Dude! Where's my baryons?

By Julianne Dalcanton | August 6, 2007 4:12 pm

Now, I like dark matter and dark energy as much as the next person (ok, maybe not quite as much as Mark). Still, I simply don’t have the temperment to spend the majority of my mental energy on ideas that are so speculative that, while interesting, they’re probably wrong. I’m not trying to disrespect the theorists, but it takes a certain mindset to enjoy the type of process Sean described. If you read the three part series, nowhere will you see the evidence that Sean or his collaborators actually believe that inflation had a preferred direction. They just decided it would be cool to explore, and will shed no tears if inflation turns out to be a nice creamy vanilla of isotropy. Me, I’m enough of pragmatist that I’m happiest spending my time working on stuff that actually exists. So, no Nobel Prize for me, but I’m ok with that.

What this has been leading me to think about over these past few weeks is where the normal matter actually wound up. Astrophysicists usually refer to “normal matter” as “baryons” — stuff like neutrons and protons which carry most of the mass of the non-exotic universe — and astronomically speaking, there’s not that many places it can wind up. Most of the baryons in the Universe start out as hot ionized gas (i.e. a plasma of free electrons and positively charged nuclei), and as time goes on, some of this gas cools into colder phases (neutral atomic or molecular gas, which are fairly easy to detect), some of which further cools into dense stars (which then heat up again due to nuclear fusion in the core) or even into solids (dust or rocky planets). So, by the present day, normal matter is either in gas (of various temperatures, densities, and ionization states) or in stars, with a smattering of solids and a tiny pinch of liquids.

Although these cooling and transformation processes are a rich field of “gastrophysics” (as the term goes), the overall landscape of baryons is sculpted by the dark matter, to first order. Dark matter has more than 80% of the mass of the Universe, so it dominates the gravitational forces in more than 99% of the volume of the Universe. Only when the barons become extremely concentrated (like in stars, or in the centers of galaxies) does normal matter significantly shape where stuff winds up. In principle.

In reality, however, the baryons actually spend quite a bit of time bossing each other around, in spite of dark matter’s best attempts to impose the kind of order a theorist would prefer. Not only does gas cool through interactions with itself (i.e. particle A gets near particle B, and an energetic photon ensues, running off with some of the particles’ energy), it also heats back up, through supernovae, which spew superheated gas outwards which in turn shock-heats any cool gas unfortunate enough to get in its way, and possibly through jets emitted by accreting black holes in the centers of galaxies. (Astrophysical theorists are all a twitter about this last idea these days, but I haven’t bought into it yet.) In other words, the observable Universe contains a level of complexity that should make any theorist shudder.

Now, if you’re not predisposed to embracing such complexity, why might you care? Well, back when I was a grad student (you know, when dinosaurs roamed the earth and the iPhone did not yet exist), light emitted by galaxies traced mass. If you knew where the galaxies were, you knew where the dark matter was. Theorists stuck in a simple numerical constant (a “bias factor”) to allow some wiggle room, but mapping the observable Universe onto the dark sector was something we assumed we could do easily. Unfortunately, we’ve since come to appreciate how drastically baryonic process change where, when, and if stars form, and thus alter the numbers and properties of galaxies. These effects are worst in the smallest dark matter halos, where the gravitational binding energies approach the typical energies of baryonic heating by supernovae. Thus, when you count galaxies, you’re no longer counting dark matter halos in a simple one-to-one mapping.

Worse still, it’s not even clear that most of the baryons wind up in galaxies at all. In the most massive gravitationally bound dark matter halos (i.e. those that surround clusters of galaxies), everything looks OK. If you add up the mass in stars and gas in the cluster, and compare it to the mass of the dark matter halo (which you’ve inferred from some combination of kinematics, gravitational lensing, or gas pressure gradients), you get about the ratio you expect from concordance cosmology. Basically, the cluster is massive enough that no matter what happened to the baryons, they got stuck in the gravitational potential well of the cluster, so they’re all still there when you count them. On the scale of galaxies like the Milky Way, however, our best accounting says that maybe 20% of the normal matter that exists actually winds up as stars or easily detectable cool gas. So, for typical galaxies, 80% of the normal matter just doesn’t seem to be around. It’s probably in some hot phase that we have a hard time detecting with current instrumentation, but its not clear if it came into the galaxy and then got shot back out, or just never made it in.

