Very nice summary. Though the Bullet Cluster technically does not prove the existence of dark matter, it's just fully consistent with the predictions of dark matter. MOND might be able to do this too, but right now no-one has a good relativistic version of it to test its predictions. Also, although MOND does require some dark matter, the amount would be consistent with the missing baryons predicted from Big Bang nucleosynthesis, so it wouldn't require a new particle.
Originally shared by Marijan-Marijan Manson (macee)
Ok. ;"even worse". :)
https://youtu.be/z3rgl-_a5C0
This is a wonderful series, but I hear what you're saying. The jury is very much out.
ReplyDeleteReally great summary, thanx for sharing!
ReplyDeleteShow me an ideology that predicts the absence of DM in globular clusters from first principles (cuspy halo problem), rather than variations on falsified exotic particle theories that require fine tuning or ad hoc secondary mechanisms to keep them viable.
ReplyDeleteDavid Carlson See section 4 and references therin :
ReplyDeletehttp://astrorhysy.blogspot.co.uk/2015/08/seven-very-good-reasons-to-be-little.html
Rhys Taylor
ReplyDeleteI see I've been wasting my time reading popular science magazine articles. Your article may be the most coolly analytical and balanced article I've read yet. In, 'The Astonishing Simplicity of Everything', Neil Turok hints that nature has apparently chosen the simplest possible schema without SUSY particles. My hat is off to anyone who can stand aloof from the partisan fray, even if it's only 80/20, but I suspect the best minds have that capability.
As many confirmations as general relativity has had through the years, I'm rather surprised that MOND got as much traction as it had before it started losing ground on galaxy cluster lensing studies. I'd have thought by now that people would have learned their lesson about betting against Einstein, but then again, maybe he's got a target on his back for that very reason.
And I do agree that the 'problems' seem marginal when comparing the conventional (problematic) lineup with one another, but I suggest that's because they haven't been compared to a truly predictive ideology.
Hint 1): Cuspy halo problem. Exotic particle models predict an asymptotic increase in DM concentrations (a cusp) up to the central SMBH in galactic bulges, which is not observed in the rotation rates of stars. I suggest that the absence of DM in globular clusters is a more poignant example of this problem.
Hint 2): The (apparent) relative absence of DM in spiral-galaxy-merger giant elliptical galaxies.
Hint 3): The Tully–Fisher relation is telegraphing the affinity of spiral galaxies to one another, and by extension, their dissimilarity to other galaxies. Galaxy variations are problematical for a bottom up ΛCDM coalescence approach to galaxy formation by the gradual coalescence of smaller galaxies to form larger galaxies. Also, the relative specific angular momentum of spiral galaxies is hard to explain from their supposed spheroidal dwarf galaxy antecedents and their spiral-galaxy-merger elliptical-galaxy descendants.
Hint 4): Early quasars prior to z = 6. We're finding earlier and earlier quasars with supermassive black holes (SMBHs), with theorists scrambling to explain their early appearance by suggesting direct collapse intermediate mass black holes as seeds for SMBHs
Hint 5): Missing satellite problem, too big to fail problem. Giant galaxies formed by the coalescence of smaller galaxies predict many more satellite galaxies. And the 'missing satellite problem' can't simply be fobbed off as dark galaxies, since some of the dark galaxies should be 'too big to fail' in forming stars. (I never understood the 'too big to fail' problem until you explained it in one sentence, which doesn't say much for other science writers.)
Hint 6): Missing baryon problem. Almost half the baryons in the universe predicted by CMB anisotropies and Big Bang nucleosynthesis D/H ratio haven't been found.
...............................
The case for baryonic DM:
Imagine that gravitationally-bound 'giant molecular clouds' come in two states,
- a dark 'normal state' with their stellar metallicity 'snowed out' (sequestered) into icy chondrules which orbit the galactic core on steeply-inclined halo orbits, and
- a familiar, opaque 'excited state', with the stellar metallicity sublimed into into the luminous gaseous state by exposure to stellar radiation in shallow orbits to the galactic disk.
