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How is molecular hydrogen detected?

  1. May 26, 2012 #1

    JDoolin

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    My textbook seems to give conflicting information on whether molecular hydrogen can or cannot be detected. On the one hand it says (p393) "Dark matter is not hydrogen gas (atomic or molecular), nor is it made up of ordinary stars. Given the amount of matter that must be accounted for, we would have been able to detect it with present-day equipment if it were in either of those forms."

    However, it also says (p302) "Molecular hydrogen...does not emit or absorb radio radiation, so it cannot easily be used as a probe of cloud structure...Instead, astronomers use radio observations of other molecules, such as carbon monoxide, hydrogen cyanide, ammonia, water, and formaldehyde, to study the dark interiors of these dusty regions", i.e. they never actually see the hydrogen--they see the other molecules in the area, and assume the molecular hydrogen must also be there.

    So on the one hand, they say "We'd be able to H2 if it were there" and on the other hand they are saying "we can't see H2 directly--we can only see the other molecules in its presence."

    If they can't detect any radio emissions of molecular hydrogen, what spectrum ARE they using to locate it?

    (Source- Astronomy-A Beginner's Guide to the Universe-Sixth Edition)
     
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  3. May 26, 2012 #2

    cepheid

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    It's true that most H2 is too cold for any of its radiative transitions to be excited, therefore we can't see it (I'm pretty sure that there are exceptions -- places where we can see warmer H2). For the most part, we need to use CO as a tracer for it. Certain empirical rules are used to determine the total amount of molecular gas (which is almost all H2) that is present based on the amount of CO emission. The accuracy of these techniques is debated, but I always got the impression that it was sort of a "factor of 2" type of problem. So the point is, when it comes to dark matter, even if you take into account that most H2 is unseen (at least in emission), anywhere where it's cold enough for there to be H2, it's also cold enough for there to be other molecules, and indeed, for there to be solid matter condensed out in the form of tiny microscopic grains, which astronomers call "dust". We can see the other molecules, and we can see dust. So, if H2 were to account for the missing mass attributed to dark matter, we would have see a LOT more emission from its visible tracers than we do see. We'd also have to explain why dynamical considerations require the DM to be everywhere in a spheroidal halo surrounding the galaxy, whereas molecular gas clearly cannot exist everywhere.

    Besides all that, there are a host of other good observational reasons why DM has be non-baryonic (ie not made of ordinary atoms), not the least of which is that it doesn't interact with visible matter through any means other than the gravitational force, and it certainly doesn't absorb or emit light.
     
  4. May 26, 2012 #3

    JDoolin

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    Well, I can see how the presence of Carbon Monoxide implies the presence of molecular hydrogen, but I don't see how the presence of molecular hydrogen implies Carbon Monoxide.

    If I understand right, Carbon can only occur as a result of nuclear fission inside a star. So if you see carbon monoxide, you're seeing the emissions of a star, a red-giant or supernova explosion. But the hydrogen was there before the star formed, and it would have existed without any Carbon or heavier atoms.
     
  5. May 26, 2012 #4

    cepheid

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    A couple of other points. I never answered your question of how H2 is detected in cases where it can be seen in emission. The intro to this paper talked about how the molecule's rotational transitions lead to emission in the mid-infrared (tens of microns):

    http://arxiv.org/abs/1109.2544

    The second point is that even if you can't see molecular hydrogen in emission, I'm pretty sure there are cases where you can see it in absorption (sillouhetted against luminous emission from nearby stars, and even seeing absorption line features from it in the spectra of other objects e.g. in the UV portion of stellar spectra). Granted, this may only allow you to see the densest clouds that happen to be in warmer surroundings (and haven't been fully dissociated by ionizing radiation), but at least it is an indication that it is there.
     
  6. May 26, 2012 #5

    cepheid

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    The ISM has been enriched with "metals" (elements heavier than helium) through billions of years (several generations) of star formation in our galaxy. So it's no longer true that these elements are localized only to supernova remnants or planetary nebulae (relics of dead stars). They've had time to spread out somewhat homogeneously. In fact, the molecular gas in the galaxy is spread out over a fairly wide area. It exists in a large ring between 3.5 kpc - 7.5 kpc from the galactic centre, in the galactic plane (although I understand that there is also some diffuse stuff at high galactic latitudes i.e. off the plane). We know this from tracing CO emission ;)
     
    Last edited: May 26, 2012
  7. May 26, 2012 #6

    JDoolin

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    (Note: I have not read post 4 and 5 yet--criss-crossed communication.)

    My real question here is whether it really is that clear that molecular hydrogen gas "cannot exist everywhere." Is it really 100% transparent to the radio waves? Precisely how much light-blocking power does it have, and at what frequencies? At what densities would it be possible to see through a billion light-years of the stuff as though it weren't even there?

    The thing is, yeah, clearly, you'd think it was unlikely that a substance could be that transparent, but on the other hand, when you think about star formation, when you look at the lobes of a radio galaxy; or the bars on a bar-galaxy, it leads me to think there seems to be something out there; a gas that everything else is running into. And when you think about star formation, it seems like you need an initial bunch of stuff to start from, and we already know it was hydrogen gas.