On the scale of the lowest mass galaxies (which are particularly interesting, because they give you the most leverage on the small scale power spectrum), we frankly don’t know what the f*@$ is up. Really. I’ve spent the past week or so working hard on this problem, and the lowest mass galaxies are just a huge freakin’ mess. We typically estimate the mass of the dark matter by using the motions of gas and stars and then assuming that if the whole mess isn’t flying apart, then there must be some amount of mass holding the galaxy together. However, these low mass guys are so puny and pathetic that (1) you can’t figure out if their motions are actually rotational or just localized gas physics pushing stuff around and (2) you can’t figure out what fraction of the galaxyies’ total motion is oriented along the line of sight. And don’t even get me started on interactions with larger galaxies. Thus, you measure a characterstic range of internal velocities for these guys, and it tells you squat about the characteristic velocities of the dark matter halo the galaxy lives in. It’s enough to make one go drink some beer.

Which I will now do.

CATEGORIZED UNDER: Science
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  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    If you know that something exists, what’s the point in thinking about it?

  • brad

    I’m guessing y’all don’t have much control over which Google ads appear in your scrollbar at right.
    I was amused at the “dangers of believing evolutionary theory” ad appearing next to this particular set of bloggers!

    Will Sean be taking Templeton money in the future? :-)

  • Myhatma Gander

    Baryons: The Boring Bits [TM]

  • loonunit

    # of commentors currently attracted to baryon discussion: 2

    # of commentors currently attracted to Harry Potter discussion: 86

    Conclusion: Harry Potter and the Deathly Hallows hard cover copies are dark matter.

    Bring me my Nobel.

  • Ijon Tichy

    If you know that something exists, what’s the point in thinking about it?

    Because it may not exist in the way you think it does.

  • Ellipsis

    Just wondering… could temporal coherence, as a function of frequency, of photons from gravitationally lensed (i.e. galaxy-galaxy) light potentially give any (clean) information on DM characteristic velocities?

    If so, perhaps a decent paper for Sean or Mark, etc.

    However, even if so, probably not enough photons for that anyway with present-day telescopes.

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  • DIS

    No Nobel Prize has ever been given for something that does not exist.

    Theorist have only been rewarded for accounting for nontrivial and far reaching experimental facts and for correct nontrivial predictions of far reaching consequence.

    So you are in very good company. Maybe you’ll even run into a Nobel prize

  • http://www.mpe.mpg.de/~erwin/ Peter Erwin

    I’m guessing y’all don’t have much control over which Google ads appear in your scrollbar at right.
    I was amused at the “dangers of believing evolutionary theory” ad appearing next to this particular set of bloggers!

    Well, I’m amused by the fact that there’s currently an ad for “Montana Dude Ranches” over in the sidebar, evidently because Julianne used “Dude” in the title…

  • http://meditations-on-an-eyeball.blogspot.com/2007/06/god-as-stop-gap.html Vance Harwood

    Could anyone post a link to how dark matter halos are formed? From what I’ve seen the evidence for existence looks pretty compelling, but I’ve never found an explanation for why dark matter doesn’t normally gravitate into the baryon rich part of galaxies. I am assuming that our solar system for example doesn’t have a lot of dark matter in it, otherwise our calculations for orbits of the planets, etc. would not match up to their observed masses.

  • http://blogs.discovermagazine.com/cosmicvariance/julianne Julianne

    Frank van den Bosch and Guinevere Kauffmann wrote a nice article for the general public in Scientific American (June 2002). It’s bound into a special issue here. A bit more sleuthing (or a trip to a library) could probably turn up a PDF of the article itself.

    From your description, you have most of the story correct. Both baryons and dark matter do tend to concentrate in the same places, but because baryons can lose energy (usually by emitting photons), while dark matter can’t (since it’s dark), the baryons can concentrate even further within the dark matter halo. Thus, we have a solar system that is very dense in baryons, but find little dark matter within the radius of the outer planets.

  • http://countiblis.blogspot.com Count Iblis

    Why wouldn’t we able to see such large quantities of hot gas?

  • http://blogs.discovermagazine.com/cosmicvariance/julianne Julianne

    Why wouldn’t we able to see such large quantities of hot gas?

    If it gets hot enough, like in clusters of galaxies, then you can detected it in the X-ray with current instruments. But, there’s a decade or so of astrophysically interesting temperatures where it doesn’t emit enough x-rays to be readily detected. You can get it in absorption, but that’s tougher to interpret except in a broad statistical sense.

  • http://www.phenix.bnl.gov/WWW/publish/stankus/Intro_Cosmology/ Paul Stankus

    Hi Julianne —

    On a related topic, I was wondering(/hoping) if you could take a few minutes to talk about the (possible) connections between galaxy formation and super-massive black holes.

    It’s been a while since I knew anything about the subject, but I recall a time when people were surprised and annoyed to learn that, in simulations, relaxing dark matter clouds tended to develop singularities of infinite density at their cores. One can try to constrain halo models to be non-singluar (“softened isothermal sphere”); but is it entirely a coincidence that when you look into the cores of real galaxies you find … singularities of infinite density? ie super-massive black holes (SMBH).