GMCs in their dark 'normal state' are in thermal (hydrostatic) equilibrium at the circa 100 K cooling temperature of molecular hydrogen, but exposure to intense stellar radiation sublimes icy chondrules, causing radiative cooling down to 10-20 K due to infrared radiation of polarized gaseous stellar-metallicity molecules (carbon monoxide, ammonia etc.), which have many more degrees of freedom than molecular hydrogen. So when stellar radiation sublimes icy chondrules, GMCs convert into their opaque excited state, cooling the clouds and eliminating the vast majority of their supporting thermal gas pressure, promoting Jeans instability, converting DM to stars and luminous gas.
ReplyDelete1) Baryonic DM that converts to stars by way of stellar radiation would dismiss the cuspy halo problem as an exotic-particle construct.
2) The relative absence of DM in spiral-galaxy-merger giant elliptical galaxies would again be due to the conversion of DM to stars in galactic collisions, making baryonic DM predictive.
The 1° BAO scale in today's universe along with the D/H ratio are touted as proof of ΛCDM cosmology, but the bandwagon had better find the missing baryons in today's universe before popping their champagne corks, otherwise they may only have succeeded in demonstrating that Big Bang nucleosynthesis is a regulated process, independent of the actual (sequestered) baryon density of the universe. Where I suggest that a majority of baryons were sequestered into warmer proto-spiral-galaxies by the end of the epoch of BBN which did not participate in 'primary (intergalactic) BBN'.
While spontaneous gravitational collapse could not occur in a radiation dominated universe without assistance, the exothermic burning of hydrogen into helium set the stage for isothermal collapse mediated by photodisintegration (helium fission), in the same way that stars over 250 solar masses are suggested to collapse directly into black holes with no supernova thermal rebound. So the universe is suggested to have undergone an episode of gravitational collapse during BBN, with supermassive black holes as a latching mechanism, largely preventing thermal rebound and trapping a majority of baryons in warm gravitationally-bound proto-spiral-galaxies which sat out primary (intergalactic) BBN. Then when proto-spiral-galaxies cooled to the BBN temperature range, they underwent 'BBN rebound' at EXACTLY the same local baryon density, photon-to-baryon ratio, temperature and pressure, forming the EXACT same D/H ratio as primary BBN.
Deuterium burning in protostars is an example of a regulated nuclear reaction in today's universe which occurs at 1 million Kelvins, regardless of star size, from brown dwarfs up to OB supergiants.
3) And fractional condensation of early BAO compressions and rarefactions would lend an asymmetry to the condensing gas which may have given spiral galaxies their typical specific angular momentum (Tully–Fisher relation).
4-6) So early quasars, the missing baryon problem, the missing satellite problem and the too big to fail problem wither away as ΛCDM model constructs.
David Carlson Those are some excellent points. While I'm about 4:1 in favour of WIMP dark matter over MOND, I'm about 8:1 in favour of WIMPS over baryonic dark matter. Neither are particularly decisive though.
ReplyDeleteAs to your points about objections to dark matter, I mostly agree but with a few caveats :
1) - the cuspy halo problem, I agree this is a difficulty. Quite honestly it's not on the list because I forgot about it. Might try and update this at some point.
2) That ellipticals lack dark matter I take as evidence that the hierarchical merging paradigm of galaxy evolution has some very large holes in it, or is perhaps wrong. I am less convinced it's evidence against dark matter but that would be a valid interpretation.
A recent paper has found that something completely unexpected happens when galaxies merge, which I describe here : http://astrorhysy.blogspot.co.uk/2015/10/the-very-interesting-gas-that-doesnt-do.html
3) TFR is MOND's biggest success, in my opinion. I am not convinced this cannot be explained in CDM, however it's even more complicated than what I wrote in the blog post. At low velocity widths, objects which are strongly gas deficient lie to the left of the TFR - they (or at least their gas component) are rotating less quickly than expected. There are also objects known of similar baryonic mass which are rotating more quickly, including some which are stellar dominated and some which are gas dominated. TFR is probably giving us a strong hint of what's going on, but working out what that is isn't easy.
