you can have a dark matter clump that doesn't pull in much baryons, or is so spread out that the baryons it pulls in don't reach the Jeans mass. I suppose the idea is basically that you have dark matter having its own tendency to clump (and I've never understood why, or why it tends to be isothermal), but that doesn't guarantee the baryons will follow. There can be a range of ratios of baryons to dark matter in local regions, so if you happen to have a low ratio, you might get a "dark galaxy." Given how rare is the evidence for dark galaxies, it might be hard to find clusters of them. On the other hand, elliptical galaxies will eventually have all their stars burn out, and by then they'll all be dark galaxies, so there should be many more in the far future.

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    Quote Originally Posted by Ken G View Post
    High velocity clouds are thought to be baryonic gas stripped out of a galaxy by tidal effects, presumably galactic collisions.
    Quote Originally Posted by Ken G View Post
    There can be a range of ratios of baryons to dark matter in local regions, so if you happen to have a low ratio, you might get a "dark galaxy."
    Tidal effects of collisions would operate on stars along with gas. In order for them to strip gas without stars, the galaxy would have to contain regions of gas but no stars.

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    Quote Originally Posted by chornedsnorkack View Post
    Tidal effects of collisions would operate on stars along with gas. In order for them to strip gas without stars, the galaxy would have to contain regions of gas but no stars.
    But aren't stars simply concentrated gas; like the thermos joke, "How do it know?"
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    Quote Originally Posted by George View Post
    But aren't stars simply concentrated gas
    And?

    Elliptical galaxies are galaxies with a lot of stars but very little gas left;
    spiral galaxies are galaxies with a lot of stars, but still a lot of gas left.
    Are there any galaxies with a lot of gas, but few stars yet formed or none at all?

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    Quote Originally Posted by chornedsnorkack View Post
    In order for them to strip gas without stars, the galaxy would have to contain regions of gas but no stars.
    I'm still not following this statement. Are you saying that gas is the primary stripper of gas and stars not so much? That certainly may be true, but if so, why the requirement of no stars?
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    Quote Originally Posted by George View Post
    why the requirement of no stars?
    Because high velocity clouds contain no stars. What do they form from? Do we see any in process of forming?

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    Quote Originally Posted by chornedsnorkack View Post
    Tidal effects of collisions would operate on stars along with gas. In order for them to strip gas without stars, the galaxy would have to contain regions of gas but no stars.
    Yes, it would probably have to be more than just tidal effects. Presumably gas pressure would be involved. Without walls.

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