    In your understanding — orders of magnitude better than mine, I trust — what are the best ideas for how SMBH’s form in galaxies? Can they grow by accreting appreciable amounts of dark matter? Should we expect SMBH’s to exist in the cores of galactic cluster halos? or just in galaxies? (this may be strongly connected to the baryon question.)

    Also, here’s my favorite question, if you’re up for it: within whatever mechanism you might favor, why would we expect a given galaxy to have just _one_ SMBH and not several?

    Thanks for whatever wisdom you can share, plus references.

    Regards,

    Paul

  • http://meditations-on-an-eyeball.blogspot.com/2007/05/meditations-on-patch-of-sky-inside-big.html Vance Harwood

    Julianne, thanks for the Frank van den Bosch and Guinevere Kauffmann link. It was very helpful. I really had been thrown off by the halo terminology –a la angelic ring above the head…

  • http://countiblis.blogspot.com Count Iblis

    If it gets hot enough, like in clusters of galaxies, then you can detected it in the X-ray with current instruments. But, there’s a decade or so of astrophysically interesting temperatures where it doesn’t emit enough x-rays to be readily detected. You can get it in absorption, but that’s tougher to interpret except in a broad statistical sense.

    Thanks! Also, is the universe invisible below certain radio frequencies? I mean, the ionized gas must have some plasma frequency below which radio-waves do not propagate….

  • Jim Graber

    The new “very dry” 4 galaxy merger:
    http://www.arxiv.org/PS_cache/arxiv/pdf/0708/0708.0011v1.pdf
    No gas , no star formation, but still enough radiation to be detected in X-rays.
    So are the extra baryons here all hot, or have they been blown completely out of the cluster?
    Or are they hiding somewhere else?
    Do “dry mergers” only happen in small clusters?
    Or can they happen in big ones, too?
    Inquiring minds want to know…
    Thanks.
    Jim Graber

  • http://skepticalcosmologist.blogspot.com/ The Skeptical Cosmologist

    Nice post Julianne! It was very accessible and well-written. It is also on a topic close to my heart, which inspired me to write my very first scientific post on my brand new weblog.

    There, I talk about the importance of bookkeeping in Science, which is also a response to Sean’s cheeky comment:
    “If you know that something exists, what’s the point in thinking about it?”
    As I mention there, and contrary to what you say, baryonic accounting doesn’t seem to exactly work out, even in galaxy clusters. The discrepancy is not quite as big as what you are working on, but it is (in my opinion) statistically significant.

  • Martin

    So, what aspect of AGN heating haven’t you bought into, then? The evidence that they do put large amounts of energy into the hot phase of the IGM is pretty overwhelming; the evidence that by doing so they solve all known structure formation problems less so, so I guess it’s the latter that you’re expressing doubt about…

  • http://bjkeefe.blogspot.com/ bjkeefe

    Gastrophysics. I like it! (And so we now know, at last, whence came gastronomy?)

    Further on, Julianne, you invoke a cliché for the distant past: “… back when I was a grad student (you know, when dinosaurs roamed the earth and the iPhone did not yet exist) …”

    I don’t know how any self-respecting person in your field neglects to add “… and when Pluto was a planet.”

    Just sayin’.

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  • http://inventing-solutions.htm sol aisenberg

    Actually there is no need for dark matter to explain the observations of Fritz Zwicky (motion of groups of galaxies) or of Vera Rubin (flat velocity rottation curves of spiral galaxies).

    The supposed need for dark matter is due to the ASSUMPTION/SPECULATION (without observational proof) that Newton’s law of gravity and gravitational constant are also valid outside our solar system. My theory of extended gravity, XTG, is based upon the fact that the equation v*v/r = M*G/(r*r) reduces to M*G=v*v*r and – in the region where the rotation velocities are constant – M*G is a linear function of distance. This led to the false ASSUMPTION/SPECULATION that the linear mass distribution – although not visible – exists as a halo around the visible matter and is dark mattter.

    When we take the alternate assution that G has the linear dependence on distance, r, the need for dark matter is removed – and researchers can spend their time and funding on more productive matters.

    Details will be on my web site (undergoing major updates). Included will be interesting clarification of dark energy, expanding universe, inflation, CMB, information in black holes, and more. Do a Google search on my name.

    Comments will be appreciated.

  • http://lablemminglounge.blogspot.com/ Lab Lemming

    How much of the baryonic matter in galaxies is in burned-out stars that are hard to see? e.g. neutron stars, black holes, etc. For that matter, where are all the neutron stars? given the short lifespans of large stars and the age of the galaxy, shouldn’t there be lots of burned out stars floating around our neighbourhood?

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