4) And there are also a few normal-ish looking galaxies which have formed early on as well, IIRC.
5) These are indeed major problems, I'd here add the planes of satellites problem as well. However, regarding TBTF, it's worth pointing out that there was a model that predicted the existence of quite large dark galaxies :
http://arxiv.org/abs/astro-ph/0609747
It massively over-predicted the numbers of such galaxies, but it did show that large dark galaxies don't necessarily contradict LCDM. Still controversial.
6) Indeed. IIRC though, the amount of missing baryons would be potentially enough to explain galaxy rotation curves but not the motions of galaxies in clusters.
On the possibility of baryonic dark matter, I would recommend reading this :
http://arxiv.org/abs/1204.4649
It's by my PhD supervisor. Neither he nor I really believe the idea but it is a possibility worth considering. In this case the idea is that the dark matter in galaxies is cold molecular hydrogen, which is extremely difficult to detect. The major problem with this is that there's no known mechanism to stop it from collapsing and forming stars, so star formation rates should be far higher than are observed. As with your 100 K GMCs - 100 K is cold. How do you propose to keep it Jeans stable ?
There are a few other reasons I'm a bit more confident that dark matter, if it exists at all, isn't baryonic :
- There is a lot more dark matter in clusters than in individual galaxies. If it's molecular hydrogen, then how did it get there and why isn't it forming galaxies ?
- I think the Bullet Cluster and similar examples provides pretty strong evidence against this idea. If there was a significant component of baryonic matter in the intracluster medium, it should collide and get stuck in the middle when the two clusters pass through each other. As expected, this is exactly what happens to the known X-ray gas. But the lensing results show that the dark matter continues to follow the galaxies, exactly as the WIMP model predicts. I don't see a way to make this work in a baryonic model.
- The missing baryons from BBN wouldn't be enough to explain all the dark matter. This is a rather more dangerous line of reasoning as it relies on the BBN model being correct, and I don't like using one model to verify another. Ideally the evidence should be independent.
ReplyDeleteAlso, I would add that for me neutrinos provide compelling evidence that the idea of an exotic particle is not so far-fetched. True, they themselves do not have the required properties to explain dark matter. But they do suggest that the idea of a WIMP is not so outlandish. I think the original video is quite right to point out that there's not really a good choice here : either gravity is wrong, or particle physics is wrong. The third option of baryonic dark mattter would mean that all of star formation theory is wrong.
A more optimistic view would be that there's definitely a lot of exciting stuff still to learn.
ReplyDeleteEpoch of Recombination:
I failed to mention the epoch of recombination in my previous abbreviated comment, when I suggest the universe underwent a second episode of nearly-isothermal gravitational collapse mediated by endothermic hydrogen and helium ionization, as soon as the photon pressure was relieved by recombination, collapsing the intergalactic medium into primordial giant (molecular) clouds of perhaps 10^5? to 10^7? Ms (solar mass). With monatomic gas (γ = 5/3), the Jeans mass increases with increasing density, so these primordial GMCs didn't fragment into smaller (Bok) globules at the time. Collapse into primordial GMCs at recombination is suggested to have thermally rebounded off of Population III stars, forming exactly one Population III star per primordial GMC; however, if so, the vast majority must have ended with pair production supernovae that left no (black hole, white dwarf or neutron star) remnants; however, the largest primordial GMCs forming the largest Population III stars underwent photodisintegration supernovae, which formed black holes, and these larger GMCs may have subsequently evolved into globular clusters or dwarf spheroidal galaxies and not paled into dark matter clouds.
"In this case the idea is that the dark matter in galaxies is cold molecular hydrogen, which is extremely difficult to detect. The major problem with this is that there's no known mechanism to stop it from collapsing and forming stars, so star formation rates should be far higher than are observed. As with your 100 K GMCs - 100 K is cold. How do you propose to keep it Jeans stable ?"