    So I'm thinking there must be a large portion of the stuff still out there that hasn't yet fallen into a clump to make stars.
     
  8. May 26, 2012 #7

    JDoolin

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    The outer radius of this ring of detectable molecular gas is about where the sun orbits the galactic center. Now on the other hand, the part of the "galactic rotation curve" where the orbits are faster than expected due to dark matter starts around radius of 15 kpc and beyond.

    Our galaxy is about 15 kpc in radius, though I'm looking at a "galactic rotation curve" in my text that extends out past 35 kpc. Its in that range of 15 to 35 kpc where the curve deviates heavily from keplerian motion, and indicates the presence of dark matter.

    The earth itself is a relic of a dead star. With an iron core, it was probably ejected from a type II supernova. Might it be possible that anything closer than 7.5 kpc to the center of the galaxy was a remnant of the same supernova? And more to the point--in the region from 15 to 35 kpc, there would be pure molecular hydrogen--so far unpolluted by supernova remnants.
     
  9. May 26, 2012 #8

    Chronos

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    It appears your textbook is slightly misleading. Atomic hydrogen is easily detected via 21 cm band emissions. Molecular hydrogen is the more common, and stable species. It does not emit in the 21 cm band. It is normally detected by indirect means, as noted by cepheid.
     
  10. May 27, 2012 #9

    JDoolin

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    So, just to reiterate... there is no known direct way to detect diffuse cold molecular hydrogen?
     
  11. May 27, 2012 #10

    Chronos

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    Molecular hydrogen has a UV signature which is difficult to detect. It is readily absorbed and easily scattered.
     
  12. May 28, 2012 #11
    It can be detected when gravitational potential energy causes it to coalesce and heat up. Any tiny variation in the density will cause the cloud to begin an isothermal and finally an adiabatic collapse.

    Now, think about where all the dark matter is. Most of it is in the halo, exactly where there are very few stars. But how could there be very few stars if there are these huge, diffuse clouds of hydrogen? The hydrogen would have to be maintained in some kind of perfect density gradient that kept it from collapsing. Now, you suggest that the collapse is just extremely slow (on the order of 10 billion years). But this flies in the face of a multitude of globular clusters that are nearly as old as the Milky Way in the Halo. So why did those clouds of molecular hydrogen in the halo collapse but not this one? That sounds like special pleading to me. How does it sound to you?
     
  13. May 28, 2012 #12
    The main evidence for dark matter is that lots of things would break down if it turns out that dark matter were made of baryons.

    Also I found this really interesting article....

    http://arxiv.org/abs/1107.3314
     
  14. Jun 15, 2012 #13

    JDoolin

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    Sorry I overlooked this response before.

    I'm not entirely sure how to answer your question, but are you taking into account the changing density over time? Are you assuming that the local conditions were the same 10 billion years ago as they are now?

    Consider that as we go back toward the Big Bang, each time you divide the age of the universe by two, you multiply the density by 8. If you agree with that reasoning, then consider, if the universe is 14 billion years old right now, at 7 billion years, it had 8 times its current density. At 3.5 billion years it had 64 times its current density.

    The globular clusters formed sometime around at least 10 billion years ago, when the universe was at most 3.5 billion years old. Which would mean they formed when the gas was at least 60 times as dense as it is now. And since ALL the globular clusters are at least 10 billion years old, it suggests that they stopped forming, at a certain time, and my suggestion is that they stopped forming because the density of the universe dropped below some certain critical level.

    Take that back another couple of steps. At 1.75 billion years, the universe would have had 64*8 = about 500 times its current density. At 900 million years, the universe would have had 500*8=4000 times its current density. At 450 million years, 32,000 times the density, etc, and you can keep going back in time and getting exponentially more and more density.

    In this extremely dense environment, A supernova explosion, for instance, at that time could have a wildly different effect than a supernova explosion now, and could have made the perturbations that made our entire galaxy possible.
     
    Last edited: Jun 15, 2012
  15. Jun 17, 2012 #14

    Bobbywhy

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    “Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is CO (carbon monoxide). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.”

    http://en.wikipedia.org/wiki/Molecular_cloud
     
  16. Jun 18, 2012 #15

    JDoolin

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    Right. I just think it is strange to ignore the possibility that there may be large amounts of H2 that is NOT accompanied by Carbon Monoxide. It seems to me, only that H2 which has interacted with supernovae and red giants should have any Carbon Monoxide in it.

    It seems to me that this explanation (the thought that the ratio of H2 to CO is constant) must be assuming that all of the H2 was emitted from stars. It completely ignores the possibility that there was H2 without carbon monoxide long before there was H2 with carbon monoxide, and that some, or even most of that pure H2 might remain.
     