I concede your point on impossibility of GMCs having lasted 13 billion years without succumbing to Jeans instability. They say you should always go with your first instinct, which was originally to suggest that primordial GMCs underwent gravitational-collapse fragmentation into (Bok) globules during the epoch of reionization, a size, I suggest, would be hydrostatically supportable against collapse by a gas temperature of 50-100 Kelvins. Even so, the largest globules collapsed to form the first Population II stars. (Conventional wisdom suggests that reionization was caused by Population III stars, while I suggest it was caused by the earliest Population II stars, with Population III stars having formed earlier during the epoch of recombination.) In that case, circa 2 to 200 Ms (solar mass) gravitationally-bound DM (Bok) globules within gravitationally-bound GMCs have persisted for 13 billion years (hydro)static equilibrium at, say, 50-100 Kelvins.
I suppose my greatest fear is the absence of an infrared invisibility window below the thermal and collisional temperature/pressure range of ro-vibrational transitions, where these transitions create a luminous infrared signature for molecular hydrogen, but in most cases, I notice that most detection of H2 has been in hot jets in molecular clouds from prestellar or protostellar cores. Is there an infrared threshold for ro-vibrational transitions in the 50 K to 100 K range? I.e. if the stellar metallicity were sequestered into the solid state, is there a phase space in which H2 could largely avoid detection. I notice that UV detection of H2 clouds rely on strong nearby sources (OB supergiants), so is it possible the Milky Way halo hasn't been extensively probed for H2 clouds?
"- I think the Bullet Cluster and similar examples provides pretty strong evidence against this idea. If there was a significant component of baryonic matter in the intracluster medium, it should collide and get stuck in the middle when the two clusters pass through each other. As expected, this is exactly what happens to the known X-ray gas. But the lensing results show that the dark matter continues to follow the galaxies, exactly as the WIMP model predicts. I don't see a way to make this work in a baryonic model."
I don't know what to think about gas cloud collisions in merging galaxies. I've heard it said that very few if any stars will collide when Andromeda meets up with the Milky Way, but circa 1/2 light year diameter (Bok) globules are vastly larger, and not only that but presumably vastly stickier, since diffuse gas clouds could internally absorb vastly more kinetic energy, linear and angular momentum than essentially point mass stars, presumably even with the addition of multiple star systems into the mix, so yes, I'll agree that slippery DM is a strike against DM globule clusters (GMCs).
ReplyDelete.....................
A Heavy Baryonic Galactic Disc
J. I. Davies, Cardiff University
(Wow, I had a couple of tentative conversations with Chandra Wickramasinghe from Cardiff U. about a possible paper on the formation of granite in the cores of scattered disk objects, but I broke it off because I wasn't ready to put something out there that would be difficult to amend tomorrow, and again next week and again next year, but it apparently ended worse than I thought because I notice I've been blacklisted from posting on Astrobiology Magazine.)
_"Our current theoretical ideas about galaxy formation are now quite different
to those of Mestel. The hierarchical theory (White and Rees, 1978) places
much more emphasis on galaxy merging as the process of large galaxy formation,
rather than a single monolithic collapse. However, although there is clear evidence
that galaxy merging does occur there is no clear evidence, for a galaxy like
the Milky Way, that it is the major physical process at the root of its formation.
Eggen, Lynden-Bell and Sandage (1962), using the metalicities and velocities
of stars, were the first to present us with good evidence that the Galaxy is the
result of the gravitational collapse of a proto-galaxy - a slowly rotating cloud
reduced to a thin disc within about 109 years. A picture of early star formation
in a spheroid (Population II) and subsequent star formation in a metal enriched
thin disc (Population I) is still a viable explanation for at least some part of
what we call the Milky Way galaxy. Based on the thickness of the Galactic disc,
Toth and Ostriker (1992) have argued that less than 4% of the mass within the
solar circle can have come from mergers and that this is inconsistent with the
hierarchical galaxy formation models. This problem with the thinness of galactic
discs has continued to provide a challenge to those who want to make galaxy
mergers the major galaxy assembly mechanism (Navarro et al., 1994, Benson et
al., 2004)."_
1. Gas disc scaling
"the required total surface density of matter in the spiral
discs he studied was, in the outer regions, approximately proportional to
the surface density of atomic hydrogen."