  17. Jun 18, 2012 #16
    I'm assuming that the Jean's Mass formula is still applicable.

    I am assuming that you are referring to the density of the universe, not an individual galaxy. While galaxy formation is still something of a mystery, I believe I am correct in stating that early in a galaxy's formation, it is in the process of overall increasing its density, not decreasing it. I do not really know how that might affect the density of molecular clouds, but you could certainly research it in the published literature.

    Remember, the density of the universe is not necessarily linearly proportional to the density of early galaxies or the density of the regions where globular clusters formed. For instance, the density of the visible universe is still decreasing, but the density of the Milky way is constant.

    It seems like a reasonable hypothesis. The question is, where is the evidence?

    Showing a correlation between density and formation of clusters does not actually support your hypothesis. You need to model how the clusters formed and how the density of the universe would affect their formation.

    I believe others have theorized this in regards to current stellar evolution. You might want to research papers on supernova-induced star formation if you have not already.
     
  18. Jun 19, 2012 #17

    Chronos

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    Twofish-Quant is our supernova expert, I'm certain he could shed light on this issue.
     
  19. Jun 19, 2012 #18
    Not much. This is an ISM question and not a supernova question. :-) :-)

    One thing that I found rather surprising is that it turns out that early universe chemistry is incredibly complicated.
     
  20. Jun 19, 2012 #19

    JDoolin

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    I bolded a few statements here; mainly I need to model how the clusters formed and how the density of the universe would affect their formation.

    I don't have a quantitative model, but I can qualitatively describe three distinct stages--perhaps four.
    Stage 1--Universe Age: Very young. Galaxy forming stage. Extremely high density. Perturbation caused by supernova results in a gravitational gradient sufficient to overcome outward Hubble-velocity.
    Stage 2--Universe Age, Less than a billion years. Globular Cluster forming stage: Medium density. Perturbation caused by supernova results in clumping of matter into stars, but insufficient to overcome outward Hubble-velocity.
    (Stage 3)--Universe Age--Current. Spiral forming stage. Superluminal jets fire into already swirling gasses, causing it to clump into stars.
    Stage 4--Universe Age--Current. Diffuse stage. Supernova explosion is not sufficient to cause clumping into stars

    I made a little video to see if I could make this clearer:
    http://screencast.com/t/QxU3YaeWAkXM

    I hope this makes clear some of the other differences between this model and your model.
    (1) in my hypothesis, the overall density of the universe equal to the overall density of a galaxy or a globular cluster at any given time. The difference is not in density but in clumpiness.
    (2) You are correct in saying that galaxy formation involves increasing the density; not decreasing it; but I'm looking for a phenomenon sufficient to reverse the Hubble flow, and clump, surrounded by a homogeneous distribution of matter. In your model, you have the distribution already starting out pre-clumped, and it becomes more clumped.
    (3) I don't have any additional evidence. You're already aware of spiral galaxies, bar galaxies, Hubble's law, and globular clusters.

    The only thing we disagree about is the level of clumpiness in the universe. You think that the universe is clumpy on the scale of galaxies, and clumpy on the scale of solar systems. I think that the universe is homogeneous on every scale right down to the cubic meter, but clumps up on the scale of stars, because of perturbations.
     
  21. Jun 19, 2012 #20
    FYI, I'm going to put on my boxing gloves. If you want to propose a serious astrophysical model, then that means that you want to get into the boxing ring and treated like a professional boxer. So I'm not going to pull punches.

    A qualitative model is useless since it's impossible to make predictions that are detailed enough to compare with observations. Now it doesn't have to be a complicated quantitative model, but you need to run some numbers.

    One quick thing to calculate is that age of the universe at which the average density of the universe reaches densities that are typical of the interstellar medium. My guess is that it's going to end up before you have any stars at all.

    What you need to be able to generate are *NUMBERS*. How many globular clusters do we expect to see? What's the density of galaxies? What's the distribution of bright matter and dark matter? What's the temperature of the gas? I want correlation functions, spectral predictions, etc. etc.

    I don't think this is going to work since you are dealing with different scales. Supernova explosions happen on length scales of kiloparsecs when you already have large local gravitational fields that overwhelm the Hubble flow. If you are talking about supernova shock waves then the Hubble flow is going to be irrelevant.

    Supernova bubbles are smaller than galaxies and can't affect Hubble flow. Supernova bubbles also have negligible gravational gradients. The shock wave is purely a gas pressure phenonmenon.

    The other thing is were did the supernova come from? If you have supernova then you already have stellar formation, and if you have stellar formation, then things are already clumping.

    (2) You are correct in saying that galaxy formation involves increasing the density; not decreasing it; but I'm looking for a phenomenon sufficient to reverse the Hubble flow, and clump, surrounded by a homogeneous distribution of matter. In your model, you have the distribution already starting out pre-clumped, and it becomes more clumped.

    Jeans instability.

    Well, you are wrong.

    The matter correlation spectrum is pretty well established, and it pretty clearly shows that things clumped top down rather than bottom up. During the 1980's it was an extremely big debate between the hot dark matter people that argued that galaxies first formed and then clustered into superclusters, and the cold dark matter people that argued that the superclusters formed first.

    The data supports the CDM people.
     
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