2. Maximum disc fitting
_"- there does
not appear to be any need for dark matter in the central regions of a
galaxy."
[Cuspy halo problem]
3. The disc halo conspiracy
_"Given that over the inner region of a galaxy
the stars and gas in a disc contribute significantly to the rotation while
it is the spherical dark halo that does this in the outer regions, it is very
strange that they ‘conspire’ to produce a flat rotation curve (van Albada
and Sancisi, 1986). There is no known physical reason for why this might
happen."_
4. The Tully-Fisher relation
_"there seems to be some conspiracy such that as surface
brightness decreases the mass-to-light ratio increases in exact proportion."_
_"It is difficult to see how these four enigmas can be accommodated within
our current theories of galaxies, galaxy formation and dark matter."_
[I.e., how can these 4 apparent congruences be explained by (essentially orthogonal) exotic particles that only interact through gravity?]
ReplyDelete_"We will show below that the column densities of the discs of
these galaxies are too high for such a sudden decline to be caused by ionization.
We alternately suggest that this drop in HI column density really does mark a
physical edge of the galactic disc (Bland-Hawthorn et al. 1997)."_
It shows how little I read that this is the first suggestion (Mestel’s disc) I've heard from elsewhere that spiral galaxies may have formed by gravitational collapse, rather than by accretion. [And could isothermal gravitational collapse have been mediated by a local endothermic reversal of a global exothermal phase change of the universe?]
I try very hard not to rely on alternative facts, only alternative interpretations, but the possibility that flat rotation rates of galaxies could arise from a concentration of DM in the disk plane, baryonic or otherwise, never occurred to me. (Then dark satellite galaxies could be drawn in from intergalactic DM medium, creating a thin DM halo that gives the mere impression of a massive halo. I'll have to see how it sits over time.)
DM still has lots of new ideas and new blood, which bodes well for its future, but I'm not sure I can say the same thing about planetary science, where the ad hoc secondary mechanisms (Jupiter migrated in before it migrated out) as enshrined in Grand Tack appear to be almost settled science. (Surely the gap between hot Jupiters and cold Jupiters is signaling something more fundamental than all or nothing planetary migration.)
....................
The Very Interesting Gas That Doesn't Do Anything
I'm always impressed by the amount of work that goes into moving the ball each millimeter down the field, which often seems to conclude with more questions than answers. I imagine the heavy lifting of scientific papers to be a lonely thankless job when I think of the desultory browsing of abstracts on arXiv by colleagues over their first cup of coffee in the morning.
David Carlson
ReplyDelete_"Brian Koberlein has gotten so many wacky theories from the uninitiated that he's recently established a policy of charging $200/hr for critiquing manuscripts, so I'm hoping to slip under the wire before you follow his lead."_
Well, you get what you pay for and I'm free... :P
"which suggests the formation of spiral galaxies by gravitational collapse, the first I'd heard of the idea from elsewhere."
Actually that's a very old idea. I did this for my undergraduate project. Start with a big spinning cloud of gas and it's pretty hard to avoid forming a really quite nice spiral galaxy.
http://astrorhysy.blogspot.cz/2015/08/what-has-dark-matter-ever-done-for-us.html
"I'm suggesting the universe has 5 times as many baryons as in ΛCDM cosmology, not merely that the 'ΛCDM missing baryons' are dark"
Ah, I see. Sorry, I missed that point the first time round.
"Even with suggested BBN sequestration of 4/5 of baryons into proto-spiral-galaxies, 1/5 would remain in the intergalactic medium, relying on ΛCDM coalescence (accretion) models to form dwarf spheroidal, elliptical and irregular galaxies. And the vast mass of superclusters would draw in vastly more of the intergalactic 1/5 of DM than island galaxies like Andromeda and the Milky Way. So this alternative ideology would necessarily incorporate ΛCDM cosmology to explain the excess DM in superclusters and to explain non-spiral dwarf galaxies."
I'm not clear on this point. My question is : why, if there is so much dark baryonic matter inside clusters (not just in the individual galaxies themselves - the motions of galaxies in clusters is too high for the clusters to be stable without dark matter), isn't it also forming stars and galaxies ? IIRC, there's considerably more dark matter in clusters than in individual galaxies. So you need an awful lots of baryons which just aren't doing anything - making the missing baryon problem far worse than in LCDM cosmology.
"With monatomic gas (γ = 5/3), the Jeans mass increases with increasing density, so these primordial GMCs didn't fragment into smaller (Bok) globules at the time."
Have you calculated the size/density of the clouds needed to avoid Jeans instability, or their freefall time ? You also need some mechanism whereby some of the baryons do collapse to form stars and galaxies, but most of them don't. It's not obvious to me what this is in your model. Plus you still need some large-scale collapse in order to explain the observed filaments and walls of galaxies.
"In that case, circa 2 to 200 Ms (solar mass) gravitationally-bound DM (Bok) globules within gravitationally-bound GMCs have persisted for 13 billion years (hydro)static equilibrium at, say, 50-100 Kelvins."
I just don't see how that is possible. Without actually doing the calculation (I will if you like though), I would guess such objects whould have to be much larger than Bok globules to be stable. Worse, they're going to be gravitationally perturbed by stars, spiral arms, and interactions with other galaxies. So I'm not sure how you could have so many dark gas clouds still surviving.
"I notice that UV detection of H2 clouds..."
Careful with you notation. HI = neutral, atomic hydrogen. H2 = molecular hydrogen, which is damn near impossible to detect directly (but see http://iopscience.iop.org/article/10.1086/312208/pdf). I would tend to associate UV emission with HII, ionized hydrogen.
https://ned.ipac.caltech.edu/level5/Combes3/Combes1.html
"I'm always impressed by the amount of work that goes into moving the ball each millimeter down the field, which often seems to conclude with more questions than answers. "
ReplyDeleteIn this case I think it's moved the ball back down the field, by quite some distance. It's an extremely surprising result, much more interesting than most press releases IMHO.
P.S. I know very little about the details of BBN, let alone BAO, so I'm not really the best person to ask about that.
ReplyDeleteRhys Taylor But I don't know that Brian Koberlein does active research like you do.
ReplyDeleteSince you're almost the very first contact I've had from an astrophysicist or astronomer, I know I have gaps in my knowledge you could drive a truck through, like never having run across the idea of spiral galaxies having formed by gravitational collapse. And almost providentially, you have expertise in my two areas of particular interest, atomic/molecular hydrogen and spiral galaxy formation by gravitational collapse. (I have a brother in law in London with an undergrad from Cambridge and a PhD from Yale who coauthored a few papers on cosmic inflation in his younger days, but he gets an indescribably pained expression on his face when he senses even a whiff of alternative science, so we talk about the weather instead.)
I suspect that the major phase changes of the universe are more significant than we're giving them credit for, particularly the epoch of Big Bang nucleosynthesis, which lasted from 10 seconds to 20 minutes after the Big Bang. So the fantastic exothermal energy released in combining protons and neutrons into (predominantly) helium-4 effectively clamped the ambient temperature of the universe to the BBN temperature range for 20 min / 10 sec = 120 lifetimes of the universe, like the phase diagram of water http://study.com/cimages/multimages/16/phasechange.jpg. So even in a radiation-dominated early universe, I suspect that gravity might get the upper hand if the radiation pressure were effectively neutralized by the isothermal nature of the epoch, with photodisintegration
It's hard to escape prejudice and I see it in my own automatic assumption that 1/5 of BBN baryons (assuming baryonic DM) escaped condensation into proto-spiral-galaxies in the intergalactic medium. The percentage of intergalactic baryons may just as well have been 50% instead of 1/5 by the end of the epoch, with virtually all of the BAO compressions condensing into proto-spiral-galaxies and none of the rarefactions. But either way, baryonic DM requires that nucleosynthesis is a regulating process that GUARANTEES the observed canonical Big Bang deuterium/hydrogen ratio (and Big Bang lithium/hydrogen, helium-3/hydrogen etc. ratios), regardless of the actual 'baryon density' of the universe, which is admittedly a HUGE unproved assumption.
The DM halo around spiral galaxies had indeed bothered me in terms of galaxy formation by gravitational collapse. I couldn't imagine why the stars would mostly confine themselves to the disk plane, with DM clouds, crashing through the disk plane on steeply-inclined halo orbits, but a heavy baryonic disc (J. I. Davies) formed by gravitational collapse with a nominal accretional DM halo gravitationally pulled in from the intergalactic realm is definitely growing on me. In fact I think I like it better than any alternative I can think of. And it would seem that disc DM would be significantly-more efficient at increasing the rotation rate of stars than halo DM, suggesting a much smaller percentage of DM in spiral galaxies than current models suggest. I think you may have won me over on this one even if you aren't convinced yourself. Rhys, did you have roll in creating the baryonic disc model?
"H2 may go unseen because its standard tracer, CO, is under-abundant (or frozen out) or because the gas is too cold for excitation to occur. Cold H2 is a candidate for hidden mass or ’dark matter’ in the universe, not at a level to change the global cosmological distribution of baryonic matter, dark matter and dark energy, but as contribution to the galactic baryonic dark matter (Combes & Pfenninger 1997; Kalberla et al. 2001 p. 297ff)."
(Kroetz et al., 2009, Direct Observations of Cold Molecular Hydrogen with Infrared Heterodyne Spectroscopy)
So that suggests two possibilities for baryonic DM:
ReplyDelete1) frozen out metallicity, and/or
2) too cold for infrared detection (presumably with gaseous carbon monoxide).
I'd tend to go with 1) rather than 2), which might conceivably allow for a sufficient thermal pressure support to prevent Jeans instability, although freezing out the high molecular weight metallicity raises the 'speed of sound' through the globule which promotes acoustic rebound, so merely subliming icy chondrules (by supergiant stars) with no temperature change also promotes Jeans instability.
"I would guess such objects would have to be much larger than Bok globules to be stable. Worse, they're going to be gravitationally perturbed by stars, spiral arms, and interactions with other galaxies. So I'm not sure how you could have so many dark gas clouds still surviving."
I do agree that invisibility and survivability of (Bok) globules over 13 billion years, particularly in a high-traffic spiral-galaxy disk plan, and I haven't a clue as to the pressure, temperature, volume and stability of a 200 solar mass globule in (hydro)static equilibrium.
"You also need some mechanism whereby some of the baryons do collapse to form stars and galaxies, but most of them don't. It's not obvious to me what this is in your model. Plus you still need some large-scale collapse in order to explain the observed filaments and walls of galaxies."
"an initially oblate spheroid tends toward a disk, and an initially prolate spheroid toward a spindle"
(Lin, Mestel and Shu, 1965, The Gravitational Collapse of a Uniform Spheroid)
I don't know if prolate spinning up into filaments or spindles is possible in gravitational collapse at a supercluster size, but if isothermal phase change epochs of the early universe promote gravitational collapse, perhaps there were one or more isothermal (phase-change-mediated) gravitational-collapse events prior to BBN which are reflected in galactic supercluster groupings. Even 13 billion years does not seem nearly long enough to form the scale of supercluster groupings in today's universe, unless they got a SIGNIFICANT early head start, so I'm leaning towards phase-change-mediated isothermal gravitational collapse prior to BBN to explain the present-day supercluster groupings. Baryon acoustic oscillations, of course, create a degree of concentration and rarefactions at a 490 million light year scale in today's universe, but that effect is far-less significant than what's observed in supercluster groupings which causes me to attribute the source of supercluster grouping to a much earlier epoch than recombination, (378,000 years after the Big Bang reflected in BAO).
"I'm not clear on this point. My question is : why, if there is so much dark baryonic matter inside clusters (not just in the individual galaxies themselves - the motions of galaxies in clusters is too high for the clusters to be stable without dark matter), isn't it also forming stars and galaxies ? IIRC, there's considerably more dark matter in clusters than in individual galaxies. So you need an awful lots of baryons which just aren't doing anything - making the missing baryon problem far worse than in LCDM cosmology."
Yes, that does imply an awful lot of DM baryons doing nothing in galactic clusters as well as in galactic halos, and perhaps only rare starburst galaxies are efficiently converting baryonic DM to stars, (but on the other hand, that bodes well for the long-term prospects of galaxy and star formation).
.........................
The Very Interesting Gas That Doesn't Do Anything
(Or The Dog That Did Nothing In The Night)
"We had a saying during the undergraduate course on general relativity : it all cancels and equals nought" Although gravitational waves supposedly travel at the speed of light, yet the Earth orbits the current position of the Sun, not where the Sun was 8 minutes ago--what ho!?
I didn't see it at first, but now I'm getting an inkling of the value of your study.
ReplyDeleteCould neutral hydrogen in the disk plane protect gravitationally-bound molecular hydrogen clouds in a baryonic galactic disc (BGD) from interstellar ultraviolet? Imagine two processes in tension with one another:
1) A BGD reservoir of molecular hydrogen clouds tend to mop up atomic hydrogen into gravitationally-bound giant molecular clouds in their invisible 'normal state', recombining atomic hydrogen (HI) into molecular hydrogen (H2), and cooling it down to invisibility.
2) Evaporation of hydrogen from giant molecular clouds and its dissociation into neutral atomic hydrogen by UV radiation, particularly by intergalactic ultraviolet radiation.
So by this suggestion, neutral atomic hydrogen would provide self-regulating galactic self-shielding of a baryonic galactic disc reservoir of dark, giant molecular clouds, suggesting why its quantity would tend to remain similar for unmerged and post-merged galaxies, and perhaps attributing a small increase in post-merger neutral hydrogen to a somewhat disrupted post-merger spiral structure, requiring slightly-more neutral hydrogen self-shielding.
So if the baryonic reservoirs of BGD spiral galaxies are primarily composed of invisible giant molecular clouds, then the visible neutral hydrogen self shielding is merely the tip of the ice berg which shouldn't be expected to be conserved before and after collision.
"The existence and detection of optically dark galaxies by 21cm surveys"
Once again, perhaps 21 cm self shielding by optically-dark spheroidal galaxies with the bulk of their baryonic DM in the form of H2.
Rhys Taylor
ReplyDeleteInterstellar Scintillation or Extreme Scattering Events or 'Paleons':
http://manlyastrophysics.org/Projects/InterstellarScintillation/index.html
The discovery of apparently gravitationally-bound Earth-mass (1E-6 solar mass) blobs of gas (circa 1 AU radius) traveling at 100s of km/s wrt the neutral gas of the galaxy and vastly outnumbering the stars in the Milky Way is to me the most astounding astrophysical proposition of my lifetime.
The following paper is much drier account of the findings:
http://arxiv.org/abs/astro-ph/0610737
Another section discusses the surprising stablity of 'hydrogen snowflakes':
http://manlyastrophysics.org/Projects/SolidHydrogen/index.html
It's almost easier to believe that these interstellar scintillation and extreme scattering events are caused by gravitational collapse explosive sublimation of hydrogen snow in the interstellar neutral hydrogen realm, than that Earth-mass gas-cloud paleons could persist from earlier epochs.
David Carlson Just a quick note to let you know I'm not ignoring you, and will respond as time permits. :)
ReplyDelete